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FU Orionis (FUor) and EX Lupi (EXor) type objects are two groups of peculiar and rare pre-main sequence low-mass stars that are undergoing powerful accretion outbursts during their early stellar evolution. Water masers are widespread in star forming regions and are powerful probes of mass accretion and ejection, but little is known about the prevalence of them toward FUors/EXors. We perform the first systematic search for the 22.2 GHz water maser line in FUors/EXors to determine its overall incidence to perform follow-up high angular resolution observations. We used the Effelsberg 100-m radio telescope to observe the 22.2 GHz H2O maser toward a sample of 51 objects. We detect 5 water masers; 3 are associated with eruptive stars, resulting in a 6% detection rate for eruptive sources. These detections include one EXor, V512 Per (also known as SVS 13 or SVS 13A), and two FUors, Z CMa and HH 354 IRS. This is the first reported detection of water maser emission towards HH 354 IRS. We detect water maser emission in our pointing towards the FUor binary RNO 1B/1C, which most likely originates from the nearby deeply embedded source IRAS 00338+6312 (~4'', from RNO 1B/1C). Emission was also detected from H$_2$O(B) (also known as SVS 13C), a Class 0 source ~30'', from the EXor V512 Per. The peak flux density of H$_2$O(B) in our observations, 498.7 Jy, is the highest observed to date. In addition to the two non-eruptive Class 0 sources (IRAS 00338+6312 and H$_2$O(B) /SVS 13C), we detect maser emission towards one Class 0/I (HH 354 IRS) and two Class I (V512 Per and Z CMa) eruptive stars. We demonstrate the presence of 22.2 GHz water maser emission in FUor/EXor systems, opening the way to radio interferometric observations to study these eruptive stars on small scales. Comparing our data with historical observations suggest that multiple water maser flares have occurred in both V512 Per and H$_2$O(B)., 12 pages, 8 figures, accepted for publication in Astronomy & Astrophysics

FU Orionis (FUor) and EX Lupi (EXor) type objects represent two small, but rather spectacular groups of low-mass, young eruptive stars. Outbursts of several magnitudes are observed, attributed to enhanced accretion from the circumstellar disk onto the central protostar. The host molecular environments of FUors/EXors are poorly explored due to the scarcity of systematic molecular line observations. We carried out the first dedicated survey of the molecular environments of a large sample of FUors/EXors, observing a total of 51 sources with the aim of studying the ammonia (NH$_3$) emission in their host environments. We observed the ammonia (J,K)=(1,1), (2,2), and (3,3) inversion transitions using the Effelsberg 100-m radio telescope. We derived H$_2$ column densities and dust temperatures using archival Herschel SPIRE data. We detected the (1,1) transition toward 28 sources and the (2,2) transition toward 12 sources, while the (3,3) transition was detected toward only two sources. We find kinetic temperatures between ~12 K and 21 K, ammonia column densities from $5.2\times10^{13}\,cm^{-2}$ to $3.2\times10^{15}\,cm^{-2}$, and fractional ammonia abundances with respect to H$_{2}$ from $4.7\times10^{-9}$ to $1.5\times10^{-7}$. The results are comparable to those found in infrared dark clouds (IRDCs). Kinetic analysis suggests that most of the eruptive stars in our sample reside in rather quiescent (sonic or transonic) host environments. Our NH$_3$ observations and analysis of the SPIRE dust-based H$_2$ column density maps confirm the presence of dense material toward 7 sources in our sample; additional sources might also harbour dense gas based on their NH$_2$ (2,2) detections, might indicate an earlier phase than originally classified. Based on our results, we suggest observations targeting additional molecular lines would help to refine the evolutionary classification of eruptive stars., 20 pages, 2 figures, accepted for publication in A&A

Young stars show variability on different time-scales from hours to decades, with a range of amplitudes. We studied two young stars, which triggered the Gaia Science Alerts system due to brightenings on a time-scale of a year. Gaia20bwa brightened by about half a magnitude, whereas Gaia20fgx brightened by about two and half magnitudes. We analyzed the Gaia light curves, additional photometry, and spectra taken with the Telescopio Nazionale Galileo and the Gran Telescopio Canarias. Several emission lines were detected toward Gaia20bwa, including hydrogen lines from H$\alpha$ to H$\delta$, Pa$\beta$, Br$\gamma$, and lines of Ca II, O I, and Na I. The H$\alpha$ and Br$\gamma$ lines were detected toward Gaia20fgx in emission in its bright state, with additional CO lines in absorption, and the Pa$\beta$ line with an inverse P Cygni profile during its fading. Based on the Br$\gamma$ lines the accretion rate was $(2.4-3.1)\times10^{-8}$ $M_\odot$ yr$^{-1}$ for Gaia20bwa and $(4.5-6.6)\times10^{-8}$ $M_\odot$ yr$^{-1}$ for Gaia20fgx during their bright state. The accretion rate of Gaia20fgx dropped by almost a factor of 10 on a time-scale of half a year. The accretion parameters of both stars were found to be similar to those of classical T Tauri stars, lower than those of young eruptive stars. However, the amplitude and time-scale of these brightenings place these stars to a region of the parameter space, which is rarely populated by young stars. This suggests a new class of young stars, which produce outbursts on a time-scale similar to young eruptive stars, but with smaller amplitudes., Comment: Accepted to MNRAS

No statistical study of COMs toward a large sample of high-mass protostars with ALMA has been carried out so far. We aim to study six N-bearing species: CH$_3$CN, HNCO, NH$_2$CHO, C$_2$H$_5$CN, C$_2$H$_3$CN and CH$_3$NH$_2$ in a large sample of high-mass protostars. From the ALMAGAL survey, 37 of the most line-rich hot molecular cores are selected. Next, we fit their spectra and find column densities and excitation temperatures of the above N-bearing species, in addition to CH$_3$OH. We (tentatively) detect CH$_3$NH$_2$ in $\sim32%$ of the sources. We find three groups of species when comparing their excitation temperatures: hot (NH$_2$CHO; Tex > 250 K), warm (C$_2$H$_3$CN, HN$^{13}$CO and CH$_{3}^{13}$CN; 100 K < Tex < 250 K) and cold species (CH$_3$OH and CH$_3$NH$_2$; Tex < 100 K). This temperature segregation reflects the trend seen in their sublimation temperatures and validates the idea of onion-like structure of COMs around protostars. Moreover, the molecules studied here show constant column density ratios across low- and high-mass protostars with scatter less than a factor $\sim3$ around the mean. The constant column density ratios point to a common formation environment of COMs or their precursors, most likely in the pre-stellar ices. The scatter around the mean of the ratios, although small, varies depending on the species considered. This spread can either have a physical origin (source structure, line or dust optical depth) or a chemical one. Formamide is most prone to the physical effects as it is tracing the closest regions to the protostars, whereas such effects are small for other species. Assuming that all molecules form in the pre-stellar ices, the scatter variations could be explained by differences in lifetimes or physical conditions of the pre-stellar clouds. If the pre-stellar lifetimes are the main factor, they should be similar for low- and high-mass protostars., 49 pages, 46 figures, Accepted for publication in A&A

A pioneering study showed that the fine structure in the luminosity function (LF) of young star clusters contains information about the evolutionary stage (age) and composition of the stellar population. The notable features include the H-peak, which is the result of the onset of hydrogen burning turning pre-main sequence stars into main sequence stars. The feature moves toward the faint end of the LF, and eventually disappears as the population evolves. Another detectable feature is the Wielen dip, a dip at M_V ~ 7 mag in the LF first identified in 1974 for stars in the solar environment. Later studies also identified this feature in the LF of star clusters. The Wielen dip is caused by the increased importance of H- opacity in a certain range of low-mass stars. We studied the detailed structure in the luminosity function using the data from Gaia DR2 and PARSEC stellar evolution models with the aim to further our understanding of young stellar populations. We analyzed the astrometric properties of stars in the solar neighborhood (< 20 pc) and in various relatively nearby (< 400 pc) young (< 50 Myr) open clusters and OB associations, and compare the features in the luminosity function with those generated by PARSEC models. The Wielen dip is confirmed in the LF of all the populations, including the solar neighborhood, at M_G ~7 mag. The H-peak is present in the LF of the field stars in the solar neighborhood. It likely signals that the population is mixed with a significant number of stars younger than 100 Myr. The H-peak is found in the LF of young open clusters and OB associations, and its location varies with age. Our observations with Gaia DR2 confirm the evolution of the H-peak from 5 Myr up to 47 Myr. The fine structure in the luminosity function in young stellar populations can be used to estimate their age., 16 pages, A&A in press

Context. Double-lined spectroscopic (SB2) binaries allow us to obtain a direct determination of the masses of their components, which is essential to test stellar models. Although these objects only provide a lower limit for the mass, they are more abundant than their eclipsing counterparts as they are not as strongly limited by the inclination of their orbit. Aims. Our aim is to derive the orbital and physical parameters of GJ 1284, a young SB2. We also revise the membership of this system and its two wide co-moving companions, GJ 898 and GJ 897AB, to a young moving group to assess, along with other youth indicators, their age. Afterwards, we compare the results from these analyses and the photometry of these systems with several pre-main-sequence evolutionary models. Methods. We use high-resolution spectra to determine the radial velocity of each component of GJ 1284 and the orbit of the system alongside its systemic velocity. Additionally, we use TESS photometry to derive the rotational period of the GJ 1284 and its two wide companions. Results. GJ 1284 is a binary system located at approximately 16 pc with an eccentric orbit (e = 0.505) of 11.83 d period made up of an M2-M2.5 + M3-M3.5 with minimum masses of M sin 3i = 0.141 ± 0.003 and 0.1189 ± 0.003 M⊙, respectively. The revised systemic velocity of γ = 0.84 ± 0.14 km s-1 suggests that it is a member of the Local Association. The kinematics together with other activity and youth indicators imply an age of 110-800 Myr for this system and its two companions. Conclusions. The isochronal ages derived from the comparison of the photometry with several evolutionary models are younger than the age estimated from the activity indicators for the three co-moving systems. The masses for the components of GJ 1284, derived from their luminosity and age using the different models, are not consistent with the masses derived from the photometry, except for the PARSEC models, but are compatible with dynamical masses of double-lined eclipsing binaries with similar ages and spectral types. The effect of magnetic activity in the form of spots can reconcile to some extent the photometric and dynamical masses, but is not considered in most of the evolutionary models.

