Search

This paper characterizes the actual science performance of the James Webb Space Telescope (JWST), as determined from the six month commissioning period. We summarize the performance of the spacecraft, telescope, science instruments, and ground system, with an emphasis on differences from pre-launch expectations. Commissioning has made clear that JWST is fully capable of achieving the discoveries for which it was built. Moreover, almost across the board, the science performance of JWST is better than expected; in most cases, JWST will go deeper faster than expected. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies., Comment: 5th version as accepted to PASP; 31 pages, 18 figures; https://iopscience.iop.org/article/10.1088/1538-3873/acb293

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST), has imaging, four coronagraphs and both low and medium resolution spectroscopic modes . Being able to simulate MIRI observations will help commissioning of the instrument, as well as get users familiar with representative data. We designed the MIRI instrument simulator (MIRISim) to mimic the on-orbit performance of the MIRI imager and spectrometers using the Calibration Data Products (CDPs) developed by the MIRI instrument team. The software encorporates accurate representations of the detectors, slicers, distortions, and noise sources along the light path including the telescope's radiative background and cosmic rays. The software also includes a module which enables users to create astronomical scenes to simulate. MIRISim is a publicly available Python package that can be run at the command line, or from within Python. The outputs of MIRISim are detector images in the same uncalibrated data format that will be delivered to MIRI users. These contain the necessary metadata for ingestion by the JWST calibration pipeline., 10 pages, 8 figures. Accepted for publication by MNRAS

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.

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.

Spectroscopic follow-up is a pre-requisite for studies of the formation and early evolution of brown dwarfs. Here we present IRTF/SpeX near-infrared spectroscopy of 30 candidate members of the young Upper Scorpius association, selected from our previous survey work. All 24 high confidence members are confirmed as young very low mass objects with spectral types from M5 to L1, 15-20 of them are likely brown dwarfs. This high yield confirms that brown dwarfs in Upper Scorpius can be identified from photometry and proper motions alone, with negligible contamination from field objects (, 12 pages, 9 figures, Accepted in MNRAS

Planet formation and clearing of protoplanetary disks is one of the long standing problems in disk evolution theory. The best test of clearing scenarios is observing systems that are most likely to be actively forming planets: the transitional disks with large inner dust cavities. We present the first results of our ALMA (Atacama Large Millimeter/submillimeter Array) Cycle 0 program using Band 9, imaging the Herbig Ae star Oph IRS 48 in CO 6−5 and the submillimeter continuum in the extended configuration. The resulting ~0.2″ spatial resolution completely resolves the cavity of this disk in the gas and the dust. The gas cavity of IRS 48 is half as large as the dust cavity, ruling out grain growth and photoevaporation as the primary cause of the truncation. On the other hand, the continuum emission reveals an unexpected large azimuthal asymmetry and steep edges in the dust distribution along the ring, suggestive of dust trapping. We will discuss the implications of the combined gas and dust distribution for planet formation at a very early stage. This is one of the first transition disks with spatially resolved gas inside the cavity, demonstrating the superb capabilities of the Band 9 receivers.

The Mid-Infrared Instrument (MIRI) Medium Resolution Spectrometer (MRS) is the only mid-IR Integral Field Spectrometer on board James Webb Space Telescope. The complexity of the MRS requires a very specialized pipeline, with some specific steps not present in other pipelines of JWST instruments, such as fringe corrections and wavelength offsets, with different algorithms for point source or extended source data. The MRS pipeline has also two different variants: the baseline pipeline, optimized for most foreseen science cases, and the optimal pipeline, where extra steps will be needed for specific science cases. This paper provides a comprehensive description of the MRS Calibration Pipeline from uncalibrated slope images to final scientific products, with brief descriptions of its algorithms, input and output data, and the accessory data and calibration data products necessary to run the pipeline., Comment: 10 pages, 8 figures

