Search

This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) Ultracool dwarfs (UCDs) encompass a wide variety of compact stellar-like objects with spectra classified as late-M, L, T, and Y. Most of them have been discovered using wide-field imaging surveys. The Virtual Observatory (VO) has proven to be of great utility to efficiently exploit these astronomical resources. We aim to validate a VO methodology designed to discover and characterize UCDs in deep extragalactic surveys like Advance Large Homogeneous Area Medium-Band Redshift Astronomical (ALHAMBRA) and Cosmological Evolution Survey (COSMOS). Three complimentary searches based on parallaxes, proper motions and colours, respectively, were carried out. A total of 897 candidate UCDs were found, with only 16 previously reported in SIMBAD. Most of the new UCDs reported here are likely late-M and L dwarfs because of the limitations imposed by the utilization of optical (Gaia DR2 and r-band) data. We complement ALHAMBRA and COSMOS photometry with other catalogues in the optical and infrared using VOSA, a VO tool that estimates effective temperatures from the spectral energy distribution fitting to collections of theoretical models. The agreement between the number of UCDs found in the COSMOS field and theoretical estimations together with the low false-negative rate (known UCDs not discovered in our search) validates the methodology proposed in this work, which will be used in the forthcoming wide and deep surveys provided by the Euclid space mission. Simulations of Euclid number counts for UCDs detectable in different photometric passbands are presented for a wide survey area of 15 000 deg2, and the limitations of applicability of Euclid data to detect UCDs using the methods employed in this paper are discussed. This research has made use of the Aladin sky atlas developed at CDS, Strasbourg Observatory, France. This publication makes use of VOSA, developed under the Spanish Virtual Observatory project supported from the Spanish MICIU through grant AyA2017-84089. This research has been partly funded by the Spanish State Research Agency (AEI) Project MDM-2017-0737 at Centro de Astrobiologia (CSIC-INTA), Unidad de Excelencia Maria de Maeztu. EM was supported by grant AYA2015-69350-C3-1-P. NL was supported by grant AYA2015-69350-C3-2-P. This work has made use of data from the European Space Agency (ESA) mission Gaia,13 processed by the Gaia Data Processing and Analysis Consortium (DPAC).14 We also extensivelymade use of TOPCAT (Taylor 2005) and STILTS (Taylor 2006) as well as the Vizier and SIMBAD services, both operated at CDS, Strasbourg, France. Peerreview

GATOS (GTC Astrophysical Transient Octuple-channel imaging Spectrograph) is a multi-channel imager and spectrograph capable of simultaneously obtaining images of the same field in 8 optical and near-infrared bands or alternatively performing spectroscopy covering the range between 3500 and 23500 Angstrom in a single shot at a resolving power of R ∼ 4000. State-of-the-art detectors envisioned for this instrument will have negligible readout times and be able to perform high-time-resolution observations. An integral-field mode covering the same range simultaneously will be a crucial element of the design. In its current design, the integral-field unit covers a field of 12" × 8" with 0.6" slitlets. Finally, we aim to include a unique spectropolarimetry unit that will give GTC the first broad-band spectropolarimeter on a 10 m class telescope. The design is an evolution of the OCTOCAM concept that was selected to be built at Gemini, and is now known as SCORPIO.

The dominant mechanism leading to the formation of brown dwarfs (BDs) remains uncertain. The most direct keys to formation, which are obtained from younger objects (pre-BD cores and proto-BDs), are limited by the very low number statistics available. We aim to identify and characterize a set of pre- and proto-BDs as well as Class II BDs in the Lupus 1 and 3 molecular clouds to test their formation mechanism. We performed ALMA band 6 (1.3 mm) continuum observations of a selection of 64 cores previously identified from AzTEC/ASTE data (1.1 mm), along with previously known Class II BDs in the Lupus 1 and 3 molecular clouds. Surveyed archival data in the optical were used to complement these observations. We expect these ALMA observations prove efficient in detecting the youngest sources in these regions, since they probe the frequency domain at which these sources emit most of their radiation. We detected 19 sources from 15 ALMA fields. Considering all the pointings in our observing setup, the ALMA detection rate was $\sim$23% and the derived masses of the detected sources were between $\sim$0.18 and 124 $\mathrm{M_{Jup}}$. We classified these sources according to their spectral energy distribution as 5 Class II sources, 2 new Class I/0 candidats, and 12 new possible pre-BD or deeply embedded protostellar candidates. We detected a promising candidate for a Class 0/I proto-BD source and inferred the disk dust mass of a bona fide Class II BD. The pre-BD cores might be the byproduct of an ongoing process of large-scale collapse. The Class II BD disks follow the correlation between disk mass and the mass of the central object that is observed at the low-mass stellar regime. We conclude that it is highly probable that the sources in the sample are formed as a scaled-down version of low-mass star formation, although disk fragmentation may be responsible for a considerable fraction of BDs., 21 pages, 10 figures, 13 tables, 3 Appendixes. Accepted in A&A

Young accreting stars drive outflows that collimate into jets, which can be seen hundreds of au from their driving sources. Accretion and outflow activity cease with system age, and it is believed that magneto-centrifugally launched disk winds are critical agents in regulating accretion through the protoplanetary disk. Protostellar jets are well studied in classical T~Tauri stars ($M_\star\lesssim2\,M_\odot$), while few nearby ($d\lesssim150\,$pc) intermediate-mass stars ($M_\star=2-10\,M_\odot$), known as Herbig Ae/Be stars, have detected jets. We report VLT/MUSE observations of the Herbig~Ae/Be star HD~100546 and the discovery of a protostellar jet. The jet is similar in appearance to jets driven by low-mass stars and compares well with the jet of HD~163296, the only other known optical jet from a nearby Herbig~Ae/Be star. We derive a (one-sided) mass-loss rate in the jet of $\log \dot{M}_{jet} \sim -9.5$ (in $M_\odot$\,yr$^{-1}$) and a ratio of outflow to accretion of roughly $3\times10^{-3}$, which is lower than that of CTTS jets. The discovery of the HD~100546 jet is particularly interesting because the protoplanetary disk around HD~100546 shows a large radial gap, spiral structure, and might host a protoplanetary system. A bar-like structure previously seen in H$\alpha$ with VLT/SPHERE shares the jet position angle, likely represents the base of the jet, and suggests a jet-launching region within about 2\,au. We conclude that the evolution of the disk at radii beyond a few au does not affect the ability of the system to launch jets., Comment: 6 pages, 6 figures, accepted for publication in A&A letters