(Abridged:) It has been hypothesized that the location of Herbig Ae/Be stars (HAeBes) within the empirical relation between the inner disk radius (r$_{in}$), inferred from K-band interferometry, and the stellar luminosity (L$_*$), is related to the presence of the innermost gas, the disk-to-star accretion mechanism, the dust disk properties inferred from the spectral energy distributions (SEDs), or a combination of these effects. This work aims to test whether the previously proposed hypotheses do, in fact, serve as a general explanation for the distribution of HAeBes in the size-luminosity diagram. GRAVITY/VLTI spectro-interferometric observations at $\sim $2.2 $ \mu$m have been obtained for five HBes representing two extreme cases concerning the presence of innermost gas and accretion modes. V590 Mon, PDS 281, and HD 94509 show no excess in the near-ultraviolet, Balmer region of the spectra ($\Delta$D$_B$), indicative of a negligible amount of inner gas and disk-to-star accretion, whereas DG Cir and HD 141926 show such strong $\Delta$D$_B$ values that cannot be reproduced from magnetospheric accretion, but probably come from the alternative boundary layer mechanism. Additional data for these and all HAeBes resolved through K-band interferometry have been compiled from the literature and updated using Gaia EDR3 distances, almost doubling previous samples used to analyze the size-luminosity relation. We find no general trend linking the presence of gas inside the dust destruction radius or the accretion mechanism with the location of HAeBes in the size-luminosity diagram. Underlying trends are present and must be taken into account when interpreting the size-luminosity correlation. Still, it is argued that the size-luminosity correlation is most likely to be physically relevant in spite of the previous statistical warning concerning dependencies on distance., Comment: 20 pages, 17 figures. The main data reduction and analysis process ia available at https://github.com/marcosarenal/gravity-data-reduction

(Abridged) Herbig Ae/Be stars (HAeBes) have so far been studied based on relatively small samples that are scattered throughout the sky. Their fundamental stellar and circumstellar parameters and statistical properties were derived with heterogeneous approaches before Gaia. Our main goal is to contribute to the study of HAeBes from the largest sample of such sources to date, for which stellar and circumstellar properties have been determined homogeneously from the analysis of the spectral energy distributions (SEDs) and Gaia EDR3 parallaxes and photometry. Multiwavelength photometry was compiled for 209 bona fide HAeBes for which Gaia EDR3 distances were estimated. Using the Virtual Observatory SED Analyser (VOSA), photospheric models were fit to the optical SEDs to derive stellar parameters, and the excesses at infrared (IR) and longer wavelengths were characterized to derive several circumstellar properties. The stellar temperature, luminosity, radius, mass, and age were derived for each star based on optical photometry. In addition, their IR SEDs were classified according to two different schemes, and their mass accretion rates, disk masses, and the sizes of the inner dust holes were also estimated uniformly. The initial mass function fits the stellar mass distribution of the sample within 2 < M_star/M_sun < 12. In this aspect, the sample is therefore representative of the HAeBe regime and can be used for statistical purposes when it is taken into account that the boundaries are not well probed. A statistical analysis was carried out mainly focused on the different circumstellar properties of Herbig Ae (HAe) and Be (HBe) stars. The complete dataset is available online through a Virtual Observatory-compliant archive, representing the most recent reference for statistical studies on the HAeBe regime., 45 pages, 13 figures, 3 tables. Accepted in A&A

Large surveys with new-generation high-contrast imaging instruments are needed to derive the frequency and properties of exoplanet populations with separations from ~5 to 300 au. A careful assessment of the stellar properties is crucial for a proper understanding of when, where, and how frequently planets form, and how they evolve. The sensitivity of detection limits to stellar age makes this a key parameter for direct imaging surveys. Aims. We describe the SpHere INfrared survey for Exoplanets (SHINE), the largest direct imaging planet-search campaign initiated at the VLT in 2015 in the context of the SPHERE Guaranteed Time Observations of the SPHERE consortium. In this first paper we present the selection and the properties of the complete sample of stars surveyed with SHINE, focusing on the targets observed during the first phase of the survey (from February 2015 to February 2017). This early sample composed of 150 stars is used to perform a preliminary statistical analysis of the SHINE data, deferred to two companion papers presenting the survey performance, main discoveries, and the preliminary statistical constraints set by SHINE. Methods. Based on a large database collecting the stellar properties of all young nearby stars in the solar vicinity (including kinematics, membership to moving groups, isochrones, lithium abundance, rotation, and activity), we selected the original sample of 800 stars that were ranked in order of priority according to their sensitivity for planet detection in direct imaging with SPHERE. The properties of the stars that are part of the early statistical sample wererevisited, including for instance measurements from the Gaia Data Release 2. Rotation periods were derived for the vast majority of the late-type objects exploiting TESS light curves and dedicated photometric observations. Results. The properties of individual targets and of the sample as a whole are presented. © ESO 2021. SPHERE is an instrument designed and built by a consortium consisting of IPAG (Grenoble, France), MPIA (Heidelberg, Germany), LAM (Marseille, France), LESIA (Paris, France), Laboratoire Lagrange (Nice, France), INAF – Osservatorio di Padova (Italy), Observatoire de Genève (Switzerland), ETH Zürich (Switzerland), NOVA (Netherlands), ONERA (France) and ASTRON (Netherlands) in collaboration with ESO. SPHERE was funded by ESO, with additional contributions from CNRS (France), MPIA (Germany), INAF (Italy), FINES (Switzerland) and NOVA (Netherlands). SPHERE also received funding from the European Commission Sixth and Seventh Framework Programmes as part of the Optical Infrared Coordination Network for Astronomy (OPTICON) under grant number RII3-Ct-2004-001566 for FP6 (2004–2008), grant number 226 604 for FP7 (2009–2012) and grant number 312430 for FP7 (2013–2016). This research has made use of the SIMBAD database and Vizier services, operated at CDS, Strasbourg, France and of the Washington Double Star Catalog maintained at the US Naval Observatory. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This paper includes data collected with the TESS mission, obtained from the MAST data archive at the Space Telescope Science Institute (STScI). Funding for the TESS mission is provided by the NASA Explorer Program. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5–26555. This paper has made use of data products available in ESO archive. Program ID: 60.A-9036(A); 072.C-0488(E) (PI Mayor), 074.C-0364(A) (PI Robichon), 074.C-0037(A) (PI Gunther), 075.C-0202(A) (PI Gunther), 075.C-0689(A) (PI Galland), 076.C-0010(A) (PI Gunther), 077.C-0012(A) (PI Gunther), 077.C-0295(D) (PI Galland), 078.D-0245(C) (PI Dall), 079.C-0046(A) (PI Gunther), 080.D-0151(A) (PI Hatzes), 080.C-0712(A) (PI Desort), 180.C-0886(A) (PI Bonfils), 082.C-0718(B) (PI Bonfils), 082.C-0427(A) (PI Doellinger), 082.C-0390(A) (PI Weise), 183.C-0437(A) (PI Bonfils), 083.C-0794(A) (PI Chauvin), 084.C-1039(A) (PI Chauvin), 184.C-0815(B) (PI Desort), 089.C-0732(A) (PI Lo Curto), 191.C-0873(D) (PI Bonfils), 192.C-0224(A) (PI Lagrange), 097.C-0864(B) (PI Lannier), 098.C-0739(A) (PI Lagrange), 099.C-0205(A) (PI Lagrange), 099.C-0458(A) (PI Lo Curto), 1101.C-0557(A) (PI Lagrange). We have used data from the WASP public archive in this research. The WASP consortium comprises of the University of Cambridge, Keele University, University of Leicester, The Open University, The Queen’s University Belfast, St. Andrews University and the Isaac Newton Group. Funding for WASP comes from the consortium universities and from the UK’s Science and Technology Facilities Council. Based on data retrieved from the SOPHIE archive at Observatoire de Haute-Provence (OHP), available at http://atlas.obs-hp.fr/sophie/. We thank the anonymous referee for useful comments. S.D., V.D., D.M. and R.G. acknowledge the support by INAF/Frontiera through the “Progetti Premiali” funding scheme of the Italian Ministry of Education, University, and Research, the PRIN-INAF 2019 “Planetary systems at young ages” (PLATEA) and the ASI-INAF agreement n.2018-16-HH.0. AV acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 757561). A.M.L. acknowledges funding from Agence Nationale de la Recherche (France) under contract number ANR-14-CE33-0018. J.C. was supported by SC Space Grant and Fulbright Colombia. M.B. acknowledges funding by the UK Science and Technology Facilities Council (STFC) grant no. ST/M001229.

We report the discovery of a mid-infrared outburst in a Young Stellar Object (YSO) with an amplitude close to 8 mag at $\lambda$$\approx$4.6 $\mu$m. WISEA J142238.82-611553.7 is one of 23 highly variable WISE sources discovered in a search of Infrared Dark Clouds (IRDCs). It lies within the small IRDC G313.671-0.309 (d$\approx$2.6 kpc), seen by the Herschel/HiGal survey as a compact, massive cloud core that may have been measurably warmed by the event. Pre-outburst data from Spitzer in 2004 suggest that it is a class I YSO, a view supported by observation of weak 2.12 $\mu$m H$_2$ emission in an otherwise featureless red continuum spectrum taken in 2019 (6 mag below the peak in K$_s$). Spitzer, WISE and VVV data indicate that the outburst began by 2006 and has a duration $>$13 yr, with a fairly flat peak from 2010--2014. The outburst luminosity of a few $\times 10^2$ Lsun is consistent with an accretion rate Mdot $\approx 10^{-4}$ Msun/yr, comparable to a classical FU Orionis event. The 4.6 $\mu$m peak in 2010 implies T = 800-1000 K and a disc radial location R$\approx$4.5 au for the emitting region. The colour evolution suggests subsequent progression outward. The apparent absence of the hotter matter expected in thermal instability or MRI models may be due to complete obscuration of the innermost disc, e.g. by an edge-on disc view. Alternatively, disc fragmentation/infalling fragment models might more naturally explain a mid-infrared peak, though this is not yet clear., Comment: Accepted by MNRAS