We used extensive ground-based multisite and archival spectroscopy to derive observational constraints for a seismic modelling of the magnetic beta Cep star V2052 Ophiuchi. The line-profile variability is dominated by a radial mode (f_1=7.14846 d^{-1}) and by rotational modulation (P_rot=3.638833 d). Two non-radial low-amplitude modes (f_2=7.75603 d^{-1} and f_3=6.82308 d^{-1}) are also detected. The four periodicities that we found are the same as the ones discovered from a companion multisite photometric campaign (Handler et al. 2012) and known in the literature. Using the photometric constraints on the degrees l of the pulsation modes, we show that both f_2 and f_3 are prograde modes with (l,m)=(4,2) or (4,3). These results allowed us to deduce ranges for the mass (M \in [8.2,9.6] M_o) and central hydrogen abundance (X_c \in [0.25,0.32]) of V2052 Oph, to identify the radial orders n_1=1, n_2=-3 and n_3=-2, and to derive an equatorial rotation velocity v_eq \in [71,75] km s^{-1}. The model parameters are in full agreement with the effective temperature and surface gravity deduced from spectroscopy. Only models with no or mild core overshooting (alpha_ov \in [0,0.15] local pressure scale heights) can account for the observed properties. Such a low overshooting is opposite to our previous modelling results for the non-magnetic beta Cep star theta Oph having very similar parameters, except for a slower surface rotation rate. We discuss whether this result can be explained by the presence of a magnetic field in V2052 Oph that inhibits mixing in its interior.

SONYC -- Substellar Objects in Nearby Young Clusters -- is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. Here we present the first results from SONYC observations of NGC1333, a ~1Myr old cluster in the Perseus star-forming complex. We have carried out extremely deep optical and near-infrared imaging in four bands (i', z', J, K) using Suprime-Cam and MOIRCS instruments at the Subaru telescope. The survey covers 0.25sqdeg and reaches completeness limits of 24.7mag in the i'-band and 20.8mag in the J-band. We select 196 candidates with colors as expected for young, very low-mass objects. Follow-up multi-object spectroscopy with MOIRCS is presented for 53 objects. We confirm 19 objects as likely brown dwarfs in NGC1333, seven of them previously known. For 11 of them, we confirm the presence of disks based on Spitzer/IRAC photometry. The effective temperatures for the brown dwarf sample range from 2500K to 3000K, which translates to masses of ~0.015 to 0.1Ms. For comparison, the completeness limit of our survey translates to mass limits of 0.004Ms for Av, Comment: 19 pages, 12 figures, accepted for publication in ApJ

PAHs have been detected toward molecular clouds and some young stars with disks, but have not yet been associated with embedded young stars. We present a sensitive mid-IR spectroscopic survey of PAH features toward a sample of low-mass embedded YSOs. The aim is to put constraints on the PAH abundance in the embedded phase of star formation using radiative transfer modeling. VLT-ISAAC L-band spectra for 39 sources and Spitzer IRS spectra for 53 sources are presented. Line intensities are compared to recent surveys of Herbig Ae/Be and T Tauri stars. The radiative transfer codes RADMC and RADICAL are used to model the PAH emission from embedded YSOs consisting of a PMS star with a circumstellar disk embedded in an envelope. The dependence of the PAH feature on PAH abundance, stellar radiation field, inclination and the extinction by the surrounding envelope is studied. The 3.3 micron PAH feature is undetected for the majority of the sample (97%), with typical upper limits of 5E-16 W/m^2. Compact 11.2 micron PAH emission is seen directly towards 1 out of the 53 Spitzer Short-High spectra, for a source that is borderline embedded. For all 12 sources with both VLT and Spitzer spectra, no PAH features are detected in either. In total, PAH features are detected toward at most 1 out of 63 (candidate) embedded protostars (, Comment: 13 pages, 9 figures, accepted for publication in A&A

With the goal to study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low luminosity YSOs using 3-38 um Spitzer and ground-based spectra. The long-known 6.0 and 6.85 um bands are detected toward all sources, with the Class 0-type YSOs showing the deepest bands ever observed. In almost all sources the 6.0 um band is deeper than expected from the bending mode of pure solid H2O. The depth and shape variations of the remaining 5-7 um absorption indicate that it consists of 5 independent components, which, by comparison to laboratory studies, must be from at least 8 different carriers. Simple species are responsible for much of the absorption in the 5-7 um region, at abundances of 1-30% for CH3OH, 3-8% for NH3, 1-5% for HCOOH, ~6% for H2CO, and ~0.3% for HCOO- with respect to solid H2O. The 6.85 um band likely consists of one or two carriers, of which one is less volatile than H2O because its abundance relative to H2O is enhanced at lower H2O/tau_9.7 ratios. It does not survive in the diffuse interstellar medium (ISM), however. The similarity of the 6.85 um bands for YSOs and background stars indicates that its carrier(s) must be formed early in the molecular cloud evolution. If an NH4+ salt is the carrier its abundance with respect to solid H2O is typically 7%, and low temperature acid-base chemistry or cosmic ray induced reactions must have been involved in its formation. Possible origins are discussed for the carrier of an enigmatic, very broad absorption between 5 and 8 um. Finally, all the phenomena observed for ices toward massive YSOs are also observed toward low mass YSOs, indicating that processing of the ices by internal ultraviolet radiation fields is a minor factor in the early chemical evolution of the ices. [abridged]