Context. Very low-mass stars are known to have jets and outflows, which is indicative of a scaled-down version of low-mass star formation. However, only very few outflows in very low-mass sources are well characterized. Aims. We characterize the bipolar molecular outflow of the very low-mass star Par-Lup3-4, a 0.12 M object known to power an optical jet. Methods. We observed Par-Lup3-4 with ALMA in Bands 6 and 7, detecting both the continuum and CO molecular gas. In particular, we studied three main emission lines: CO(2-1), CO(3-2), and 13 CO(3-2). Results. Our observations reveal for the first time the base of a bipolar molecular outflow in a very low-mass star, as well as a stream of material moving perpendicular to the primary outflow of this source. The primary outflow morphology is consistent with the previously determined jet orientation and disk inclination. The outflow mass is 9.5 × 10 -7 M , with an outflow rate of 4.3 × 10 -9 M yr -1 . A new fitting to the spectral energy distribution suggests that Par-Lup3-4 may be a binary system. Conclusions. We have characterized Par-Lup3-4 in detail, and its properties are consistent with those reported in other very low-mass sources. This source provides further evidence that very low-mass sources form as a scaled-down version of low-mass stars., With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (MDM-2017-0737)

Context. The mechanisms of planet formation are still under debate. We know little about how planets form, even if more than 4000 exoplanets have been detected to date. Recent investigations target the cot of newly born planets: the protoplanetary disk. At the first stages of their life, exoplanets still accrete material from the gas-rich disk in which they are embedded. Transitional disks are indeed disks that show peculiarities, such as gaps, spiral arms, and rings, which can be connected to the presence of substellar companions. Aims. To investigate what is responsible for these features, we selected all the known transitional disks in the solar neighborhood (, Astronomy & Astrophysics, 633, ISSN:0004-6361, ISSN:1432-0746

Context: Debris discs orbiting young stars are key to understand dust evolution and the planetary formation process. We take advantage of a recent membership analysis of the 30 Myr nearby open cluster IC 4665 based on the Gaia and DANCe surveys to revisit the disc population of this cluster. Aims: We aim to study the disc population of IC 4665 using Spitzer (MIPS and IRAC) and WISE photometry. Methods: We use several colour-colour diagrams with empirical photospheric sequences to detect the sources with an infrared excess. Independently, we also fit the spectral energy distribution (SED) of our debris disc candidates with the Virtual Observatory SED analyser (VOSA) which is capable of automatically detecting infrared excesses and provides effective temperature estimates. Results: We find six candidates debris disc host-stars (five with MIPS and one with WISE) and two of them are new candidates. We estimate a disc fraction of 24$\pm$10\% for the B-A stars, where our sample is expected to be complete. This is similar to what has been reported in other clusters of similar ages (Upper Centaurus Lupus, Lower Centaurus Crux, the $\beta$ Pictoris moving group, and the Pleiades). For solar type stars we find a disk fraction of 9$\pm$9%, lower than that observed in regions with comparable ages. Conclusions: Our candidates debris disc host-stars are excellent targets to be studied with ALMA or the future James Webb Space Telescope (JWST)., Comment: Accepted for publication in A&A

We present sensitive and high angular-resolution (similar to 02-03) (sub)millimeter (230 and 345 GHz) continuum and CO(2-1)/CO(3-2) line archive observations of the disk star system in UX Tauri carried out with the Atacama Large Millimeter/Submillimeter Array. These observations reveal the gas and dusty disk surrounding the young star UX Tauri A with a large signal-to-noise ratio (>400 in the continuum and >50 in the line), and for the first time we detect the molecular gas emission associated with the disk of UX Tauri C (with a size for the disk of, This paper makes use of the following ALMA data: ADS/JAO. ALMA#2015.1.00888.S, and ADS/JAO.ALMA#2013.1.00498. S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. L.F.R., is grateful to CONACyT, Mexico, and DGAPA, UNAM for the financial support. L.A.Z. acknowledges financial support from CONACyT-280775 and UNAM-PAPIIT IN110618 grants, Mexico. A.P. acknowledges financial support from CONACyT and UNAM-PAPIIT IN113119 grant, Mexico. G.A. and M.O. 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) and through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709).

The primary objectives of the ExoplANETS-A project are to: establish new knowledge on exoplanet atmospheres; establish new insight on influence of the host star on the planet atmosphere; disseminate knowledge, using online, web-based platforms. The project, funded under the EU's Horizon-2020 programme, started in January 2018 and has a duration ~3 years. We present an overview of the project, the activities concerning the host stars and some early results on the host stars., To appear in the Proceedings of IAU Symposium No. 345 - Origins: from the Protosun to the First Steps of Life

Context. Ruprecht 147 is the oldest (2.5 Gyr) open cluster in the solar vicinity (< 300 pc), making it an important target for stellar evolution studies and exoplanet searches. Aims. Derive a census of members and the luminosity, mass, and spatial distributions of the cluster. Methods. We use an astro-photometric data set including all available information from the literature together with our own observations. We process the data with an updated version of an existent membership selection methodology. Results. We identify 259 high-probability candidate members, including 58 previously unreported. All these candidates cover the luminosity interval between G > 6 mag to i< 21 mag. The cluster luminosity and mass distributions are derived with an unprecedented level of details allowing us to recognize, among other features, the Wielen dip. The mass distribution in the low-mass regime drops sharply at 0.4 $M_{\odot}$ even though our data are sensitive to stellar masses down to 0.1 $M_{\odot}$, suggesting that most very-low-mass members left the cluster as the result of its dynamical evolution. In addition, the cluster is highly elongated (ellipticity $\sim$ 0.5) towards the galactic plane, and mass segregated. Conclusions. Our combined Gaia+DANCe data set allows us to obtain an extended list of cluster candidate members, and to derive luminosity, mass and projected spatial distributions in the oldest open cluster of the solar vicinity., 25 pages

Context. The study of star formation is extremely challenging, due to the lack of complete and clean samples of young nearby clusters and star-forming regions. The recent Gaia DR2 catalogue complemented with the deep ground-based COSMIC DANCe catalogue offers a new database of unprecedented accuracy to revisit the membership of clusters and star-forming regions. The 30 Myr open cluster IC 4665 is one of the few well-known clusters of this age and it is an excellent target where evolutionary models can be tested and planetary formation studied. Aims. We provide a comprehensive membership analysis of IC 4665 and study the following properties: empirical isochrones, distance, magnitude distribution, present-day system mass function, and spatial distribution. Methods. We used the Gaia DR2 catalogue together with the DANCe catalogue to look for members via a probabilistic model of the distribution of the observable quantities in both the cluster and background populations. Results. We obtained a final list of 819 candidate members that cover a 12.4 magnitude range (7 < J < 19.4). We find that 50% are new candidates, and we estimate a conservative contamination rate of 20%. This unique sample of members allows us to obtain a present-day system mass function in the range of 0.02–6 M⊙, which reveals a number of details not seen in previous studies. In addition, we find that a spherically symmetric spatial distribution is favoured by our final list of members for this young open cluster. Conclusions. Our membership analysis represents a significant increase in the quantity and quality (low contamination) with respect to previous studies. It offers an excellent opportunity to revisit other fundamental parameters such as age.