Stellar components in binaries are subject to tidal forces which influence asteroseismic properties. Tidally pertubed pulsations have been reported for different objects but none of these are in their pre-main sequence phase of evolution. This makes RS Cha, consisting of two $\delta$ Scuti stars and with pulsational characteristics influenced by tidal effects , the first such object observed. We aim to investigate the pulsational properties of the eclipsing binary RS Cha in terms of the theory of tidally perturbed pulsations. Based on photometric time series obtained from the TESS satellite, we performed binary modelling using PHOEBE to interpret the binary light curve and to allow the investigation of the pulsations of both components in RS Cha. We modelled the detrended light curve with the superposition of linear modes. The frequencies were then interpreted as self excited modes perturbed by tidal forces. We find evidence for tidally perturbed modes, which enables the identification of pulsation modes. RS Cha mainly exhibits dipole modes, while one prominent $l=2$ or $l=3$ mode is also inferred. The latter verifies previous results from spectroscopic time series. This work shows that RS Cha is an ideal candidate to test the theory of tidally perturbed pulsations within the framework of asteroseismic modelling. The identification of multiple pulsation modes using this theory is unprecedented and will be a keystone in the future of pre-main sequence asteroseismology. However, amplitude modulation caused by the changing light ratio during the orbital phase in an eclipsing binary also plays a significant role, which can complicate mode identification., Comment: 11 pages plus 7 for the appendix, 8 pages plus 8 for the appendix, accepted for publication in Astronomy & Astrophysics

We studied the stellar populations and their associated interstellar medium structures located in the region of IRAS 16571-4029 and 16575-4023 sources, aiming to evaluate their distance and their main physical parameters. We focused our attention on the embedded cluster DBS 113 and its associated HII region RCW 116B. For the study, optical images were obtained using wide band and narrow band filters (ugri and OIII, OIIIC, Hα, HαC, SII respectively) together with infrared (K band) spectra of five stars. in the region of DBS 113. These data were complemented with available bibliographic information and photometric multi-band data from several surveys at optical (VPHAS+), infrared (2MASS, VVV, WISE) and radio (SGPS, PMN) spectral ranges. Astrometric and photometric data from GAIA DR2 in the region were also taken into account. Our speectroscopic and photometric study allowed to identify near to 20 early type (OB) stars, about 20 objects with infrared excess, and 10 candidates to young stellar objects in the region of DBS 113. We built their stellar energy distributions ranging from 0.3 to 22 µm. Our data also suggest an abnormal behavior of the reddening law (RV=4.0) in the region of this cluster. Besides, we obtained a corresponding spectrophotometric distance from optical/infrared data which was consistent with the distance value computed from GAIA measurements (d ~ 2 kpc). This value locates this cluster at the inner side of the Sagittarius-Carina Galactic arm and it is different from the traditionally adopted kinematic distance of 1-1.2 kpc, based on hydrogen recombination line at millimeter wavelengths. This difference was explained by the presence of a hydrogen expanding shell with a central velocity relative to the molecular cloud. The new proposed distance allowed to estimate more precisely the parameters of the HII region RCW 116B and to describe the energetic balance among the emission of most massive stars, the ionization of the region and the heated dust. Regarding the IRAS 16575-4023 source, we found that the brightest star in this region is a foreground object. We also identified a set of OB-stars candidates with high absorption values (AV ~ 23 ? 26), located behind the molecular cloud AGAL G345.336+01.021. Fil: Baume, Gustavo Luis. Universidad Nacional de La Plata; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina Fil: Corti, Mariela Alejandra. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomía; Argentina. Universidad Nacional de La Plata; Argentina Fil: Borissova, J.. Universidad de Valparaiso; Chile. Instituto Milenio de Astrofísica; Chile Fil: Ramirez Alegria, S.. Universidad de Antofagasta; Chile Fil: Corvera, A. V.. Universidad Nacional de La Plata; Argentina

Determining the mechanisms that drive the evolution of protoplanetary disks is a necessary step to understand how planets form. Here we measured the mass accretion rate for young stellar objects at age >5 Myr, a critical test for the current models of disk evolution. We present the analysis of the spectra of 36 targets in the ~5-10 Myr old Upper Scorpius region for which disk masses were measured with ALMA. We find that the mass accretion rates in this sample of old but still survived disks are similarly high as those of the younger (10^-9 Msun/yr while having low disk masses. Furthermore, the median values of the measured mass accretion rates in the disk mass ranges where our sample is complete at a level ~60-80% are compatible in these three regions. At the same time, the spread of mass accretion rates at any given disk mass is still >0.9 dex even at age>5 Myr. These results are in contrast with simple models of viscous evolution, which would predict that the values of the mass accretion rate diminish with time, and a tighter correlation with disk mass at age>5 Myr. Similarly, simple models of internal photoevaporation cannot reproduce the observed mass accretion rates, while external photoevaporation might explain the low disk masses and high accretion rates. A partial possible solution to the discrepancy with the viscous models is that the gas-to-dust ratio of the disks at >5 Myr is significantly different and higher than the canonical 100, as suggested by some dust and gas disk evolution models. The results shown here require the presence of several inter-playing processes, such as detailed dust evolution, external photoevaporation and possibly MHD winds, to explain the secular evolution of protoplanetary disks., Comment: Accepted on Astronomy & Astrophysics. Language edited version

The intermediate-mass pre-main sequence Herbig Ae/Be stars are key to understanding the differences in formation mechanisms between low- and high-mass stars. The study of the general properties of these objects is hampered by the fact that few and mostly serendipitously discovered sources are known. Our goal is to identify new Herbig Ae/Be candidates to create a homogeneous and well defined catalogue of these objects. We have applied machine learning techniques to 4,150,983 sources with data from Gaia DR2, 2MASS, WISE, and IPHAS or VPHAS+. Several observables were chosen to identify new Herbig Ae/Be candidates based on our current knowledge of this class, which is characterised by infrared excesses, photometric variabilities, and H$\alpha$ emission lines. Classical techniques are not efficient for identifying new Herbig Ae/Be stars mainly because of their similarity with classical Be stars, with which they share many characteristics. By focusing on disentangling these two types of objects, our algorithm has also identified new classical Be stars. We have obtained a large catalogue of 8470 new pre-main sequence candidates and another catalogue of 693 new classical Be candidates with a completeness of $78.8\pm1.4\%$ and $85.5\pm1.2\%$, respectively. Of the catalogue of pre-main sequence candidates, at least 1361 sources are potentially new Herbig Ae/Be candidates according to their position in the Hertzsprung-Russell diagram. In this study we present the methodology used, evaluate the quality of the catalogues, and perform an analysis of their flaws and biases. For this assessment, we make use of observables that have not been accounted for by the algorithm and hence are selection-independent, such as coordinates and parallax based distances. The catalogue of new Herbig Ae/Be stars that we present here increases the number of known objects of the class by an order of magnitude., Comment: Accepted for publication in Astronomy & Astrophysics. Received 14 February 2020 / Accepted 27 April 2020 (18 pages, 9 figures)

We investigate ongoing accretion activity in young stars in the TW Hydrae association (TWA, ~8-10 Myr), an ideal target to probe the final stages of disk accretion down to brown dwarf masses. Our sample comprises eleven TWA members with infrared excess, amounting to 85% of the total TWA population with disks, with spectral types between M0 and M9, and masses between 0.58 and 0.02 Msol. We employed homogeneous spectroscopic data from 300 to 2500 nm, obtained with X-shooter, to derive individual extinction, stellar parameters, and accretion parameters simultaneously. We then examined Balmer lines and forbidden emission lines to probe the physics of the star-disk interaction environment. We detected signatures of ongoing accretion for 70% of our TWA targets. This implies a fraction of accretors of 13-17% across the entire TWA (accounting for potentially accreting members not included in our survey). The spectral emission associated with these stars reveals a more evolved stage of these accretors compared to younger PMS populations: (i) a large fraction (~50%) exhibit nearly symmetric, narrow H_alpha line profiles; (ii) over 80% exhibit Balmer decrements consistent with moderate accretion and optically thin emission; (iii) less than a third exhibit forbidden line emission in [O I] 6300A, indicative of winds and outflows activity. However, the distribution in accretion rates (Mdot) derived for the TWA sample follows closely that of younger regions for Mstar~0.1-0.3 Msun. An overall correlation between Mdot and Mstar is detected, best reproduced by Mdot~Mstar^(2.1+/-0.5). At least in the lowest Mstar regimes, stars that still retain a disk at ages ~8-10 Myr are found to exhibit statistically similar, albeit moderate, accretion levels as those measured around younger objects. This slow Mdot evolution may be associated with longer evolutionary timescales of disks around low-mass stars., 27 pages (19 in the main text plus Appendices), 7 tables, 19 figures. Accepted for publication in Astronomy & Astrophysics. Abstract shortened for the arXiv listing

5 pags., 3 figs., AB Aur is a Herbig Ae star hosting a well-known transitional disk. Because of its proximity and low inclination angle, it is an excellent object to study planet formation. Our goal is to investigate the chemistry and dynamics of the molecular gas component in the AB Aur disk, and its relation with the prominent horseshoe shape observed in continuum mm emission. We used the Northern Extended Milimeter Array interferometer to map with high angular resolution the J = 3-2 lines of HCO and HCN. By combining both, we can gain insight into the AB Aur disk structure. Chemical segregation is observed in the AB Aur disk: HCO shows intense emission toward the star position, at least one bright molecular bridge within the dust cavity, and ring-like emission at larger radii, while HCN is only detected in an annular ring that is coincident with the dust ring and presents an intense peak close to the dust trap. We use HCO to investigate the gas dynamics inside the cavity. The observed bright HCO bridge connects the compact central source with the outer dusty ring. This bridge can be interpreted as an accretion flow from the outer ring to the inner disk/jet system proving gas accretion through the cavity., We thank the Spanish MINECO for funding support from AYA2016- 75066-C2-1/2-P, AYA2012-32032 and ERC under ERC- 2013-SyG, G. A. 610256 NANOCOSMOS. This work was supported by the Programme National “Physique et Chimie du Milieu Interstellaire” (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES. S.P.T.M. acknowledges to the European Union’s Horizon 2020 research and innovation program for funding support given under grant agreement No. 639459 (PROMISE).