Context. Dust sedimentation is known to affect the infrared spectra and images of disks. In particular the far-infrared emission may be reduced by strong sedimentation. However, dust grains of different sizes sediment to different depths in the disk. Spectral features of one species may thus be enhanced, while those of other species may be suppressed. Aims. Polycyclic aromatic hydrocarbons (PAHs) are among the smallest “dust grains”. We investigate how the presence of PAHs in protoplanetary disks affects the disk’s spectral energy distribution (SED) and feature strengths when the thermal (large) grains are allowed to sediment. Methods. We calculate the vertical distribution of dust grains for both the PAH “dust” and the thermal dust grains.We include vertical settling and vertical mixing via turbulence. The results are inserted into a Monte-Carlo radiative transfer code to compute the SEDs. Results. For high turbulence the sedimentation barely affects the spectrum. For low turbulence, however, the PAHs still stay well-mixed in the disk’s surface layer, while the 0.1 μm size grains sediment deep into the disk. This strongly enhances the PAH features relative to the continuum (by factors of 2 to 10), while the far-infrared flux is reduced. This predicts that sources with weak far-infrared flux have stronger PAH features, which is – at least among Herbig Ae stars – opposite to what is observed, suggesting that sedimentation is not the only factor responsible for the weak mid- to far-infrared excess in some disks. We speculate that coagulation might be a solution, reducing both the mid- to far-infrared flux and the PAH features.

We present mid-infrared Spitzer IRAC and MIPS images of the UX Orionis star VV Ser and the surrounding cloud. The 5.6--70 micron images show bright, localized and nebulous emission extended over 4 arcmin centered on VV Ser. We interpret the nebulosity as being due to transiently heated grains excited by UV photons emitted by VV Ser. A companion paper describes how the physical structure of the VV Ser disk has been constrained using a wide range of observational data modeled by an axisymmetric Monte Carlo radiative transfer code. In this paper we employ the model to study the nebulosity surrounding VV Ser using quantum-heated PAH molecules and Very Small Grains (VSGs) consisting of amorphous carbon in the thermal cooling approximation. Imprinted on the nebulosity is a wedge-shaped dark band, centered on the star. We interpret this dark wedge as the shadow cast by the inner regions of a near-edge-on disk in UV light, allowing the PAHs to be excited only outside of this shadow. The presence of a disk shadow strongly constrains the inclination as well as the position angle of the disk. Although depending on the adopted PAH opacity, the abundance of PAHs in the surrounding cloud is constrained to 5+/-2% of the total dust mass, given the opacity. The extent of the nebulosity constrains the density of the gas surrounding the VV Ser disk to 500+/-200 cm-3 for a gas-to-dust ratio of 100. This low density suggests that the quantum heated material is not part of the original envelope of VV Ser and that it is rather a quiescent part of the Serpens molecular cloud that the star has passed into after being formed. [Abstract abridged], 11 pages, accepted for publication in ApJ

Substellar Objects in Nearby Young Clusters -- SONYC -- is a survey program to investigate the frequency and properties of substellar objects in nearby star-forming regions. We present new spectroscopic follow-up of candidate members in Chamaeleon-I (~2 Myr, 160 pc) and Lupus 3 (~1 Myr, 200 pc), identified in our earlier works. We obtained 34 new spectra (1.5 - 2.4 mum, R~600), and identified two probable members in each of the two regions. These include a new probable brown dwarf in Lupus 3 (NIR spectral type M7.5 and Teff=2800 K), and an L3 (Teff=2200 K) brown dwarf in Cha-I, with the mass below the deuterium-burning limit. Spectroscopic follow-up of our photometric and proper motion candidates in Lupus 3 is almost complete (>90%), and we conclude that there are very few new substellar objects left to be found in this region, down to 0.01 - 0.02 MSun and Av \leq 5. The low-mass portion of the mass function in the two clusters can be expressed in the power-law form dN/dM \propto M^{-\alpha}, with \alpha~0.7, in agreement with surveys in other regions. In Lupus 3 we observe a possible flattening of the power-law IMF in the substellar regime: this region seems to produce fewer brown dwarfs relative to other clusters. The IMF in Cha-I shows a monotonic behavior across the deuterium-burning limit, consistent with the same power law extending down to 4 - 9 Jupiter masses. We estimate that objects below the deuterium-burning limit contribute of the order 5 - 15% to the total number of Cha-I members., Comment: Accepted for publication by Astrophysical Journal