Although advances in exoplanet detection techniques have seen an increase in discoveries, observing a planet in the earliest stages of formation still remains a difficult task. Here four epochs of spectra of the transitional disk object T Cha are analysed to determine whether spectro-astrometry can be used to detect a signal from its proposed protoplanet, T Cha b. The unique properties of T Cha are also further constrained. H{\alpha} and [O I]{\lambda} 6300, the most prominent lines, were analysed using spectro-astrometry. H{\alpha} being a direct accretion tracer is the target for the T Cha b detection while [O I]{\lambda} 6300 is considered to be an indirect tracer of accretion. [O I]{\lambda} 6300 is classified as a broad low velocity component (BC LVC). The mass accretion rate was derived for all epochs using new [O I]{\lambda} 6300 LVC relationships and the H{\alpha} line luminosity. It is shown that a wind is the likely origin of the [O I]{\lambda} 6300 line and that the [O I]{\lambda} 6300 line serves as a better accretion tracer than H{\alpha} in this case. From the comparison between M acc([OI]) and M acc(H{\alpha}) it is concluded that T Cha is not an intrinsically weak accretor but rather that a significant proportion of the H{\alpha} emission tracing accretion is obscured. T Cha b is not detected in the spectro-astrometric analysis yet a detection limit of 0.5 mas is derived. The analysis in this case was hampered by spectro-astrometric artefacts and by the unique properties of T Cha. While it seems that spectro-astrometry as a means of detecting exoplanets in TDs can be challenging it can be used to put an limit on the strength of the H{\alpha} emission from accreting planetary companions and thus can have an important input into the planning of high angular resolution observations.

We seek to update the spectroscopy binary fraction of the SACY (Search for Associations Containing Young stars) sample taking in consideration all possible biases in our identification of binary candidates, such as activity and rotation. Using high-resolution spectroscopic observations we have produced $\sim$1300 cross-correlation functions (CCFs) to disentangle the previously mentioned sources of contamination. The radial velocity values obtained were cross-matched with the literature and were used to revise and update the spectroscopic binary (SB) fraction in each of the SACY association. In order to better describe the CCF profile, we calculated a set of high-order cross-correlation features to determine the origin of the variations in radial velocities. We identified 68 SB candidates from our sample of 410 objects. Our results hint that the youngest associations have a higher SB fraction. Specifically, we found sensitivity-corrected SB fractions of $22 \substack{+15 \\ -11} \%$ for $\epsilon$~Cha , $31 \substack{+16 \\ -14} \%$ for TW Hya and $32 \substack{+9 \\ -8} \%$ for $\beta$~Pictoris, in contrast with the five oldest ($\sim 35-125$ Myr) associations we have sampled which are $\sim 10\%$ or lower. This result seems independent of the methodology used to asses membership to the associations. The new CCF analysis, radial velocity estimates and SB candidates are particularly relevant for membership revision of targets in young stellar associations. These targets would be ideal candidates for follow-up campaigns using high-resolution techniques in order to confirm binarity, resolve the orbits, and ideally calculate dynamical masses. Additionally, if the results on SB fraction in the youngest associations are confirmed, it could hint of non-universal multiplicity among SACY associations., Comment: The paper has been accepted in A&A

(Abridged) The detection of forming planets in disks around young stars remains elusive, and state-of-the-art observational techniques provide somewhat ambiguous results. It has been reported that the pre-transitional T Tauri star LkCa 15 could host three planets; candidate planet b is in the process of formation, as inferred from its H$\alpha$ emission. However, a more recent work casts doubts on the planetary nature of the previous detections. We have observed LkCa 15 with ISIS/WHT. The spectrograph's slit was oriented towards the last reported position of LkCa 15 b (parallel direction) and 90degr from that (perpendicular). The photocenter and full width half maximum (FWHM) of the Gaussians fitting the spatial distribution at H$\alpha$ and the adjacent continuum were measured. A well-known binary (GU CMa) was used as a calibrator to test the spectro-astrometric performance of ISIS/WHT, recovering consistent photocenter and FWHM signals. However, the photocenter shift predicted for LkCa 15 b is not detected, but the FWHM in H$\alpha$ is broader than in the continuum for both slit positions. Our simulations show that the photocenter and FWHM observations cannot be explained simultaneously by an accreting planet. In turn, both spectro-astrometric observations are naturally reproduced from a roughly symmetric Halpha emitting region centered on the star and extent comparable to the orbit originally attributed to the planet at several au. The extended H$\alpha$ emission around LkCa 15 could be related to a variable disk wind, but additional multi-epoch data and detailed modeling are necessary to understand its physical nature. Spectro-astrometry in H$\alpha$ is able to test the presence of accreting planets and can be used as a complementary technique to survey planet formation in circumstellar disks., Comment: Accepted as a Letter in A&A. 4 appendices, 10 figures, 2 tables

“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).

Aims. We develop, test and characterise of a new statistical tool (intelligent system) for the sifting and analysis of nearby young open cluster (NYOC) populations. Methods. Using a Bayesian formalism, this statistical tool is able to obtain the posterior distributions of parameters governing the cluster model. It also uses hierarchical bayesian models to establish weakly informative priors, and incorporates the treatment of missing values and non-homogeneous (heteroscedastic) observational uncertainties. Results. From simulations, we estimate that this statistical tool renders kinematic (proper motion) and photometric (luminosity) distributions of the cluster population with a contamination rate of $5.8 \pm 0.2$ %. The luminosity distributions and present day mass function agree with the ones found by Bouy et al. (2015b) on the completeness interval of the survey. At the probability threshold of maximum accuracy, the classifier recovers $\sim$ 90% of Bouy et al. (2015b) candidate members and finds 10% of new ones. Conclusions. A new statistical tool for the analysis of NYOC is introduced, tested and characterised. Its comprehensive modelling of the data properties allows it to get rid of the biases present in previous works. In particular, those resulting from the use of only completely observed (non-missing) data and the assumption of homoskedastic uncertainties. Also, its Bayesian framework allows it to properly propagate observational uncertainties into membership probabilities and cluster velocity and luminosity distributions. Our results are in a general agreement with those from the literature, although we provide the most up-to-date and extended list of candidate members of the Pleiades cluster., Comment: 30 pages, 21 figures, 1 online table

On June 25th 2017, the new intermediate-resolution optical IFU and MOS of the 10.4-m GTC had its first light. As part of the tests carried out to verify the performance of the instrument in its two modes (IFU and MOS) and 18 spectral setups (identical number of VPHs with resolutions R=6000-20000 from 0.36 to 1 micron) a number of astronomical objects were observed. These observations show that MEGARA@GTC is called to fill a niche of high-throughput, intermediateresolution IFU and MOS observations of extremely-faint narrow-lined objects. Lyman-¿ absorbers, star-forming dwarfs or even weak absorptions in stellar spectra in our Galaxy or in the Local Group can now be explored to a new level. Thus, the versatility of MEGARA in terms of observing modes and spectral resolution and coverage will allow GTC to go beyond current observational limits in either depth or precision for all these objects. The results to be presented in this talk clearly demonstrate the potential of MEGARA in this regard. © 2018 SPIE.