Aims. R Coronae Australis (R CrA) is the brightest star of the Coronet nebula of the Corona Australis (CrA) star forming region. It has very red colors, probably due to dust absorption and it is strongly variable. High contrast instruments allow for an unprecedented direct exploration of the immediate circumstellar environment of this star. Methods. We observed R CrA with the near-IR channels (IFS and IRDIS) of SPHERE at VLT. In this paper, we used four different epochs, three of them from open time observations while one is from the SPHERE guaranteed time. The data were reduced using the DRH pipeline and the SPHERE Data Center. On the reduced data we implemented custom IDL routines with the aim to subtract the speckle halo.We have also obtained pupil-tracking H-band (1.45-1.85 micron) observations with the VLT/SINFONI near-infrared medium-resolution (R~3000) spectrograph. Results. A companion was found at a separation of 0.156" from the star in the first epoch and increasing to 0.18400 in the final one. Furthermore, several extended structures were found around the star, the most noteworthy of which is a very bright jet-like structure North-East from the star. The astrometric measurements of the companion in the four epochs confirm that it is gravitationally bound to the star. The SPHERE photometry and the SINFONI spectrum, once corrected for extinction, point toward an early M spectral type object with a mass between 0.3 and 0.55 M?. The astrometric analyis provides constraints on the orbit paramenters: e~0.4, semi-major axis at 27-28 au, inclination of ~ 70{\deg} and a period larger than 30 years. We were also able to put constraints of few MJup on the mass of possible other companions down to separations of few tens of au., Comment: 11 pages, 13 figures, accepted by A&A

Context. In very young clusters, stellar age distribution is empirical proof of the duration of star cluster formation and thus it gives indications of the physical mechanisms involved in the star formation process. Determining the amount of interstellar extinction and the correct reddening law are crucial steps to derive fundamental stellar parameters and in particular accurate ages from the Hertzsprung-Russell diagram. Aims. In this context, we seek to derive accurate stellar ages for NGC 6530, the young cluster associated with the Lagoon Nebula to infer the star formation history of this region. Methods. We used the Gaia-ESO survey observations of the Lagoon Nebula, together with photometric literature data and Gaia DR2 kinematics, to derive cluster membership and fundamental stellar parameters. Using spectroscopic effective temperatures, we analysed the reddening properties of all objects and derived accurate stellar ages for cluster members. Results. We identified 652 confirmed and 9 probable members. The reddening inferred for members and non-members allows us to distinguish foreground objects, mainly main-sequence stars, and background objects, mainly giants, and to trace the three-dimensional structure of the nebula. This classification is in agreement with the distances inferred from Gaia DR2 parallaxes for these objects. Finally, we derive stellar ages for 382 confirmed cluster members for which we obtained the individual reddening values. In addition, we find that the gravity-sensitive γ index distribution for the M-type stars is correlated with stellar age. Conclusions. For all members with Teff < 5500 K, the mean logarithmic age is 5.84 (units of years) with a dispersion of 0.36 dex. The age distribution of stars with accretion or discs, i.e. classical T Tauri stars with excess (CTTSe), is similar to that of stars without accretion and without discs, i.e. weak T Tauri stars with photospheric emission (WTTSp). We interpret this dispersion as evidence of a real age spread since the total uncertainties on age determinations, derived from Monte Carlo simulations, are significantly smaller than the observed spread. This conclusion is supported by evidence of the decrease of the gravity-sensitive γ index as a function of stellar ages. The presence of a small age spread is also supported by the spatial distribution and kinematics of old and young members. In particular, members with accretion or discs, formed in the last 1 Myr, show evidence of subclustering around the cluster centre, in the Hourglass Nebula and in the M8-E region, suggesting a possible triggering of star formation events by the O-type star ionization fronts.© ESO 2019., The authors acknowledge support through the PRIN INAF 2014 funding scheme of the National Institute for Astrophysics (INAF) of the Italian Ministry of Education, University and Research (>The Gaia-ESO Survey>, P.I.: S. Randich). This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541.

We present long baseline Atacama Large Millimeter/submillimeter Array (ALMA) observations of the 870$\,\mu$m dust continuum emission and CO (3-2) from the protoplanetary disk around the Herbig Ae/Be star HD 100546, which is one of the few systems claimed to have two young embedded planets. These observations achieve a resolution of 4 au (3.8 mas), an rms noise of 66$\mu$Jy/beam, and reveal an asymmetric ring between $\sim$20-40 au with largely optically thin dust continuum emission. This ring is well fit by two concentric and overlapping Gaussian rings of different widths and a Vortex. In addition, an unresolved component is detected at a position consistent with the central star, which may trace the central inner disk ($, Comment: 14 pages, accepted by ApJ

Several detections of wide-orbit planet-mass/sub-stellar companions around young solar-like stars were reported in the last decade. The origin of those possible planets is still unclear but accretion tracers and VLT/SPHERE observations indicate that they are surrounded by circumplanetary material or even a circumplanetary disk. We want to investigate if the gas component of disks around wide-orbit companions is detectable with current and future (sub)mm telescopes and what constraints such gas observations can provide on the nature of the circumplanetary material and on the mass of the companion. We applied the radiation thermo-chemical disk code ProDiMo to model the dust and gas component of passive circumplanetary disks and produced realistic synthetic observables. We considered different companion properties, disk parameters and radiative environments and compared the resulting synthetic observables to telescope sensitivities and to existing dust observations. The main criterion for a successful detection is the size of the circumplanetary disk. At a distance of about 150 pc, a circumplanetary disk with an outer radius of about 10 au is detectable with ALMA in about 6 hours in optically thick CO lines. Other aspects such as the companion's luminosity, disk inclination and background radiation fields are also relevant and should be considered to optimize the observing strategy for detection experiments. For most of the known wide-orbit planet-mass companions, their maximum theoretical disk size of one third of the Hill radius would be sufficient to allow detection of CO lines. It is therefore feasible to detect their gas disks and constrain the mass of the companion through the kinematic signature. Even in the case of non-detections such observations will provide stringent constraints on disk size and gas mass, information crucial for formation theories., Comment: 13 pages, 11 figures, accepted for publication in A&A

We present new 890 $\mu m$ continuum ALMA observations of 5 brown dwarfs (BDs) with infrared excess in Lupus I and III -- which, in combination with 4 BDs previously observed, allowed us to study the mm properties of the full known BD disk population of one star-forming region. Emission is detected in 5 out of the 9 BD disks. Dust disk mass, brightness profiles and characteristic sizes of the BD population are inferred from continuum flux and modeling of the observations. Only one source is marginally resolved, allowing for the determination of its disk characteristic size. We conduct a demographic comparison between the properties of disks around BDs and stars in Lupus. Due to the small sample size, we cannot confirm or disprove if the disk mass over stellar mass ratio drops for BDs, as suggested for Ophiuchus. Nevertheless, we find that all detected BD disks have an estimated dust mass between 0.2 and 3.2 $M_{\bigoplus}$; these results suggest that the measured solid masses in BD disks can not explain the observed exoplanet population, analogous to earlier findings on disks around more massive stars. Combined with the low estimated accretion rates, and assuming that the mm-continuum emission is a reliable proxy for the total disk mass, we derive ratios of $\dot{M}_{\mathrm{acc}} / M_{\mathrm{disk}}$ significantly lower than in disks around more massive stars. If confirmed with more accurate measurements of disk gas masses, this result could imply a qualitatively different relationship between disk masses and inward gas transport in BD disks., Comment: Accepted for publication on A&A, 14 pages of main text with 13 figures, and 9 pages of appendices A, B, C and D with 14 figures

We present a detailed multiwavelength characterization of the multi-ring disk of HD 169142. We report new Atacama Large Millimeter/submillimeter Array (ALMA) observations at 3 mm and analyze them together with archival 0.89 and 1.3 mm data. Our observations resolve three out of the four rings in the disk previously seen in high-resolution ALMA data. A simple parametric model is used to estimate the radial profile of the dust optical depth, temperature, density, and particle size distribution. We find that the multiple ring features of the disk are produced by annular accumulations of large particles, probably associated with gas pressure bumps. Our model indicates that the maximum dust grain size in the rings is ∼1 cm, with slightly flatter power-law size distributions than the interstellar medium-like size distribution (p ∼ 3.5) found in the gaps. In particular, the inner ring (∼26 au) is associated with a strong and narrow buildup of dust particles that could harbor the necessary conditions to trigger the streaming instability. According to our analysis, the snowlines of the most important volatiles do not coincide with the observed substructures. We explore different ring formation mechanisms and find that planet-disk interactions are the most likely scenario to explain the main features of HD 169142. Overall, our multiwavelength analysis provides some of the first unambiguous evidence of the presence of radial dust traps in the rings of HD 169142. A similar analysis in a larger sample of disks could provide key insights on the impact that disk substructures have on the dust evolution and planet formation processes.© 2019. The American Astronomical Society. All rights reserved.., E.M., and C.C.E. acknowledge support from the National Science Foundation under CAREER grant No. AST-1455042 and the Sloan Foundation. M.O., G.A., J.M.T., and J.F.G. acknowledge financial support from the State Agency for Research of the Spanish MCIU through the AYA2017-84390-C2-1-R grant (co-funded by FEDER). M.O., G. A., and J.F.G. acknowledge support from the >Center of Excellence Severo Ochoa> award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). M. F. and G.H.M.B. acknowledge support from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no 757957).

As part of the ongoing effort to characterize the low-mass (sub)stellar population in a sample of massive young clusters, we have targeted the ∼2 Myr old cluster NGC 2244. The distance to NGC 2244 from Gaia DR2 parallaxes is 1.59 kpc, with errors of 1% (statistical) and 11% (systematic). We used the Flamingos-2 near-infrared camera at the Gemini-South telescope for deep multi-band imaging of the central portion of the cluster (∼2.4 pc). We determined membership in a statistical manner, through a comparison of the cluster's color-magnitude diagram to that of a control field. Masses and extinctions of the candidate members are then calculated with the help of evolutionary models, leading to the first initial mass function (IMF) of the cluster extending into the substellar regime, with the 90% completeness limit around 0.02 M . The IMF is well represented by a broken power law (dN/dM ∝ M ) with a break at ∼0.4 M . The slope on the high-mass side (0.4-7 M ) is α = 2.12 ±0.08, close to the standard Salpeter slope. In the low-mass range (0.02-0.4 M ), we find a slope α = 1.03 ±0.02, which is at the high end of the typical values obtained in nearby star-forming regions (α = 0.5-1.0), but still in agreement within the uncertainties. Our results reveal no clear evidence for variations in the formation efficiency of brown dwarfs (BDs) and very low-mass stars due to the presence of OB stars, or for a change in stellar densities. Our finding rules out photoevaporation and fragmentation of infalling filaments as substantial pathways for BD formation.© 2019. The American Astronomical Society. All rights reserved., K.M. acknowledges funding by the Science and Technology Foundation of Portugal (FCT), grants No. IF/00194/2015 and PTDC/FIS-AST/28731/2017. Part of the research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework program (FP7/2007-2013)/ERC grant agreement No. [614922]. R.J. acknowledges support from NSERC grants. A. S.' s work is supported by the STFC grant No. ST/R000824/1. L.C. acknowledges support from CONICYT-FONDECYT grant No. 1171246. K.P.R. acknowledges CONICYT PAI Concurso Nacional de Insercion en la Academia, Convocatoria 2016 Folio PAI79160052. This research has made use of the Spanish Virtual Observatory (http://svo.cab.inta-csic.es) supported from the Spanish MINECO/FEDER through grant AYA2014-55216.