International audience; Aims.We search for Polycyclic Aromatic Hydrocarbon (PAH) features towards young low-mass (T Tauri) stars and compare them with surveys of intermediate mass (Herbig Ae/Be) stars. The presence and strength of the PAH features are interpreted with disk radiative transfer models exploring the PAH feature dependence on the incident UV radiation, PAH abundance and disk parameters. Methods: .Spitzer Space Telescope 5-35 μm spectra of 54 pre-main sequence stars with disks were obtained, consisting of 38 T Tauri, 7 Herbig Ae/Be and 9 stars with unknown spectral type. Results: .Compact PAH emission is detected towards at least 8 sources of which 5 are Herbig Ae/Be stars. The 11.2 μm PAH feature is detected in all of these sources, as is the 6.2 μm PAH feature for the 4 sources for which short wavelength data are available. However, the 7.7 and 8.6 μm features appear strongly in only 1 of these 4 sources. Based on the 11.2 μm feature, PAH emission is observed towards at least 3 T Tauri stars, with 14 tentative detections, resulting in a lower limit to the PAH detection rate of 8%. The lowest mass source with PAH emission in our sample is T Cha with a spectral type G8. All 4 sources in our sample with evidence for dust holes in their inner disk show PAH emission, increasing the feature/continuum ratio. Typical 11.2 μm line intensities are an order of magnitude lower than those observed for the more massive Herbig Ae/Be stars. Measured line fluxes indicate PAH abundances that are factors of 10-100 lower than standard interstellar values. Conversely, PAH features from disks exposed to stars with T_eff≤ 4200 K without enhanced UV are predicted to be below the current detection limit, even for high PAH abundances. Disk modeling shows that the 6.2 and 11.2 μm features are the best PAH tracers for T Tauri stars, whereas the 7.7 and 8.6 μm bands have low feature over continuum ratios due to the strongly rising silicate emission.

Infrared ~5--35 um spectra for 40 solar-mass T Tauri stars and 7 intermediate-mass Herbig Ae stars with circumstellar disks were obtained using the Spitzer Space Telescope as part of the c2d IRS survey. This work complements prior spectroscopic studies of silicate infrared emission from disks, which were focused on intermediate-mass stars, with observations of solar-mass stars limited primarily to the 10 um region. The observed 10 and 20 um silicate feature strengths/shapes are consistent with source-to-source variations in grain size. A large fraction of the features are weak and flat, consistent with um-sized grains indicating fast grain growth (from 0.1--1.0 um in radius). In addition, approximately half of the T Tauri star spectra show crystalline silicate features near 28 and 33 um indicating significant processing when compared to interstellar grains. A few sources show large 10-to-20 um ratios and require even larger grains emitting at 20 um than at 10 um. This size difference may arise from the difference in the depth into the disk probed by the two silicate emission bands in disks where dust settling has occurred. The 10 um feature strength vs. shape trend is not correlated with age or Halpha equivalent width, suggesting that some amount of turbulent mixing and regeneration of small grains is occurring. The strength vs. shape trend is related to spectral type, however, with M stars showing significantly flatter 10 um features (larger grain sizes) than A/B stars. The connection between spectral type and grain size is interpreted in terms of the variation in the silicate emission radius as a function of stellar luminosity, but could also be indicative of other spectral-type dependent factors (e.g, X-rays, UV radiation, stellar/disk winds, etc.)., 17 pages, 13 figures, accepted for publication by ApJ, formatted with emulateapj using revtex4 v4.0

Sensitive 5-38 um Spitzer Space Telescope (SST) and ground based 3-5 um spectra of the embedded low mass protostars B5 IRS1 and HH46 IRS show deep ice absorption bands superposed on steeply rising mid-infrared continua. The ices likely originate in the circumstellar envelopes. The CO2 bending mode at 15 um is a particularly powerful tracer of the ice composition and processing history. Toward these protostars, this band shows little evidence for thermal processing at temperatures above 50 K. Signatures of lower temperature processing are present in the CO and OCN- bands, however. The observed CO2 profile indicates an intimate mixture with H2O, but not necessarily with CH3OH, in contrast to some high mass protostars. This is consistent with the low CH3OH abundance derived from the ground based L band spectra. The CO2/H2O column density ratios are high in both B5 IRS1 and HH46 IRS (~35%). Clearly, the SST spectra are essential to study ice evolution in low mass protostellar environments, and to eventually determine the relation between interstellar and solar system ices., Scheduled for publication in ApJS Sept. 2004, v154 [Spitzer Special issue]. 5 pages, 1 b&w figure, 1 color figure