MWC758 is a young star surrounded by a transitional disk. Recently, a protoplanet candidate has been detected around MWC758 through high-resolution $L'$-band observations. The candidate is located inside the disk cavity at a separation of $\sim$111 mas from the central star, and at an average position angle of $\sim$165.5 degrees. We have performed simultaneous adaptive optics observations of MWC758 in the H$_{\alpha}$ line and the adjacent continuum using SPHERE/ZIMPOL at the Very Large Telescope (VLT). We aim at detecting accreting protoplanet candidates through spectral angular differential imaging observations. The data analysis does not reveal any H$_{\alpha}$ signal around the target. The derived contrast curve in the B_Ha filter allows us to derive a 5$\sigma$ upper limit of $\sim$7.6 mag at 111 mas, the separation of the previously detected planet candidate. This contrast translates into a H$_{\alpha}$ line luminosity of $L_{\rm H_{\alpha}}\lesssim$ 5$\times$10$^{-5}$ $L_{\odot}$ at 111 mas, and an accretion luminosity of $L_{acc}, Comment: 5 pages, accepted for publication in A&A letters

For one decade, the spectral-type and age of the $��$ Oph object IRS-48 were subject to debates and mysteries. Modelling its disk with mid-infrared to millimeter observations led to various explanations to account for the complex intricacy of dust-holes and gas-depleted regions. We present multi-epoch high-angular-resolution interferometric near-infrared data of spatially-resolved emissions in its first 15AU, known to have very strong Polycyclic Aromatic Hydrocarbon (PAH) emissions within this dust-depleted region. We make use of new Sparse-Aperture-Masking data to instruct a revised radiative-transfer model where SED fluxes and interferometry are jointly fitted. Neutral and ionized PAH, Very Small Grains (VSG) and classical silicates are incorporated into the model; new stellar parameters and extinction laws are explored. A bright (42L$_{\odot}$) central-star with A$_v$=12.5mag and R$_v$=6.5 requires less near-infrared excess: the inner-most disk at $\approx$1AU is incompatible with the data. The revised stellar parameters place this system on a 4 Myr evolutionary track, 4 times younger than previous estimations, in better agreement with the surrounding $��$ Oph region and disk-lifetimes observations. The disk-structure converges to a classical-grains outer-disk from 55AU combined with a fully resolved VSG\&PAH-ring, at 11-26 AU. We find two over-luminosities in the PAH-ring at color-temperatures consistent with the radiative transfer simulations; one follows a Keplerian circular orbit at 14AU. We show a depletion of a factor $\approx$5 of classical dust grains compared to VSG\&PAH: the IRS-48 disk is nearly void of dust-grains in the first 55 AU. A 3.5M$_{Jup}$ planet on a 40AU orbit qualitatively explains the new disk-structure.

In this work we present ALMA continuum observations at 880 $\mu$m of 30 sub-mm cores previously identified with APEX/LABOCA at 870$\mu$m in the Barnard 30 cloud. The main goal is to characterize the youngest and lowest mass population in the cloud. As a result, we report the detection of five (out of 30) spatially unresolved sources with ALMA, with estimated masses between 0.9 and 67 M$_{\rm Jup}$. From these five sources, only two show gas emission. The analysis of multi-wavelength photometry from these two objects, namely B30-LB14 and B30-LB19, is consistent with one Class II- and one Class I low-mass stellar object, respectively. The gas emission is consistent with a rotating disk in the case of B30-LB14, and with an oblate rotating envelope with infall signatures in the case of LB19. The remaining three ALMA detections do not have infrared counterparts and can be classified as either deeply embedded objects or as starless cores if B30 members. In the former case, two of them (LB08 and LB31) show internal luminosity upper limits consistent with Very Low Luminosity objects, while we do not have enough information for LB10. In the starless core scenario, and taking into account the estimated masses from ALMA and the APEX/LABOCA cores, we estimate final masses for the central objects in the substellar domain, so they could be classified as pre-BD core candidates., Comment: Published in A&A

In an attempt to study whether the formation of brown dwarfs (BDs) takes place as a scaled-down version of low-mass stars, we conducted IRAM30m/MAMBO-II observations at 1.2 mm in a sample of 12 proto-BD candidates selected from Spitzer/IRAC data in the B213-L1495 clouds in Taurus. Subsequent observations with the CSO at 350 micron, VLA at 3.6 and 6 cm, and IRAM30m/EMIR in the 12CO(1-0), 13CO(1-0), and N2H+(1-0) transitions were carried out toward the two most promising Spitzer/IRAC source(s), J042118 and J041757. J042118 is associated with a compact (

High-precision radial velocity measurements have suffered from stellar spots effects for more than one decade. With the advent of high-resolution infra-red spectrographs, one is allowed to move into a new spectral domain where the influence of these stellar phenomena on measurements is significantly reduced. We present the first results of our CRIRES campaign on TW Hya, around which a periodic optical radial velocity variation was found and attributed to a planet. Our work showed that the signal is not present in the infra-red, pointing to a cold spot instead of to a planet as the explanation for the different data sets. This campaign demonstrates the power of this new approach and shows that CRIRES can deliver high-precision radial velocity measurements.

Extrasolar planet searches blossomed and prospered under the auspices of the radial velocity method. However, efficient as it is, it can misinterpret a periodic signal created by stellar phenomena as being originated from a planet. The most common of these is the presence of a cold spot, frequent in active stars. Here we review the representative cases of HD 166435, GJ 674 and TW Hya and the different methods used to pinpoint the spots' presence. We finish by underlining the importance of infrared measurements, as done in the TW Hya campaign, as a way of reducing the effect of spots on radial velocity measurements to a negligible level.

HD 100546 is one of the few known pre-main-sequence stars that may host a planetary system in its disk. We analyze new VLT/SPHERE/ZIMPOL polarimetric images of HD 100546 with filters in H$\alpha$ and the adjacent continuum. We have probed the disk gap and the surface layers of the outer disk, covering a region < 500 mas (< 55 au at 109 pc) from the star, at an angular resolution of ~ 20 mas. Our data show an asymmetry: the SE and NW regions of the outer disk are more polarized than the SW and NE. This can be explained from a preferential scattering angle close to 90$^o$, consistent with previous polarization images. The outer disk extends from 13 $\pm$ 2 to 45 $\pm$ 9 au, with a position angle and inclination of 137 $\pm$ 5$^o$ and 44 $\pm$ 8$^o$. The comparison with previous estimates suggests that the disk inclination could increase with the stellocentric distance, although the different measurements are still consistent within the error bars. In addition, no direct signature of the innermost candidate companion is detected from polarimetry, confirming recent results based on intensity imagery. We set an upper limit to its mass accretion rate < 10$^{-8}$ M$_{\odot}$/yr for a sub-stellar mass of 15M$_{Jup}$. Finally, we report the first detection (> 3$\sigma$) of a ~ 20 au bar-like structure that crosses the gap through the central region of HD 100546. It is tentatively suggested that the bar could be dust dragged by infalling gas that radially flows from the outer disk. This could represent an exceptional case in which a small-scale radial inflow is observed in a single system. If this scenario is confirmed, it could explain the presence of atomic gas in the inner disk that would otherwise accrete on to the central star on a timescale of a few months/years, as previously indicated from spectro-interferometric data, and could be related with additional (undetected) planets., Comment: 8 pages, 7 figures. Accepted in A&A