Context. We find an extended source in the direct vicinity of Sgr A∗ with an approximate projected mean distance of 425 ± 26 mas. Its sky-projected elongated shape can be described by an averaged spatial extension of x = 110 ± 20 mas and y = 180 ± 20 mas. With this, the observed object points in the analyzed SINFONI data sets between 2006 and 2016 directly toward the supermassive black hole. We discuss different possible scenarios that could explain the detected blueshifted line emission source. Aims. Here we present a detailed and extensive analysis of the adaptive optics corrected SINFONI data between 2006 and 2016 with a spatial pixel scale of 0 . ″ 025 and a corresponding field of view of 0 . ″ 8 × 0 . ″ 8 per single data cube with the focus on the newly discovered source. We spectroscopically identify the source, which we name X8, in the blueshifted Brγ line maps. Additionally, an upper limit for the continuum magnitude can be derived from the close-by S-star S41. Methods. We applied the standard reduction procedure with the SINFONI/EsoRex pipeline for the analysis. We applied pre- and post-data correction in order to establish various calibration procedures. For the sharpened images, we used the Lucy-Richardson algorithm with a low iteration number. For the high-pass filtered images, we used the smooth-subtracting process in order to increase the signal-to-noise ratio. Results. We are able to detect the elongated line emission source in quantified data sets between 2006 and 2016. We find a lower limit for the infrared continuum magnitude of K s 17.0 ± 0.1. The alignment of X8 toward Sgr A∗ can be detected in data sets that fulfill a sufficient number of observations with a defined quality level. A more detailed analysis of the results shows indications of a bipolar outflow source that might be associated with either a young stellar object, or with a post-AGB star or young planetary nebula. Conclusions. The near-infrared excess source X8 close to S24, S25, and S41 can be detected between 2006 and 2016. In addition to an apparent bow-shock morphology, the source shows clear signatures of a bipolar outflow that is consistent with both a young stellar object and a post-AGB star. If confirmed, this would be the closest ever detected bipolar outflow source to the supermassive black hole. Similar to the case of the DSO/G2 source and other dusty sources, it further supports the in situ star formation in the direct vicinity of Sgr A∗. If X8 were a bow-shock source, it would be the third object of this type that can be found in projection in the mini-cavity. This scenario would support the idea that the cavity is created by a wind from Sgr A∗. © ESO 2019., We received funding from the European Union Seventh Framework Program (FP7/2013-2017) under grant agreement no 312789 - Strong gravity: Probing Strong Gravity by Black Holes Across the Range of Masses. Michal Zajacek acknowledges the support by the National Science Centre, Poland, grant No. 2017/26/A/ST9/00756 (Maestro 9). This work was supported in part by the Deutsche Forschungsgemeinschaft (DFG) via the Cologne Bonn Graduate School (BCGS), the Max Planck Society through the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics, as well as special funds through the University of Cologne and SFB 956 - Conditions and Impact of Star Formation.

We have extended and refined the existing young stellar object (YSO) catalogs for the Orion A molecular cloud, the closest massive star-forming region to Earth. This updated catalog is driven by the large spatial coverage (18.3 deg$^2$, $\sim$950 pc$^2$), seeing limited resolution ($\sim$0.7$"$), and sensitivity ($K_s, 39 pages, 25 figures, Accepted for publication by A&A

There have been several recent detections of candidate Keplerian discs around massive young protostars. Given the relatively large disc-to-star mass ratios in these systems, and their young ages, it is worth investigating their propensity to becoming self-gravitating. To this end, we compute self-consistent, semi-analytic models of putative self-gravitating discs for five candidate disc systems. Our aim is not to fit exactly the observations, but to demonstrate that the expected dust continuum emission from marginally unstable self-gravitating discs can be quite weak, due to high optical depth at the midplane even at millimetre wavelengths. In the best cases, the models produce "observable" disc masses within a factor of, 9 pages, 20 figures, accepted for publication in MNRAS

Context. V1118 Ori is a classical EXor source whose light curve has been monitored, although not continuously, over the past 30 years. This source underwent a powerful outburst in 2005, followed by 10 years of quiescence and a less intense outburst in 2015. In 2019, a new intense brightness increase was observed (Δg ∼ 3 mag). Aims. This new accretion episode offers the opportunity to compare the photometric and spectroscopic properties of multiple outbursts of the same source. This allows us to highlight the differences and similarities among various events by removing any possible bias related to the intrinsic properties of the star-disk system. Methods. We discovered the 2019 V1118 Ori outburst by examining the g-band light curve acquired by the Zwicky Transient Facility and followed the declining phase with the Rapid Eye Mount telescope in the griz bands. Two near-infrared spectra were also acquired at different brightness stages with the Large Binocular Telescope. Results. The last event shows the following characteristics: (1) an amplitude similar to that of 2015 and lower than that of 2005; (2) a duration of less than one year as in previous events; (3) a increasing (decreasing) speed of 0.018 (0.031) mag day−1, which is different from previous cases; (4) a gradually blueing of the [g − r] color is observed over time, while the [r − i] color remains roughly unchanged; (5) with few exceptions, the near-infrared lines (mainly H I recombination) are the same as those observed in 2015; and (6) the mass accretion rate peaks at Ṁacc ∼ 10−7 M⊙ yr−1 and decreases in about a month down to a few 10−8 M⊙ yr−1. Conclusions. Our analysis shows that the comparison of data from different outbursts of the same source is a nontrivial exercise, which allows us to obtain important clues that are useful to drive theoretical efforts toward a better understanding of the EXor phenomenon.

Context. Near-IR observations of protoplanetary disks provide information about the properties of the inner disk. High-resolution spectra of abundant molecules such as CO can be used to determine the disk structure in the warm inner parts. The v2∕v1 ro-vibrational ratio of v1−0 and v2−1 transitions has recently been observed to follow distinct trends with the CO emitting radius in a sample of TTauri and Herbig disks; these trends have empirically been interpreted as due to depletion of the inner disk from gas and dust. Aims. We use thermochemical disk models to explore the to interpret the trends of these CO ro-vibrational CO emission. Methods. We used the radiation thermochemical code ProDiMo to explore a set of previously published models with different disk properties and varying one parameter at a time: the inner radius, the dust-to-gas mass ratio, and the gas mass. In addition, we used models in which we changed the surface density power-law index, and employed a larger set of CO ro-vibrational levels that also include fluorescence from the first electronic state. We investigated these models for TTauri and Herbig star disks. Finally, we included a set of DIANA models for individual TTauri and Herbig disks that were constructed to reproduce a large set of multiwavelength observations. Results. This modeling exploration highlights promising parameters that may explain the observed trends in ro-vibrational CO emission. Our models with an increasing inner radius match the observed trend for TTauri disks, in which we were also able to account for the vertical spread in the data by different values for the dust-to-gas mass ratio and for the disk gas mass in different disks. Our models instead match the CO vibrational ratio observed in Herbig disks only in the case of large inner holes and cannot produce the low ratios that are measured in many disks. The models do produce an inversion in the trend, where v2−1∕v1−0 increases and does not decrease for CO radii larger than a few au. The reason for this is that the P(4) v2−1 line becomes optically thin and superthermally excited. In our models, this does not require invoking UV fluorescence pumping. Conclusions. Our modeling explorations suggest that the observed decrease in v2−1∕v1−0 with CO radius in TTauri disks might be a consequence of inside-out disk depletion. For the Herbig disks, a more complex inner disk structure may instead be needed to explain the observed trends in the excitation of CO emission as a function of emitting radius: disk gaps emptied of dust, partially depleted in gas, and/or possibly a disk structure with an inverted surface density profile. These structures need to be further investigated in future work.

The process of massive star ($M\geq8~M_\odot$) formation is still poorly understood. Observations of massive young stellar objects (MYSOs) are challenging due to their rarity, short formation timescale, large distances, and high circumstellar extinction. Here, we present the results of a spectroscopic analysis of a population of MYSOs in the Large Magellanic Cloud (LMC). We took advantage of the spectral resolution and wavelength coverage of X-shooter (300-2500 nm), mounted on the European Southern Observatory Very Large Telescope, to detect characteristic spectral features in a dozen MYSO candidates near 30 Doradus, the largest starburst region in the Local Group hosting the most massive stars known. The X-shooter spectra are strongly contaminated by nebular emission. We used a scaling method to subtract the nebular contamination from our objects. We detect H$\alpha,\beta$, [O I] 630.0 nm, Ca II infrared triplet, [Fe II] 1643.5 nm, fluorescent Fe II 1687.8 nm, H$_2$ 2121.8 nm, Br$\gamma$, and CO bandhead emission in the spectra of multiple candidates. This leads to the spectroscopic confirmation of 10 candidates as bona fide MYSOs. We compare our observations with photometric observations from the literature and find all MYSOs to have a strong near-infrared excess. We compute lower limits to the brightness and luminosity of the MYSO candidates, confirming the near-infrared excess and the massive nature of the objects. No clear correlation is seen between the Br$\gamma$ luminosity and metallicity. Combining our sample with other LMC samples results in a combined detection rate of disk features such as fluorescent Fe II and CO bandheads which is consistent with the Galactic rate (40\%). Most of our MYSOs show outflow features., Comment: Accepted to A&A, 26 pages, 19 figures, 4 tables

Context. We investigate small-amplitude light variations in FU Ori occurring in timescales of days and weeks. Aims. We seek to determine the mechanisms that lead to these light changes. Methods. The visual light curve of FU Ori gathered by the MOST satellite continuously for 55 days in the 2013-2014 winter season and simultaneously obtained ground-based multi-colour data were compared with the results from a disc and star light synthesis model. Results. Hotspots on the star are not responsible for the majority of observed light variations. Instead, we found that the long periodic family of 10.5-11.4 d (presumably) quasi-periods showing light variations up to 0.07 mag may arise owing to the rotational revolution of disc inhomogeneities located between 16-20 solar radii. The same distance is obtained by assuming that these light variations arise because of a purely Keplerian revolution of these inhomogeneities for a stellar mass of 0.7 solar mass. The short-periodic (3-1.38 d) small amplitude (0.01 mag) light variations show a clear sign of period shortening, similar to what was discovered in the first MOST observations of FU Ori. Our data indicate that these short-periodic oscillations may arise because of changing visibility of plasma tongues (not included in our model), revolving in the magnetospheric gap and/or likely related hotspots as well. Conclusions. Results obtained for the long-periodic 10-11 d family of light variations appear to be roughly in line with the colour-period relation, which assumes that longer periods are produced by more external and cooler parts of the disc. Coordinated observations in a broad spectral range are still necessary to fully understand the nature of the short-periodic 1-3 d family of light variations and their period changes., Accepted to A&A