ISO-Oph-50 is a young low-mass object in the ~Myr old Ophiuchus star forming region undergoing dramatic changes in its optical/near/mid-infrared brightness by 2-4 mag. We present new multi-band photometry and near-infrared spectra, combined with a synopsis of the existing literature data. Based on the spectroscopy, the source is confirmed as a mid M dwarf, with evidence for ongoing accretion. The near-infrared lightcurves show large-scale variations, with 2-4 mag amplitude in the bands IJHK, with the object generally being bluer when faint. Near its brightest state, the object shows colour changes consistent with variable extinction of dAV~7 mag. High-cadence monitoring at 3.6mu reveals quasi-periodic variations with a typical timescale of 1-2 weeks. The best explanation for these characteristics is a low-mass star seen through circumstellar matter, whose complex variability is caused by changing inhomogeneities in the inner parts of the disk. When faint, the direct stellar emission is blocked; the near-infrared radiation is dominated by scattered light. When bright, the emission is consistent with a photosphere strongly reddened by circumstellar dust. Based on the available constraints, the inhomogeneities have to be located at or beyond ~0.1 AU distance from the star. If this scenario turns out to be correct, a major portion of the inner disk has to be clumpy, structured, and/or in turmoil. In its observational characteristics, this object resembles other types of YSOs with variability caused in the inner disk. Compared to other objects, however, ISO-Oph-50 is clearly an extreme case, given the large amplitude of the brightness and colour changes combined with the erratic behaviour. ISO-Oph-50 has been near its brightest state since 2013; further monitoring is highly encouraged., Comment: 11 pages, 7 colour figures, appendix with all new photometry, accepted for publication in MNRAS

SONYC -- Substellar Objects in Nearby Young Clusters -- is a survey program to investigate the frequency and properties of substellar objects in nearby star-forming regions. We present a new imaging and spectroscopic survey conducted in the young (~1 Myr), nearby (~200 pc) star-forming region Lupus 3. Deep optical and near-infrared images were obtained with MOSAIC-II and NEWFIRM at the CTIO-4m telescope, covering ~1.4 sqdeg on the sky. The i-band completeness limit of 20.3 mag is equivalent to 0.009-0.02 MSun, for Av \leq 5. Photometry and 11-12 yr baseline proper motions were used to select candidate low-mass members of Lupus 3. We performed spectroscopic follow-up of 123 candidates, using VIMOS at the Very Large Telescope (VLT), and identify 7 probable members, among which 4 have spectral type later than M6.0 and Teff \leq 3000K, i.e. are probably substellar in nature. Two of the new probable members of Lupus 3 appear underluminous for their spectral class and exhibit emission line spectrum with strong Halpha or forbidden lines associated with active accretion. We derive a relation between the spectral type and effective temperature: Teff=(4120 +- 175) - (172 +- 26) x SpT, where SpT refers to the M spectral subtype between 1 and 9. Combining our results with the previous works on Lupus 3, we show that the spectral type distribution is consistent with that in other star forming regions, as well as is the derived star-to-BD ratio of 2.0-3.3. We compile a census of all spectroscopically confirmed low-mass members with spectral type M0 or later., Comment: Accepted by Astrophysical Journal