Multi-Conjugate Adaptive Optics (MCAO) combines the advantages of standard adaptive optics, which provides high contrast and high spatial resolution, and of wide field ~1' imaging. Up to recently, MCAO for astronomy was limited to laboratory experiments. In this paper, we present the first scientific results obtained with the first MCAO instrument put on the sky. We present a new study of the Trapezium cluster using deep MCAO images with a field of view of 1'x1' obtained at the VLT. We have used deep J, H and Ks images recently obtained with the prototype MCAO facility MAD at the VLT in order to search for new members and new multiple systems in the Trapezium cluster. On bright targets (Ks~9mag), these images allow us to reach DeltaKs~6mag as close as 0.4" We report the detection of 128 sources, including 10 new faint objects in the magnitude range between 16.1, Comment: High resolution version available on http://arrakeen.free.fr/pub/madorion.pdf Accepted 25 Sep. 2007 for publication in A&A, 14 pages, 11 figures

We look for wide, faint companions around members of the 5 Myr Lambda Orionis open cluster. We used optical, near-infrared, and Spitzer/IRAC photometry. We report the discovery of a very wide very low mass visual binary, LOri167, formed by a brown dwarf and a planetary-mass candidate located at 5 arcsec, which seems to belong to the cluster. We derive Teff of 2125 and 1750 K. If they are members, comparisons with theoretical models indicate masses of 17 (20-15) Mjup and 8 (13-7) Mjup, with a projected separation of 2000 AU. Such a binary system would be difficult to explain in most models, particularly those where substellar objects form in the disks surrounding higher mass stars.

Context. The multiplicity rate of the youngest stellar objects is directly linked to their formation process, and therefore represents one of the most critical parameters to constrain theories of star formation. Multiplicities of embedded protostars are, however, not well determined empirically. Aims. The aim of this work is to study the multiplicity of three embedded proto-stellar systems, namely SSV63, L1551 NE, and L1551-IRS5, through mid-IR imaging. All of them are suspected binary or multiple systems, but prone to ambiguities in the literature. Methods. We present high spatial resolution mid-infrared observations obtained with VISIR at the VLT. Results. For the SSV 63 system, we report the discovery of a new Class I companion, approx 2"6 N of SSV63 E, which was never seen in previous near-IR and radio continuum studies. The companion to SSV 63 W, which was detected at near-IR wavelengths, is also present in the mid-IR regime. In the case of L1551 NE, we have confirmed the presence of a companion at 0''5 NW from the central star, as previously reported by Reipurth et al. (2002, AJ, 124, 1045). Finally, we have not detected any companion to L1551-IRS5. However, we find signs of extended emission around the target. In total, we have detected three binaries. The SEDs of the primaries are consistent with their Class I/flat-spectrum classification. For the secondaries, ancillary data are required to derive their evolutionary status. Conclusions. The geometrical arrangement of sources in the SSV63 system strongly supports the view of hierarchical fragmentation of cloud cores.

T Chamaeleontis is a young star surrounded by a transitional disk, and a plausible candidate for ongoing planet formation. Recently, a substellar companion candidate was reported within the disk gap of this star. However, its existence remains controversial, with the counter-hypothesis that light from a high inclination disk may also be consistent with the observed data. The aim of this work is to investigate the origin of the observed closure phase signal to determine if it is best explained by a compact companion. We observed T Cha in the L and K s filters with sparse aperture masking, with 7 datasets covering a period of 3 years. A consistent closure phase signal is recovered in all L and K s datasets. Data were fit with a companion model and an inclined circumstellar disk model based on known disk parameters: both were shown to provide an adequate fit. However, the absence of expected relative motion for an orbiting body over the 3-year time baseline spanned by the observations rules out the companion model. Applying image reconstruction techniques to each dataset reveals a stationary structure consistent with forward scattering from the near edge of an inclined disk., Comment: 6 pages, 3 figures, accepted for publication in MNRAS Letters

T and Y-dwarfs are among the coolest and least luminous objects detected, and they can help to understand the properties of giant planets. Their multiplicity properties can shed light on the formation process. We observed a sample six T dwarfs and one L9 dwarf with the Laser Guide Star (LGS) and NAOS-CONICA, the adaptive optics (AO) facility, and the near infrared camera at the ESO Very Large Telescope. From the seven observed objects, we have identified a subarcsecond binary system, WISE J0612-3036, composed of two similar components with spectral types of T6. We measure a separation of $\rho$ = 350$\pm$5 mas and a position angle of $PA = 235\pm1^{\circ}$. Using the mean absolute magnitudes of T6 dwarfs in the 2MASS $JHK_s$ bands, we estimate a distance of $d$=31$\pm$6 pc and derive a projected separation of $\rho$ = 11$\pm$2 au. Another target, WISE J2255-3118, shows a very faint object at 1.3 arcsec in the $K_{\rm s}$ image. The object is marginally detected in $H$, and we derive a near infrared color of $H-K_{\rm s}$$>$ 0.1\,mag. $HST/WFC3$ public archival data reveals that the companion candidate is an extended source.Together with the derived color, this suggests that the source is most probably a background galaxy. The five other sources are apparently single, with 3-$\sigma$ sensitivity limits between $H$=19-21 for companions at separations $\geq$ 0.5 arcsec. WISE 0612-3036 is probably a new T-dwarf binary composed of two T6 dwarfs. As in the case of other late T-dwarf binaries, it shows a mass ratio close to 1, although its projected separation, $\sim$11 au, is larger than the average ($\sim$ 5 au). Additional observations are needed to confirm that the system is bound., Comment: 8 pages, 7 figures, A&A, matches published version

Planet formation is one explanation for the partial clearing of dust observed in the disks of some T Tauri stars. Indeed studies using state-of-the-art high angular resolution techniques have very recently begun to observe planetary companions in these so-called transitional disks. The goal of this work is to use spectra of the transitional disk object LkCa 15 obtained with X-Shooter on the Very Large Telescope to investigate the possibility of using spectro-astrometry to detect planetary companions to T Tauri stars. It is argued that an accreting planet should contribute to the total emission of accretion tracers such as H$\alpha$ and therefore planetary companions could be detected with spectro-astrometry in the same way as it has been used to detect stellar companions to young stars. A probable planetary-mass companion was recently detected in the disk of LkCa 15. Therefore, it is an ideal target for this pilot study. We studied several key accretion lines in the wavelength range 300 nm to 2.2 $\mu$m with spectro-astrometry. While no spectro-astrometric signal is measured for any emission lines the accuracy achieved in the technique is used to place an upper limit on the contribution of the planet to the flux of the H$\alpha$, Pa$\gamma$, and Pa$\beta$ lines. The derived upper limits on the flux allows an upper limit of the mass accretion rate, log($\dot{M}_{acc}$) = -8.9 to -9.3 for the mass of the companion between 6 M$_{Jup}$ and 15 M$_{Jup}$, respectively, to be estimated (with some assumptions)., Comment: Accepted by Astronomy & Astrophysics