Time-varying phenomena are one of the most substantial sources of astrophysical information and their study has led to many fundamental discoveries in modern astronomy. We have developed an automated tool to search and analyze variable sources in the near infrared $\rm K_{s}$-band, using the data from the Vista Variables in the V\'ia L\'actea (VVV) ESO Public Large Survey. This process relies on the characterization of variable sources using different variability indices, calculated from time series generated with Point Spread Function photometry of sources under analysis. In particular, we used two main indices: the total amplitude $\rm \Delta K_s$ and the eta index, $\eta$, to identify variable sources. Once variable objects are identified, periods are determined with Generalized Lomb-Scargle periodograms, and the Information Potential Metric. Variability classes are assigned according to a compromise between comparisons with VVV Templates and the period of the variability. The automated tool is applied on VVV tiles d001 and d002 and led to discovery of 200 variable sources. We detected 70 irregular variable sources and 130 periodic ones. In addition nine open cluster candidates projected in the region are analyzed, the infrared variable candidates found around these clusters are further scrutinized by cross-matching their locations against emission star candidates from VPHAS+ survey $\rm H_{\alpha}$ color cuts., Comment: 27 pages, 19 figures

“Credit: Barrado, D. et al., A&A, 612 (2018): A79, reproduced with permission, c ESO”, Aims. The early evolutionary stage of brown dwarfs (BDs) is not very well characterized, especially during the embedded phase. Our goal is to gain insight into the dominant formation mechanism of very low-mass objects and BDs. Methods. We have conducted deep observations at 870 μm obtained with the LABOCA bolometer at the APEX telescope in order to identify young submillimeter (submm) sources in the Barnard 30 dark cloud. We have complemented these data with multi-wavelength observations from the optical to the far-IR and compiled complete spectral energy distributions in order to identify the counterparts, characterize the sources and to assess their membership to the association and stellar or substellar status based on the available photometric information. Results. We have identified 34 submm sources and a substantial number of possible and probable Barnard 30 members within each individual APEX/LABOCA beam. They can be classified into three distinct groups. First, 15 of these 34 have a clear optical or IR counterpart to the submm peak and nine of them are potential proto-BD candidates. Moreover, a substantial number of them could be multiple systems. A second group of 13 sources comprises candidate members with significant infrared excesses located away from the central submm emission. All of them include BD candidates, some displaying IR excess, but their association with submm emission is unclear. In addition, we have found six starless cores and, based on the total dust mass estimate, three might be pre-substellar (or pre-BDs) cores. Finally, the complete characterization of our APEX/LABOCA sources, focusing on those detected at 24 and/or 70 μm, indicates that in our sample of 34 submm sources there are, at least: two WTTs, four CTTs, five young stellar objects, eight proto-BD candidates (with another three dubious cases), and one very low luminosity objects. Conclusions. Our findings provide additional evidence concerning the BD formation mechanism, which seems to be a downsized version of the stellar formation., This research has been funded by the Spanish grants AYA 2014-55840-P, and AYA2014- 57369-C3-3-P, ESP2015-65712-C5-1-R, and ESP2017-87676-C5-1-R. AP and MTR acknowledge financial support from UNAM-DGAPA-PAPIIT IA102815 grant, México, and from CATA (PB06) CONICYT, Chile, respectively. AB was financed by proyecto Fondecyt Iniciación 11140572 and the Millennium Science Initiative (Chilean Ministry of Economy), through grant Nucleus P10-022-F. It makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MICINN through grant AyA2008-02156, and of the SIMBAD database, operated at CDS, Strasbourg, France. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. The paper was finished during a stay at ALMA and ESO headquarters in Santiago de Chile by DB and NH, which were supported by ALMA & ESO and by the BBVA foundation, respectively, and another at Universidad de Valparaiso by DB, supported by proyect Proyecto Fondecyt de Iniciación 11140572 (Chile).

Results from BVRI photometric observations of the pre-main sequence star V1704 Cyg collected during the time period from August 2010 to December 2017 are presented. The star is located in the star-forming HII region IC 5070 and it exhibits photometric variability in all-optical passbands. After analyzing the obtained data, V1704 Cyg is classified as a classical T Tauri star., Accepted for publication in BAJ

Our understanding of protoplanetary disks is rapidly departing from the classical view of a smooth, axisymmetric disk. This is in part thanks to the high angular resolution that (sub)millimeter observations can provide. Here, we present the combined results of Atacama Large Millimeter/submillimeter Array (ALMA) (0.9 mm) and Very Large Array (VLA) (7 mm) dust continuum observations toward the protoplanetary disk around the solar analog GM Aur. Both images clearly resolve the ∼35 au inner cavity. The ALMA observations also reveal a fainter disk that extends up to ∼250 au. We model our observations using two approaches: an analytical fit to the observed deprojected visibilities, and a physical disk model that fits the spectral energy distribution as well as the VLA and ALMA observations. Despite not being evident in the deconvolved images, the VLA and ALMA visibilities can only be fitted with two bright rings of radii ∼40 and ∼80 au. Our physical model indicates that this morphology is the result of an accumulation or trapping of large dust grains, probably due to the presence of two pressure bumps in the disk. Even though alternative mechanisms cannot be discarded, the multiple rings suggest that forming planets may have cleared at least two gaps in the disk. Finally, our analysis suggests that the inner cavity might display different sizes at 0.9 and 7 mm. This discrepancy could be caused by the presence of free-free emission close to the star at 7 mm, or by a more compact accumulation of the large dust grains at the edge of the cavity.© 2018. The American Astronomical Society. All rights reserved., E.M., C.C.E., and A.R. acknowledge support from the National Science Foundation under CAREER grant Number AST-1455042 and the Sloan Foundation. G.A., M.O., and J.F.G. are supported by the MINECO (Spain) AYA2014-57369-C3 and AYA2017-84390-C2 grants (co-funded by FEDER).

Evolutionary models are widely used to infer the mass of stars, brown dwarfs, and giant planets. Their predictions are thought to be less reliable at young ages ($, Comment: Accepted for publication in A&A. 26 pages, 22 figures, 12 tables

Context. The central molecular zone (CMZ) is a similar to 200 pc region around the Galactic centre. The study of star formation in the central part of the Milky Way is of great interest as it provides a template for the closest galactic nuclei. Aims. We present a spectroscopic follow-up of photometrically selected young stellar object (YSO) candidates in the CMZ of the Galactic centre. Our goal is to quantify the contamination of this YSO sample by reddened giant stars with circumstellar envelopes and to determine the star formation rate (SFR) in the CMZ. Methods. We obtained KMOS low-resolution near-infrared spectra (R similar to 4000) between 2.0 and 2.5 mu m of sources, many of which have been previously identified by mid-infrared photometric criteria as massive YSOs in the Galactic centre. Our final sample consists of 91 stars with good signal-to-noise ratio. We separated YSOs from cool late-type stars based on spectral features of CO and Br gamma at 2.3 mu m and 2.16 mu m, respectively. We made use of spectral energy distribution (SED) model fits to the observed photometric data points from 1.25 to 24 mu m to estimate approximate masses for the YSOs. Results. Using the spectroscopically identified YSOs in our sample, we confirm that existing colour-colour diagrams and colour-magnitude diagrams are unable to efficiently separate YSOs and cool late-type stars. In addition, we define a new colour-colour criterion that separates YSOs from cool late-type stars in the H - K-S vs. H-[8.0] diagram. We use this new criterion to identify YSO candidates in the vertical bar l vertical bar < 1.5, vertical bar b vertical bar < 0.5 region and use model SED fits to estimate their approximate masses. By assuming an appropriate initial mass function (IMF) and extrapolating the stellar IMF down to lower masses, we determine a SFR of similar to 0.046 +/- 0.026 M-circle dot yr (1) assuming an average age of 0.75 +/- 0.25 Myr for the YSOs. This value is lower than estimates found using the YSO counting method in the literature. Conclusions. Our SFR estimate in the CMZ agrees with the previous estimates from various methods and reaffirms that star formation in the CMZ is proceeding at a lower rate than predicted by various star forming models.© ESO, 2018., G.N. and M.S. acknowledges the Programme National de Cosmologie et Galaxies (PNCG) of CNRS/INSU, France, for financial support. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement No. [614922].

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Jets and outflows are ubiquitous in the process of formation of stars since outflow is intimately associated with accretion. Free–free (thermal) radio continuum emission in the centimeter domain is associated with these jets. The emission is relatively weak and compact, and sensitive radio interferometers of high angular resolution are required to detect and study it. One of the key problems in the study of outflows is to determine how they are accelerated and collimated. Observations in the cm range are most useful to trace the base of the ionized jets, close to the young central object and the inner parts of its accretion disk, where optical or near-IR imaging is made difficult by the high extinction present. Radio recombination lines in jets (in combination with proper motions) should provide their 3D kinematics at very small scale (near their origin). Future instruments such as the Square Kilometre Array (SKA) and the Next Generation Very Large Array (ngVLA) will be crucial to perform this kind of sensitive observations. Thermal jets are associated with both high and low mass protostars and possibly even with objects in the substellar domain. The ionizing mechanism of these radio jets appears to be related to shocks in the associated outflows, as suggested by the observed correlation between the centimeter luminosity and the outflow momentum rate. From this correlation and that of the centimeter luminosity with the bolometric luminosity of the system it will be possible to discriminate between unresolved HII regions and jets, and to infer additional physical properties of the embedded objects. Some jets associated with young stellar objects (YSOs) show indications of non-thermal emission (negative spectral indices) in part of their lobes. Linearly polarized synchrotron emission has been found in the jet of HH 80–81, allowing one to measure the direction and intensity of the jet magnetic field, a key ingredient to determine the collimation and ejection mechanisms. As only a fraction of the emission is polarized, very sensitive observations such as those that will be feasible with the interferometers previously mentioned are required to perform studies in a large sample of sources. Jets are present in many kinds of astrophysical scenarios. Characterizing radio jets in YSOs, where thermal emission allows one to determine their physical conditions in a reliable way, would also be useful in understanding acceleration and collimation mechanisms in all kinds of astrophysical jets, such as those associated with stellar and supermassive black holes and planetary nebulae.© 2018, The Author(s). GA acknowledges support from MINECO (Spain) AYA2014-57369-C3-3-P and AYA2017-84390-C2-1-R grants (co-funded by FEDER). LFR acknowledges support from CONACyT, Mexico and DGAPA, UNAM. CC-G acknowledges support from UNAM-DGAPA-PAPIIT grant numbers IA102816 and IN108218. We thank an anonymous referee for his/her useful comments and suggestions.