Although the star itself is not He enriched, the periodicity and the variability in the UV wind lines of the pulsating B1 IV star beta Cep are similar to what is observed in magnetic He-peculiar B stars, suggesting that beta Cep is magnetic. We searched for a magnetic field using spectropolarimetry. From UV spectroscopy, we analysed the wind variability and investigated the correlation with the magnetic data. Using 130 time-resolved circular polarisation spectra, obtained with the MuSiCoS spectropolarimeter at the 2m TBL from 1998 until 2005, we applied the least-squares deconvolution method on the Stokes V spectra and derived the longitudinal component of the integrated magnetic field over the visible hemisphere of the star. We performed a period analysis on the magnetic data and on EW measurements of UV wind lines obtained over 17 years. We also analysed the short- and long-term radial velocity variations, which are due to the pulsations and the 90-year binary motion. beta Cep hosts a sinusoidally varying magnetic field with an amplitude 97(4) G and an average value -6(3) G. From the UV wind line variability, we derive a period of 12.00075(11) days, which is the rotation period of the star, and is compatible with the observed magnetic modulation. Phases of maximum and minimum field match those of maximum emission in the UV wind lines, strongly supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour as a function of pulsation behaviour and UV line variability. This paper presents the analysis of the first confirmed detection of a dipolar magnetic field in an upper main-sequence pulsating star. Maximum wind absorption originates in the magnetic equatorial plane. Maximum emission occurs when the magnetic north pole points to the Earth. Radial velocities agree with the ~90-y orbit around its Be-star binary companion., 14 pages, 10 figures, 5 tables

The statistics of discovered exoplanets suggest that planets form efficiently. However, there are fundamental unsolved problems, such as excessive inward drift of particles in protoplanetary disks during planet formation. Recent theories invoke dust traps to overcome this problem. We report the detection of a dust trap in the disk around the star Oph IRS 48 using observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast crescent-shaped emission on one side of the star originating from millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest rings centered on the star. The difference in distribution of big grains versus small grains/gas can be modeled with a vortex-shaped dust trap triggered by a companion., 25 pages, 7 figures (accepted version prior to language editing)

Within the SONYC - Substellar Objects in Nearby Young Clusters - survey, we investigate the frequency of free-floating planetary-mass objects (planemos) in the young cluster NGC1333. Building upon our extensive previous work, we present spectra for 12 of the faintest candidates from our deep multi-band imaging, plus seven random objects in the same fields, using MOIRCS on Subaru. We confirm seven new sources as young very low mass objects (VLMOs), with Teff of 2400-3100K and mid-M to early-L spectral types. These objects add to the growing census of VLMOs in NGC1333, now totaling 58. Three confirmed objects (one found in this study) have masses below 15 MJup, according to evolutionary models, thus are likely planemos. We estimate the total planemo population with 5-15 MJup in NGC1333 is, Comment: 14 pages, 11 figures. Updated version after proof corrections, additional comment in Sect. 5.4

We present Herschel PACS spectroscopy of the [OI] 63 micron gas-line for three circumstellar disk systems showing signs of significant disk evolution and/or planet formation: HR 8799, HD 377 and RX J1852.3-3700. [OI] is undetected toward HR 8799 and HD 377 with 3 sigma upper limits of 6.8 x 10^-18 W m^-2 and 9.9 x 10^-18 W m^-2 respectively. We find an [OI] detection for RX J1852.3-3700 at 12.3 +- 1.8 x 10^-18 W m^-2. We use thermo-chemical disk models to model the gas emission, using constraints on the [OI] 63 micron, and ancillary data to derive gas mass upper limits and constrain gas-to-dust ratios. For HD 377 and HR 8799, we find 3 sigma upper limits on the gas mass of 0.1-20 Mearth. For RX J1852.3-3700, we find two distinct disk scenarios that could explain the detection of [OI] 63 micron and CO(2-1) upper limits reported from the literature: (i) a large disk with gas co-located with the dust (16-500 AU), resulting in a large tenuous disk with ~16 Mearth of gas, or (ii) an optically thick gas disk, truncated at ~70 AU, with a gas mass of 150 Mearth. We discuss the implications of these results for the formation and evolution of planets in these three systems., Accepted for publication in ApJ, 8 pages ApJ style (incl. references), 2 figures, 4 tables

SONYC -- Substellar Objects in Nearby Young Clusters -- is a program to investigate the frequency and properties of young substellar objects with masses down to a few times that of Jupiter. Here we present a census of very low mass objects in the ~1 Myr old cluster NGC1333. We analyze near-infrared spectra taken with FMOS/Subaru for 100 candidates from our deep, wide-field survey and find 10 new likely brown dwarfs with spectral types of M6 or later. Among them, there are three with >~M9 and one with early L spectral type, corresponding to masses of 0.006 to, Comment: 16 pages, 7 tables, 12 figures, accepted for publication in ApJ