T Cha is a young star surrounded by a transitional disk with signatures of planet formation. We have obtained high-resolution and high-sensitivity ALMA observations of T Cha in the ${\rm CO}(3$--$2)$, ${\rm ^{13}CO}(3$--$2)$, and ${\rm CS}(7$--$6)$ emission lines to reveal the spatial distribution of the gaseous disk around the star. In order to study the dust within the disk we have also obtained continuum images at 850$\mu$m from the line-free channels. We have spatially resolved the outer disk around T Cha. Using the CO(3-2) emission we derive a radius of $\sim$230 AU. We also report the detection of the $^{13}$CO(3-2) and the CS(7-8) molecular emissions, which show smaller radii than the CO(3-2) detection. The continuum observations at 850$\mu$m allow the spatial resolution of the dusty disk, which shows two emission bumps separated by $\sim$40AU, consistent with the presence of a dust gap in the inner regions of the disk, and an outer radius of $\sim$80AU. Therefore, T Cha is surrounded by a compact dusty disk and a larger and more diffuse gaseous disk, as previously observed in other young stars. The continuum intensity profiles are different at both sides of the disk suggesting possible dust asymmetries. We derive an inclination of i(deg)=67$\pm$5, and a position angle of PA (deg)= 113$\pm$6, for both the gas and dust disks. The comparison of the ALMA data with radiative transfer models shows that the gas and dust components can only be simultaneously reproduced when we include a tapered edge prescription for the surface density profile. The best model suggests that most of the disk mass is placed within a radius of $R

Astronomy and Astrophysics 590. (2016): A79 reproduced with permission from Astronomy & Astrophysics, The Chamaeleon II molecular cloud is an active star-forming region that offers an excellent opportunity to study the formation of brown dwarfs in the southern hemisphere. Aims. Our aims are to identify a population of pre-and proto-brown dwarfs (5σ mass limit threshold of ~0.015 M⊙) and provide information on the formation mechanisms of substellar objects. Methods. We performed high sensitivity observations at 870 μm using the LABOCA bolometer at the APEX telescope towards an active star-forming region in Chamaeleon II. The data are complemented by an extensive multiwavelength catalogue of sources, which covers the optical to the far-infrared, to study the nature of the LABOCA detections. Results. We detect 15 cores at 870 μm, and 11 of them show masses in the substellar regime. The most intense objects in the surveyed field correspond to the submillimetre counterparts of the well-known young stellar objects DK Cha and IRAS 12500-7658. We identify a possible proto-brown dwarf candidate (ChaII-APEX-L) with IRAC emission at 3.6 and 4.5 μm. Conclusions. Our analysis indicates that most of the spatially resolved cores are transient, and that the point-like starless cores in the substellar regime (with masses between 0.016 M⊙ and 0.066 M⊙) could be pre-brown dwarfs cores that are gravitationally unstable if they have radii less than 220 AU to 907 AU (1.2′′ to 5′′ at 178 pc), respectively, for different masses. ALMA observations will be key to revealing the energetic state of these pre-brown dwarfs candidates, I.d.G. acknowledge support from MICINN (Spain) AYA2011-30228-C03 grant (including FEDER funds). H.B. is funded by the Ramón y Cajal fellowship program number RYC-2009-04497. A.B. acknowledges financial support from the Proyecto Fondecyt Iniciación 11140572. A.P. acknowledges the financial support from UNAM, and CONACyT (México). C.E. is partly supported by Spanish Grants AYA 2011-26202 and AYA 2014- 55840-P.This research has been funded by Spanish grants AYA 2012-38897-C02-01 and PRICIT-S2009/ESP-1496

Context. Some of transition disks show asymmetric structures in thermal sub-millimetre emission and optical scattered light. These structures can be the result of planet(s) or companions embedded in the disk. Aims. We aim to detect and analyse the scattered light of the transition disk J160421.7-213028, identify disk structures, and compare the results with previous observations of this disk at other wavelengths. Methods. We obtained and analysed new polarised intensity observations of the transition disk J160421.7-213028 with VLT/SPHERE using the visible light instrument ZIMPOL at $R'$ band (0.626$\mu$m). We probe the disk gap down to a radius of confidence of 0.1'' (${\sim}15$ AU at 145 pc). We interpret the results in the context of dust evolution when planets interact with the parental disk. Results. We observe a gap from 0.1 to 0.3'' (${\sim}15$ to 40AU) and a bright annulus as previously detected by HiCIAO $H$ band observations at $1.65\mu$m. The radial width of the annulus is around $40$AU, and its centre is at ${\sim}61$AU from the central star. The peak of the reflected light at 0.626$\mu$m is located 20 AU inward of the cavity detected in the submillimetre. In addition, we detect a dip at a position angle of ${\sim}46.2 \pm 5.4^\circ$. A dip was also detected with HiCIAO, but located at ${\sim}85^\circ$. If the dip observed with HiCIAO is the same, this suggests an average dip rotation of ${\sim}12^\circ/$year, which is inconsistent with the local Keplerian angular velocity of $\sim$0.8$^\circ$/yr at $\sim$61AU. Conclusions. The spatial discrepancy in the radial emission in J160421.7-213028 at different wavelengths is consistent with dust filtration at the outer edge of a gap carved by a massive planet. The dip rotation can be interpreted by fast variability of the inner disk and/or the presence of a warp or circumplanetary material of a planet at ${\sim}9.6$ AU., Comment: Accepted for publication in A&A Letters

We report the initial results of a VLT/NACO high spatial resolution imaging survey for multiple systems among 58 M-type members of the nearby Upper Scorpius OB association. Nine pairs with separations below 100 have been resolved. Their small angular separations and the similarity in the brightness of the components (DMagK, 11 pages, 1 table, 7 figures, request high resolution copies to bouy@iac.es

We present the first results on variability of very low mass stars and brown dwarfs belonging to the 5 Myr Lambda Orionis cluster (Collinder 69). We have monitored almost continuously in the J filter a small area of the cluster which includes 12 possible members of the cluster during one night. Some members have turned to be short-term variable. One of them, LOri167, has a mass close to the planetary mass limit and its variability might be due to instabilities produced by the deuterium burning, although other mechanism cannot be ruled out., Comment: Accepted for Astronomische Nachrichten. Workshop "Ultra-low-mass star formation and evolution", La Palma June 2005

The observational characterization of the stellar mass function in galactic clusters, the field, and even other galaxies, has resulted in a wealth of parameters that must be reproduced and explained by any successful star formation theory. To extend our knowledge on the mass function to the substellar regime, we have conducted a large observational survey of nearby young clusters (ρ Oph, IC 348, Serpens, and λ Ori). We adopted a twofold approach to this study, using both near-IR photometry to find all candidate substellar members and extensive spectroscopic follow-up to derive a spectroscopic IMF, and deep methane imaging to search for the lowest mass members. I will present the results of this longterm endeavor, where we have found no evidence for variations in the substellar mass function across different clusters, and have uncovered a handful of candidate young T dwarfs likely to represent the minimum mass for star formation.