The spiral waves detected in the protostellar disc surrounding Elias 2-27 have been suggested as evidence of the disc being gravitationally unstable. However, previous work has shown that a massive, stable disc undergoing an encounter with a massive companion are also consistent with the observations. We compare the spiral morphology of smoothed particle hydrodynamic simulations modelling both cases. The gravitationally unstable disc produces symmetric, tightly wound spiral arms with constant pitch angle, as predicted by the literature. The companion disc's arms are asymmetric, with pitch angles that increase with radius. However, these arms are not well-fitted by standard analytic expressions, due to the high disc mass and relatively low companion mass. We note that differences (or indeed similarities) in morphology between pairs of spirals is a crucial discriminant between scenarios for Elias 2-27, and hence future studies must fit spiral arms individually. If Elias 2-27 continues to show symmetric tightly wound spiral arms in future observations, then we posit that it is the first observed example of a gravitationally unstable protostellar disc., Comment: 9 pages, 3 figures, accepted for publication in ApJL

We present high angular resolution 0.2 arcsec continuum and molecular emission line Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of G17.64+0.16 in Band 6 (220GHz) taken as part of a campaign in search of circumstellar discs around (proto)-O-stars. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. At a resolution of 400au the main continuum core is essentially unresolved and isolated from other strong and compact emission peaks. We detect SiO (5-4) emission that is marginally resolved and elongated in a direction perpendicular to the large-scale outflow seen in the 13CO (2-1) line using the main ALMA array in conjunction with the Atacama Compact Array (ACA). Morphologically, the SiO appears to represent a disc-like structure. Using parametric models we show that the position-velocity profile of the SiO is consistent with the Keplerian rotation of a disc around an object between 10-30Mo in mass, only if there is also radial expansion from a separate structure. The radial motion component can be interpreted as a disc wind from the disc surface. Models with a central stellar object mass between 20 and 30Mo are the most consistent with the stellar luminosity (100000 Lo) and indicative of an O-type star. The H30a millimetre recombination line (231.9GHz) is also detected, but spatially unresolved, and is indicative of a very compact, hot, ionised region co-spatial with the dust continuum core. Accounting for all observables, we suggest that G17.64 is consistent with a O-type young stellar object in the final stages of protostellar assembly, driving a wind, but that has not yet developed into a compact HII region. The existance and detection of the disc in G17.64 is likely related to its isolated and possibly more evolved nature, traits which may underpin discs in similar sources., Comment: 23 pages, 13 fig (inc 3 Appendix figures)

13 pags., 18 figs., 2 tabs., Aims. This paper investigates how the far-IR water ice features can be used to infer properties of disks around T Tauri stars and the water ice thermal history. We explore the power of future observations with SOFIA/HIRMES and SPICA's proposed far-IR instrument SAFARI. Methods. A series of detailed radiative transfer disk models around a representative T Tauri star are used to investigate how the far-IR water ice features at 45 and 63 μm change with key disk properties: disk size, grain sizes, disk dust mass, dust settling, and ice thickness. In addition, a series of models is devised to calculate the water ice emission features from warmup, direct deposit, and cooldown scenarios of the water ice in disks. Results. Photodesorption from icy grains in disk surfaces weakens the mid-IR water ice features by factors of 4-5. The far-IR water ice emission features originate from small grains at the surface snow line in disks at distance of 10-100 au. Unless this reservoir is missing in disks (e.g., transitional disks with large cavities), the feature strength does not change. Grains larger than 10 μm do not contribute to the features. Grain settling (using turbulent description) affects the strength of the ice features by at most 15%. The strength of the ice feature scales with the disk dust mass and water ice fraction on the grains, but saturates for dust masses higher than 10 M and for ice mantles that increase the dust mass by more than 50%. The various thermal histories of water ice leave an imprint on the shape of the features (crystalline and/or amorphous) and on the peak strength and position of the 45 μm feature. SOFIA/HIRMES can only detect crystalline ice features that are much stronger than those simulated in our standard T Tauri disk model in deep exposures (1 h). SPICA/SAFARI can detect the typical ice features in our standard T Tauri disk model in short exposures (10 min). Conclusions. The sensitivity of SPICA/SAFARI will allow the detailed study of the 45 and 63 μm water ice feature in unbiased surveys of T Tauri stars in nearby star forming regions and an estimate of the mass of their ice reservoir. The water ice emission features carry an imprint of the thermal history of the ice, and thus can distinguish between various formation and transport scenarios. Amorphous ice at 45 μm that has a much broader and flatter peak could be detected in deep surveys if the underlying continuum can be well characterized and the baseline stability of SAFARI is better than a few percent., IK and MM acknowledge funding from the EU FP7-2011 under Grant Agreement no. 284405. LK is supported by a grant from the Netherlands Research School for Astronomy (NOVA).

Investigating variability at the earliest stages of low-mass star formation is fundamental in understanding how a protostar assembles mass. While many simulations of protostellar disks predict non-steady accretion onto protostars, deeper investigation requires robust observational constraints on the frequency and amplitude of variability events characterised across the observable SED. In this study, we develop methods to robustly analyse repeated observations of an area of the sky for submillimetre variability in order to determine constraints on the magnitude and frequency of deeply embedded protostars. We compare \mbox{850 $\mu$m} JCMT Transient Survey data with archival JCMT Gould Belt Survey data to investigate variability over 2-4 year timescales. Out of 175 bright, independent emission sources identified in the overlapping fields, we find 7 variable candidates, 5 of which we classify as \textit{Strong} and the remaining 2 as \textit{Extended} to indicate the latter are associated with larger-scale structure. For the \textit{Strong} variable candidates, we find an average fractional peak brightness change per year of |4.0|\% yr$^{-1}$ with a standard deviation of $2.7\%\mathrm{\:yr}^{-1}$. In total, 7\% of the protostars associated with \mbox{850 $\mu$m} emission in our sample show signs of variability. Four of the five \textit{Strong} sources are associated with a known protostar. The remaining source is a good follow-up target for an object that is anticipated to contain an enshrouded, deeply embedded protostar. In addition, we estimate the \mbox{850 $\mu$m} periodicity of the submillimetre variable source, EC 53, to be \mbox{567 $\pm$ 32 days} based on the archival Gould Belt Survey data., Comment: 33 Pages, 19 figures, Accepted in ApJ

Far-infrared and (sub)millimeter fluxes can be used to study dust in protoplanetary disks, the building blocks of planets. Here, we combine observations from the Herschel Space Observatory with ancillary data of 284 protoplanetary disks in the Taurus, Chamaeleon I, and Ophiuchus star-forming regions, covering from the optical to mm/cm wavelengths. We analyze their spectral indices as a function of wavelength and determine their (sub)millimeter slopes when possible. Most disks display observational evidence of grain growth, in agreement with previous studies. No correlation is found between other tracers of disk evolution and the millimeter spectral indices. A simple disk model is used to fit these sources, and we derive posterior distributions for the optical depth at 1.3 mm and 10 au, the disk temperature at this same radius, and the dust opacity spectral index. We find the fluxes at 70 microns to correlate strongly with disk temperatures at 10 au, as derived from these simple models. We find tentative evidence for spectral indices in Chamaeleon I being steeper than those of disks in Taurus/Ophiuchus, although more millimeter observations are needed to confirm this trend and identify its possible origin. Additionally, we determine the median spectral energy distribution of each region and find them to be similar across the entire wavelength range studied, possibly due to the large scatter in disk properties and morphologies., Comment: ApJ, accepted. 38 pages, 11 figures, 16 tables, 3 appendices. The associated data and tables can be found in the Zenodo repository https://doi.org/10.5281/zenodo.889053

RCW38 is a deeply embedded young (~1 Myr), massive star cluster located at a distance of 1.7 kpc. Twice as dense as the Orion nebula cluster, orders of magnitude denser than other nearby star-forming regions and rich in massive stars, RCW38 is an ideal place to look for potential differences in brown dwarf formation efficiency as a function of environment. We present deep, high-resolution adaptive optics data of the central ~0.5 × 0.5 pc2 obtained with NACO at the Very Large Telescope. Through comparison with evolutionary models, we determine masses and extinction for ~480 candidate members, and derive the first initial mass function (IMF) of the cluster extending into the substellar regime. Representing the IMF as a set of power laws in the form dN/dMαM-α, we derive the slope α = 1.60 ± 0.13 for the mass range 0.5-20M⊙,which is shallower than the Salpeter slope, but in agreement with results in several other young massive clusters. At the low-mass side, we find α =0.71±0.11 formasses between 0.02 and 0.5M⊙, or α =0.81±0.08 for masses between 0.02 and 1M⊙. Our result is in agreement with the values found in other young star-forming regions, revealing no evidence that a combination of high stellar densities and the presence of numerous massive stars affects the formation efficiency of brown dwarfs and very-low-mass stars.We estimate that the Milky Way galaxy contains between 25 and 100 billion brown dwarfs (with masses > 0.03M⊙). © 2017 The Authors., This work is based on observations collected at the European Southern Observatory under programmes 090.C-0270 and 70.C-0400. KM acknowledges funding by the Joint Committee of ESO/Government of Chile, and by the Science and Technology Foundation of Portugal (FCT), grant no. IF/00194/2015. Part of the research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no. [614922]. RJ acknowledges support from NSERC grants. JA acknowledges funding by the Science and Technology Foundation of Portugal (FCT), grant no. SFRH/BPD/101562/2014.