SONYC - Substellar Objects in Nearby Young Clusters - is a survey program to investigate the frequency and properties of brown dwarfs down to masses below the Deuterium burning limit in nearby star forming regions. In this second paper, we present results on the ~1 Myr old cluster Rho Ophiuchi, combining our own deep optical and near-infrared imaging using Subaru with photometry from the 2-Micron All Sky Survey and the Spitzer Space Telescope. Of the candidates selected from iJKs photometry, we have confirmed three -- including a new brown dwarf with a mass close to the Deuterium limit -- as likely cluster members through low-resolution infrared spectroscopy. We also identify 27 sub-stellar candidates with mid-infrared excess consistent with disk emission, of which 16 are new and 11 are previously spectroscopically confirmed brown dwarfs. The high and variable extinction makes it difficult to obtain the complete sub-stellar population in this region. However, current data suggest that its ratio of low-mass stars to brown dwarfs in similar to those reported for several other clusters, though higher than what was found for NGC 1333 in Scholz et al. 2009., 32 pages, 9 figures, accepted for publication in ApJ

Context. The first magnetic fields in O- and B-type stars that do not belong to the Bp-star class, have been discovered. The cyclic UV wind-line variability, which has been observed in a significant fraction of early-type stars, is likely to be related to such magnetic fields. Aims. We attempt to improve our understanding of massive-star magnetic fields, and observe twenty-five carefully-selected, OB-type stars. Methods. Of these stars we obtain 136 magnetic field strength measurements. We present the UV wind-line variability of all selected targets and summarise spectropolarimetric observations acquired using the MUSICOS spectropolarimeter, mounted at the TBL, Pic du Midi, between December 1998 and November 2004. From the average Stokes I and V line profiles, derived using the LSD method, we measure the magnetic field strengths, radial velocities, and first moment of the line profiles. Results. No significant magnetic field is detected in any OB-type star that we observed. Typical 1{\sigma} errors are between 15 and 200 G. A possible magnetic-field detection for the O9V star 10 Lac remains uncertain, because the field measurements depend critically on the fringe- effect correction in the Stokes V spectra. We find excess emission in UV-wind lines, centred about the rest wavelength, to be a new indirect indicator of the presence of a magnetic field in early B-type stars. The most promising candidates to host magnetic fields are the B-type stars {\delta} Cet and 6 Cep, and a number of O stars. Conclusions. Although some O and B stars have strong dipolar field, which cause periodic variability in the UV wind-lines, such strong fields are not widespread. If the variability observed in the UV wind-lines of OB stars is generally caused by surface magnetic fields, these fields are either weak (, Comment: A&A published

Understanding how disks dissipate is essential to studies of planet formation. However, identifying exactly how dust and gas dissipates is complicated due to difficulty in finding objects clearly in the transition of losing their surrounding material. We use Spitzer IRS spectra to examine 35 photometrically-selected candidate cold disks (disks with large inner dust holes). The infrared spectra are supplemented with optical spectra to determine stellar and accretion properties and 1.3mm photometry to measure disk masses. Based on detailed SED modeling, we identify 15 new cold disks. The remaining 20 objects have IRS spectra that are consistent with disks without holes, disks that are observed close to edge-on, or stars with background emission. Based on these results, we determine reliable criteria for identifying disks with inner holes from Spitzer photometry and examine criteria already in the literature. Applying these criteria to the c2d surveyed star-forming regions gives a frequency of such objects of at least 4% and most likely of order 12% of the YSO population identified by Spitzer. We also examine the properties of these new cold disks in combination with cold disks from the literature. Hole sizes in this sample are generally smaller than for previously discovered disks and reflect a distribution in better agreement with exoplanet orbit radii. We find correlations between hole size and both disk and stellar masses. Silicate features, including crystalline features, are present in the overwhelming majority of the sample although 10 micron feature strength above the continuum declines for holes with radii larger than ~7 AU. In contrast, PAHs are only detected in 2 out of 15 sources. Only a quarter of the cold disk sample shows no signs of accretion, making it unlikely that photoevaporation is the dominant hole forming process in most cases., Comment: 24 pages, 18 figures and 8 tables. Fixed a typo in Table 8