The far-UV spectrum of the T Tauri stars (TTSs) provides important clues about the structure of the stellar atmospheres, winds and accretion shocks. The IUE (InternationalUltravioletExplorer) Final Archive contains the most complete data base for such studies. A new extraction system, the IUE Newly Extracted Spectra (ines), has been developed to overcome the disadvantages of the extraction system used in the IUE Final Archive, the Signal Weighted Extraction Technique (swet). We have compared the ines spectra of the whole sample of TTSs in the far-UV range (1200–2000 A) with the swet low-resolution spectra available in the IUE Final Archive. Although in most of the cases there is a good agreement between both samples, an important enhancement of the ines line fluxes with respect to the swet line fluxes is reported for particular spectra. The line fluxes are enhanced by as much as a factor of ∼2.5 in some objects, which is significant for variability studies of TTSs because the variations of the UV lines are typically of this order. The emission-measure distributions built to study the atmospheres of these stars are based on the UV emission line fluxes, so the new system is susceptible to introduce changes in these models. Moreover, the non-linear enhancement of the ines line fluxes produces variations in diagnostic line ratios usually taken as temperature and density tracers in late-type stars. These line ratios can vary by as much as a factor of 3 when the ines data are compared with the swet, with the subsequent variation of the physical parameters derived from them.

GASPS is a far-infrared line and continuum survey of protoplanetary and young debris disks using PACS on the Herschel Space Observatory. The survey includes [OI] at 63 microns, as well as 70, 100 and 160um continuum, with the brightest objects also studied in [OI]145um, [CII]157um, H2O and CO. Targets included T Tauri stars and debris disks in 7 nearby young associations, and a sample of isolated Herbig AeBe stars. The aim was to study the global gas and dust content in a wide disk sample, systemically comparing the results with models. In this paper we review the main aims, target selection and observing strategy. We show initial results, including line identifications, sources detected, and a first statistical study. [OI]63um was the brightest line in most objects, by a factor of ~10. Detection rates were 49%, including 100% of HAeBe stars and 43% of T Tauri stars. Comparison with published dust masses show a dust threshold for [OI]63um detection of ~1e-5 M_solar. Normalising to 140pc distance, 32% with mass 1e-6 - 1e-5 M_solar, and a small number with lower mass were also detected. This is consistent with moderate UV excess and disk flaring. In most cases, continuum and line emission is spatially and spectrally unresolved, suggesting disk emission. ~10 objects were resolved, likely from outflows. Detection rates in [OI]145um, [CII]157um and CO J=18-17 were 20-40%, but [CII] was not correlated with disk mass, suggesting it arises instead from a compact envelope. [OI] detection rates in T Tauri associations of ages 0.3-4Myr were ~50%. ~2 stars were detectable in associations of 5-20Myr, with no detections in associations of age >20Myr. Comparing with the total number of young stars, and assuming a ISM-like gas/dust ratio, this indicates that ~18% of stars retain a gas-rich disk of total mass >1M_Jupiter for 1-4Myr, 1-7% keep such disks for 5-10Myr, and none remain beyond 10-20Myr., Accepted for PASP

Context: Observations of transitional disks give us an understanding of the formation of planets and planetary systems such as our own. But care must be taken in the identification of such sources: the higher spatial resolution of the Herschel Space Observatory provides a new view on the origin of the far-infrared and sub-millimeter excesses observed. Aims: We review the nature of previously known transitional disks in the Chamaeleon I star-forming region with Herschel data. Methods: We analyze Herschel PACS and SPIRE images of the young star T54 together with ancillary images. We also analyze its spectral energy distribution and indications from optical and mid-infrared spectroscopy. Results: We detect extended emission in the PACS 70 \mu m image ~6" off source at a position angle of 196{\deg} from T54. The emission detected at longer wavelength (PACS 100, 160, SPIRE 250 and 350 \mu m) is also offset from the position of the star. This suggests that the excess observed in the far-infrared part of the SED is not fully associated with T54. Conclusions: Herschel images show that the far-infrared excess seen in T54 is not due to a transitional disk but to extended emission south-west of the source. The object still shows point-like and now downscaled excess at mid-infrared wavelengths, but its origin cannot be constrained without higher spatial resolution data. However, different indications point towards an evolved disk or extended unresolved emission close to the source., Comment: 5 pages, 2 figures, Accepted for publication in Astronomy & Astrophysics on 25 October 2012

Observational data on companion statistics around young stellar systems is needed to flesh out the formation pathways for extrasolar planets and brown dwarfs. Aperture masking is a new technique that is able to address an important part of this discovery space. We observed the two debris disk systems HD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offered on the NaCo instrument at the VLT. A search for faint companions was performed using a detection strategy based on the analysis of closure phases recovered from interferograms recorded on the Conica camera. Our results demonstrate that SAM is a very competitive mode in the field of companion detection. We obtained 5 sigma high-contrast detection limits at lambda/D of 2.5x10^{-3} (\Delta L' = 6.5) for HD 92945 and 4.6x10^{-3} (\Delta L' = 5.8) for HD 141569. According to brown dwarf evolutionary models, our data impose an upper mass boundary for any companion for the two stars to, respectively, 18 and 22 Jupiter masses at minimum separations of 1.5 and 7 AU. The detection limits is mostly independent of angular separation, until reaching the diffraction limit of the telescope. We have placed upper limits on the existence of companions to our target systems that fall close to the planetary mass regime. This demonstrates the potential for SAM mode to contribute to studies of faint companions. We furthermore show that the final dynamic range obtained is directly proportional to the error on the closure phase measurement. At the present performance levels of 0.28 degree closure phase error, SAM is among the most competitive techniques for recovering companions at scales of one to several times the diffraction limit of the telescope. Further improvements to the detection threshold can be expected with more accurate phase calibration., Comment: accepted in A&A, 9 pages

This paper discusses benchmark brown dwarfs in various environments, and focuses on those in wide binary systems. We present a summary of the recently discovered T dwarf population from the UKIDSS Large Area Survey, and describe the constraints that it places on our knowledge of the sub-stellar initial mass function. We also present some exciting results from our ongoing search for wide companions to this sample, that has so far revealed an M4-T8.5 binary system at ∼12 parsecs and also the first ever Tdwarf-white dwarf binary system. The T dwarfs in these binaries have their properties constrained by the primary object and are thus benchmark objects that are already testing the predictions of theoretical model atmospheres.