Context. Precise determination of stellar masses is necessary to test the validity of pre-main-sequence (PMS) stellar evolutionary models, whose predictions are in disagreement with measurements for masses below 1.2 M. To improve such a test, and based on our previous studies, we selected the AB Doradus moving group (AB Dor-MG) as the best-suited association on which to apply radio-based high-precision astrometric techniques to study binary systems. Aims. We seek to determine precise estimates of the masses of a set of stars belonging to the AB Dor-MG using radio and infrared observations. Methods. We observed in phase-reference mode with the Very Large Array (VLA) at 5 GHz and with the European VLBI Network (EVN) at 8.4 GHz the stars HD 160934, EK Dra, PW And, and LO Peg. We also observed some of these stars with the near-infrared CCD AstraLux camera at the Calar Alto observatory to complement the radio observations. Results. We determine model-independent dynamical masses of both components of the star HD 160934, A and c, which are 0.70 ± 0.07 M and 0.45 ± 0.04 M, respectively. We revised the orbital parameters of EK Dra and we determine a sum of the masses of the system of 1.38 ± 0.08 M. We also explored the binarity of the stars LO Peg and PW And. Conclusions. We found observational evidence that PMS evolutionary models underpredict the mass of PMS stars by 10%-40%, as previously reported by other authors. We also inferred that the origin of the radio emission must be similar in all observed stars, that is, extreme magnetic activity of the stellar corona that triggers gyrosynchrotron emission from non-Thermal, accelerated electrons. © ESO, 2017., R.A., J.C.G., J.M.M., and E.R. were partially supported by the Spanish MINECO projects AYA2012-38491-C02-01 and AYA2015-63939-C2-2-P and by the Generalitat Valenciana project PROMETEO/2009/104 and PROMETEOII/2014/057. R.A. acknowledges the Max-Planck-Institute fur Radioastronomie for its hospitality. E.T. was supported by the "PRA 2016 Universita di Pisa". E.T. also acknowledges the INFN iniziativa specifica TAsP.

Full list of authors: Spina, L.; Randich, S.; Magrini, L.; Jeffries, R. D.; Friel, E. D.; Sacco, G. G.; Pancino, E.; Bonito, R.; Bravi, L.; Franciosini, E.; Klutsch, A.; Montes, D.; Gilmore, G.; Vallenari, A.; Bensby, T.; Bragaglia, A.; Flaccomio, E.; Koposov, S. E.; Korn, A. J.; Lanzafame, A. C. Smiljanic, R.; Bayo, A.; Carraro, G.; Casey, A. R.; Costado, M. T.; Damiani, F.; Donati, P.; Frasca, A.; Hourihane, A.; Jofré, P.; Lewis, J.; Lind, K.; Monaco, L.; Morbidelli, L.; Prisinzano, L.; Sousa, S. G.; Worley, C. C.; Zaggia, S., Context. The radial metallicity distribution in the Galactic thin disc represents a crucial constraint for modelling disc formation and evolution. Open star clusters allow us to derive both the radial metallicity distribution and its evolution over time. Aims. In this paper we perform the first investigation of the present-day radial metallicity distribution based on [Fe/H] determinations in late type members of pre-main-sequence clusters. Because of their youth, these clusters are therefore essential for tracing the current interstellar medium metallicity. Methods. We used the products of the Gaia-ESO Survey analysis of 12 young regions (age < 100 Myr), covering Galactocentric distances from 6.67 to 8.70 kpc. For the first time, we derived the metal content of star forming regions farther than 500 pc from the Sun. Median metallicities were determined through samples of reliable cluster members. For ten clusters the membership analysis is discussed in the present paper, while for other two clusters (i.e. Chamaeleon I and Gamma Velorum) we adopted the members identified in our previous works. Results. All the pre-main-sequence clusters considered in this paper have close-To-solar or slightly sub-solar metallicities. The radial metallicity distribution traced by these clusters is almost flat, with the innermost star forming regions having [Fe/H] values that are 0.10-0.15 dex lower than the majority of the older clusters located at similar Galactocentric radii. Conclusions. This homogeneous study of the present-day radial metallicity distribution in the Galactic thin disc favours models that predict a flattening of the radial gradient over time. On the other hand, the decrease of the average [Fe/H] at young ages is not easily explained by the models. Our results reveal a complex interplay of several processes (e.g. star formation activity, initial mass function, supernova yields, gas flows) that controlled the recent evolution of the Milky Way. © ESO, 2017., Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 188.B-3002. These data products have been processed by the Cambridge Astronomy Survey Unit (CASU) at the Institute of Astronomy, University of Cambridge, and by the FLAMES/UVES reduction team at INAF/Osservatorio Astrofisico di Arcetri. These data have been obtained from the Gaia-ESO Survey Data Archive, prepared and hosted by the Wide Field Astronomy Unit, Institute for Astronomy, University of Edinburgh, which is funded by the UK Science and Technology Facilities Council. This work was partly supported by the European Union FP7 programme through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541. We acknowledge the support from INAF and Ministero dell' Istruzione, dell' Universita' e della Ricerca (MIUR) in the form of the grant "Premiale VLT 2012" and PRIN-2014. The results presented here benefit from discussions held during the Gaia-ESO workshops and conferences supported by the ESF (European Science Foundation) through the GREAT Research Network Programme. L.S. acknowledges the support from FAPESP (2014/15706-9).

We perform a spectroscopic survey of the foreground population in Orion A with MMT/Hectospec. We use these data, along with archival spectroscopic data and photometric data, to derive spectral types, extinction values, and masses for 691 stars. Using the Spitzer Space Telescope data, we characterize the disk properties of these sources. We identify 37 new transition disk (TD) objects, one globally depleted disk candidate, and 7 probable young debris disks. We discover an object with a mass less than 0.018-0.030 M$_{\odot}$, which harbors a flaring disk. Using the H$\alpha$ emission line, we characterize the accretion activity of the sources with disks, and confirm that fraction of accreting TDs is lower than that of optically thick disks (46$\pm$7% versus 73$\pm$9%, respectively). Using kinematic data from the Sloan Digital Sky Survey and APOGEE INfrared Spectroscopy of Young Nebulous Clusters program (IN-SYNC), we confirm that the foreground population shows similar kinematics to their local molecular clouds and other young stars in the same regions. Using the isochronal ages, we find that the foreground population has a median age around 1-2 Myr, which is similar to the one of other young stars in Orion A. Therefore, our results argue against the presence of a large and old foreground cluster in front of Orion A., Comment: 34 pages, 23 figures, and 6 tables, accepted by AJ

Using K2 data, we have identified 23 very low mass members of the $\rho$ Oph and Upper Scorpius star-forming region as having periodic photometric variability not easily explained by well-established physical mechanisms such as star spots, eclipsing binaries, or pulsation. All of these unusual stars are mid-to-late M dwarfs without evidence of active accretion, and with photometric periods generally $, Comment: Accepted by AJ. 45 pages, 41 figures

Context: Quantifying the gas content inside the dust gaps of transition disks is important to establish their origin. Aims: We seek to constrain the surface density of warm gas in the disk of HD 139614, a Herbig Ae star with a transition disk exhibiting a dust gap from 2.3 to 6 AU. Methods: We have obtained ESO/VLT CRIRES high-resolution spectra of CO ro-vibrational emission. We derived constraints on the disk's structure by modeling the line-profiles, the spectroastrometric signal, and the rotational diagrams using flat Keplerian disk models. Results: We detected v=1-0 12CO, 2-1 12CO, 1-0 13CO, 1-0 C18O, and 1-0 C17O ro-vibrational lines. 12CO v=1-0 lines have an average width of 14 km/s, Tgas of 450 K and an emitting region from 1 to 15 AU. 13CO and C18O lines are on average 70 and 100 K colder, 1 and 4 km/s narrower, and are dominated by emission at R>6 AU. The 12CO v=1-0 line-profile indicates that if there is a gap in the gas it must be narrower than 2 AU. We find that a drop in the gas surface density (delta_gas) at R2 AU) gas gap suggest the presence of an embedded, Comment: accepted for publication in A&A; 30 pages (including Appendix), 24 figure; v2-v4: text corrections

We find transient, transit-like dimming events within the K2 time series photometry of the young star RIK-210 in the Upper Scorpius OB association. These dimming events are variable in depth, duration, and morphology. High spatial resolution imaging revealed the star is single, and radial velocity monitoring indicated that the dimming events can not be due to an eclipsing stellar or brown dwarf companion. Archival and follow-up photometry suggest the dimming events are transient in nature. The variable morphology of the dimming events suggests they are not due to a single, spherical body. The ingress of each dimming event is always shallower than egress, as one would expect for an orbiting body with a leading tail. The dimming events are periodic and synchronous with the stellar rotation. However, we argue it is unlikely the dimming events could be attributed to anything on the stellar surface based on the observed depths and durations. Variable obscuration by a protoplanetary disk is unlikely on the basis that the star is not actively accreting and lacks the infrared excess associated with an inner disk. Rather, we explore the possibilities that the dimming events are due to magnetospheric clouds, a transiting protoplanet surrounded by circumplanetary dust and debris, eccentric orbiting bodies undergoing periodic tidal disruption, or an extended field of dust or debris near the corotation radius., Comment: accepted to ApJ, 25 pages, 19 figures, 4 tables

Aims: We present the first extensive spectroscopic study of the global population in star clusters Trumpler 16, Trumpler 14, and Collinder 232 in the Carina nebula, using data from the Gaia-ESO Survey, down to solar-mass stars. Methods: In addition to the standard homogeneous survey data reduction, a special processing was applied here because of the bright nebulosity surrounding Carina stars. Results: We find about 400 good candidate members ranging from OB types down to slightly subsolar masses. About 100 heavily reddened early-type Carina members found here were previously unrecognized or poorly classified, including two candidate O stars and several candidate Herbig Ae/Be stars. Their large brightness makes them useful tracers of the obscured Carina population. The spectroscopically derived temperatures for nearly 300 low-mass members enables the inference of individual extinction values and the study of the relative placement of stars along the line of sight. Conclusions: We find a complex spatial structure with definite clustering of low-mass members around the most massive stars and spatially variable extinction. By combining the new data with existing X-ray data, we obtain a more complete picture of the three-dimensional spatial structure of the Carina clusters and of their connection to bright and dark nebulosity and UV sources. The identification of tens of background giants also enables us to determine the total optical depth of the Carina nebula along many sightlines. We are also able to put constraints on the star formation history of the region with Trumpler 14 stars found to be systematically younger than stars in other subclusters. We find a large percentage of fast-rotating stars among Carina solar-mass members, which provide new constraints on the rotational evolution of pre-main-sequence stars in this mass range.© 2017 ESO., This work was partly supported by the European Union FP7 program through ERC grant number 320360 and by the Leverhulme Trust through grant RPG-2012-541.