This is the first paper of a series devoted to the Lambda Orionis star-forming region, from the X-ray perspective, which will provide a comprehensive view of this complex region. In this paper we focus in uncovering the population of the central, young cluster Collinder 69 (C69), and in particular those diskless members not identified by previous near- and mid-infrared surveys, and to establish the X-ray luminosity function for the association. We have combined two exposures taken with the XMM-Newton satellite with an exhaustive data set of optical, near- and mid-infrared photometry to assess the membership of the X-ray sources based on color-color and color-magnitude diagrams, as well as other properties, such as effective temperatures, masses and bolometric luminosities. We detected a total of 164 X-ray sources, of which 66 are probable and possible cluster members. A total of 16 are newly identified probable members. The two XMM-Newton pointings east and west of the cluster center have allowed us to verify the heterogeneous spatial distribution of young stars, probably related to the large scale structure of the region. The disk fraction of the X-ray detected cluster sample is very low, close to 10%, in remarkable contrast to the low-mass stellar and substellar population (mostly undetected in X-rays) where the disk fraction reaches about 50%. The X-ray luminosity function of C69 provides support for an age of several Myr when compared with other well known young associations. With our improved cluster census we confirm previous reports on the untypically low disk fraction compared to other clusters of several Myr age. The different disk fractions of X-ray detected (essentially solar-like) and undetected (mostly low-mass stars and brown dwarfs) members can be understood as a consequence of a mass-dependence of the time-scale for disk evolution., 38 pages, 16 figures

International audience; Context. T Cha is a young star surrounded by a cold disk. The presence of a gap within its disk, inferred from fitting to the spectral energy distribution, has suggested on-going planetary formation. Aims: The aim of this work is to look for very low-mass companions within the disk gap of T Cha. Methods: We observed T Cha in L' and Ks with NAOS-CONICA, the adaptive optics system at the VLT, using sparse aperture masking. Results: We detected a source in the L' data at a separation of 62 ± 7 mas, position angle of ~78 ± 1 degrees, and a contrast of DeltaL' = 5.1 ± 0.2 mag. The object is not detected in the Ks band data, which show a 3-sigma contrast limit of 5.2 mag at the position of the detected L' source. For a distance of 108 pc, the detected companion candidate is located at 6.7 AU from the primary, well within the disk gap. If T Cha and the companion candidate are bound, the comparison of the L' and Ks photometry with evolutionary tracks shows that the photometry is inconsistent with any unextincted photosphere at the age and distance of T Cha. The detected object shows a very red Ks - L' color, for which a possible explanation would be a significant amount of dust around it. This would imply that the companion candidate is young, which would strengthen the case for a physical companion, and moreover that the object would be in the substellar regime, according to the Ks upper limit. Another exciting possibility would be that this companion is a recently formed planet within the disk. Additional observations are mandatory to confirm that the object is bound and to properly characterize it.

Whilst there is a generally accepted evolutionary scheme for the formation of low-mass stars, the analogous processes when moving down in mass to the brown dwarfs regime are not yet well understood. In this first paper we try to build the most complete and unbiased spectroscopically confirmed census of the population of Collinder 69, the central cluster of the Lambda Orionis star forming region, as a first step in addressing the question of how brown dwarfs and planetary mass objects form. We have studied age dependent features in optical and near-infrared spectra of candidate members to the cluster (such as alkali lines and accretion associated indicators). In addition, we have complemented that study with the analysis of other youth indicators like X-ray emission or mid-infrared excess. We have confirmed the membership to Collinder 69 of \sim90 photometric candidate members. As a byproduct we have determined a temperature scale for young M, very low-mass stars and brown dwarfs. We have assembled one of the most complete Initial Mass Functions from 0.016 to 20 M\odot. And, finally, we have studied the implications of the spatial distribution of the confirmed members on the proposed mechanisms of brown dwarfs formation., Comment: Accepted for publication in A&A

Context. Circumstellar discs are ubiquitous around young stars, but rapidly dissipate their gas and dust on timescales of a few Myr. The Herschel Space Observatory allows for the study of the warm disc atmosphere, using far-infrared spectroscopy to measure gas content and excitation conditions, and far-IR photometry to constrain the dust distribution.Aims. We aim to detect and characterize the gas content of circumstellar discs in four targets as part of the Herschel science demonstration phase.Methods. We carried out sensitive medium resolution spectroscopy and high sensitivity photometry at gimel similar to 60-190 mu m using the Photodetector Array Camera and Spectrometer instrument on the Herschel Space Observatory.Results. We detect [OI] 63 mu m emission from the young stars HD 169142, TW Hydrae, and RECX 15, but not HD 181327. No other lines, including [CII] 158 and [OI] 145, are significantly detected. All four stars are detected in photometry at 70 and 160 mu m. Extensive models are presented in associated papers.

Planets are formed in disks around young stars. With an age of ~10 Myr, TW Hya is one of the nearest T Tauri stars that is still surrounded by a relatively massive disk. In addition a large number of molecules has been found in the TW Hya disk, making TW Hya the perfect test case in a large survey of disks with Herschel-PACS to directly study their gaseous component. We aim to constrain the gas and dust mass of the circumstellar disk around TW Hya. We observed the fine-structure lines of [OI] and [CII] as part of the Open-time large program GASPS. We complement this with continuum data and ground-based 12CO 3-2 and 13CO 3-2 observations. We simultaneously model the continuum and the line fluxes with the 3D Monte-Carlo code MCFOST and the thermo-chemical code ProDiMo to derive the gas and dust masses. We detect the [OI] line at 63 micron. The other lines that were observed, [OI] at 145 micron and [CII] at 157 micron, are not detected. No extended emission has been found. Preliminary modeling of the photometric and line data assuming [12CO]/[13CO]=69 suggests a dust mass for grains with radius < 1 mm of ~1.9 times 10^-4 Msun (total solid mass of 3 times 10^-3 Msun) and a gas mass of (0.5--5) times 10^-3 Msun. The gas-to-dust mass may be lower than the standard interstellar value of 100., 6 pages, 6 figures, accepted for publication in the A&A Herschel special issue

In the course of an imaging survey we have detected a visual companion to the intermediate-mass star HR 6037. In this letter, we present two epoch observations of the binary with NACO/VLT, and near-IR spectroscopy of the secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as a co-moving companion. Its J and H band ISAAC spectra suggest the object has an spectral type of M9+-1, with a surface gravity intermediate between that of 10 Myr dwarfs and field dwarfs with identical spectral type. The comparison of its Ks-band photometry with evolutionary tracks allows us to derive a mass, effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K, and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03, and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary system., Comment: (5 pages, 4 figures, accepted for publication in A&A Letters)