We present Spitzer IRS mid-infrared (5-35 micron) spectra of a complete flux-limited sample (> 3 mJy at 8 micron) of young stellar object (YSO) candidates selected on the basis of their infrared colors in the Serpens Molecular Cloud. Spectra of 147 sources are presented and classified. Background stars (with slope consistent with a reddened stellar spectrum and silicate features in absorption), galaxies (with redshifted PAH features) and a planetary nebula (with high ionization lines) amount to 22% of contamination in this sample, leaving 115 true YSOs. Sources with rising spectra and ice absorption features, classified as embedded Stage I protostars, amount to 18% of the sample. The remaining 82% (94) of the disk sources are analyzed in terms of spectral energy distribution shapes, PAHs and silicate features. The presence, strength and shape of these silicate features are used to infer disk properties for these systems. About 8% of the disks have 30/13 micron flux ratios consistent with cold disks with inner holes or gaps, and 3% of the disks show PAH emission. Comparison with models indicates that dust grains in the surface of these disks have sizes of at least a few ��\m. The 20 micron silicate feature is sometimes seen in absence of the 10 micron feature, which may be indicative of very small holes in these disks. No significant difference is found in the distribution of silicate feature shapes and strengths between sources in clusters and in the field. Moreover, the results in Serpens are compared with other well-studied samples: the c2d IRS sample distributed over 5 clouds and a large sample of disks in the Taurus star-forming region. The remarkably similar distributions of silicate feature characteristics in samples with different environment and median ages - if significant - imply that the dust population in the disk surface results from an equilibrium between dust growth and destructive collision processes that are maintained over a few million years for any YSO population irrespective of environment., accepted by ApJ

International audience; Aims: Dust grains in the planet-forming regions around young stars are expected to be heavily processed due to coagulation, fragmentation, and crystallization. This paper focuses on the crystalline silicate dust grains in protoplanetary disks for a statistically significant number of TTauri stars (96). Methods: As part of the cores to disks (c2d) legacy program, we obtained more than a hundred Spitzer/IRS spectra of TTauri stars, over a spectral range of 5-35 μm where many silicate amorphous and crystalline solid-state features are present. At these wavelengths, observations probe the upper layers of accretion disks up to distances of a dozen AU from the central object. Results: More than 3/4 of our objects show at least one crystalline silicate emission feature that can be essentially attributed to Mg-rich silicates. The Fe-rich crystalline silicates are largely absent in the c2d IRS spectra. The strength and detection frequency of the crystalline features seen at λ > 20 μm correlate with each other, while they are largely uncorrelated with the observational properties of the amorphous silicate 10 μm feature. This supports the idea that the IRS spectra essentially probe two independent disk regions: a warm zone (≤1 AU) emitting at λ ~ 10 μm and a much colder region emitting at λ > 20 μm (≤10 AU). We identify a crystallinity paradox, as the long-wavelength (λ > 20 μm) crystalline silicate features are detected 3.5 times more frequently (~55% vs. ~15%) than the crystalline features arising from much warmer disk regions (λ ~10 μm). This suggests that the disk has an inhomogeneous dust composition within ~10 AU. The analysis of the shape and strength of both the amorphous 10 μm feature and the crystalline feature around 23 μm provides evidence for the prevalence of μm-sized (amorphous and crystalline) grains in upper layers of disks. Conclusions: The abundant crystalline silicates found far from their presumed formation regions suggest efficient outward radial transport mechanisms in the disks around TTauri stars. The presence of μm-sized grains in disk atmospheres, despite the short timescales for settling to the midplane, suggests efficient (turbulent) vertical diffusion, probably accompanied by grain-grain fragmentation to balance the expected efficient growth. In this scenario, the depletion of submicron-sized grains in the upper layers of the disks points toward removal mechanisms such as stellar winds or radiation pressure. Tables [see full textsee full textsee full textsee full text], [see full textsee full textsee full textsee full text] and Figs. [see full textsee full textsee full textsee full text]-[see full textsee full textsee full textsee full text] are only available in electronic form at http://www.aanda.org

We announce the discovery of SST-Lup3-1, a very low mass star close to the brown dwarf boundary in Lupus III with a circum(sub)stellar disk, discovered by the `Cores to Disks' Spitzer Legacy Program from mid-, near-infrared and optical data, with very conspicuous crystalline silicate features in its spectrum. It is the first of such objects with a full 5 to 35 micron spectrum taken with the IRS and it shows strong 10 and 20 micron silicate features with high feature to continuum ratios and clear crystalline features out to 33 micron. The dust in the disk upper layer has a crystalline silicate grain fraction between 15% and 33%, depending on the assumed dust continuum. The availability of the full Spitzer infrared spectrum allows an analysis of the dust composition as a function of temperature and position in the disk. The hot (~ 300 K) dust responsible for the 10 micron feature consists of a roughly equal mix of small (~ 0.1 micron) and large (~ 1.5 micron) grains, whereas the cold (~ 70 K) dust responsible for the longer wavelength silicate features contains primarily large grains (> 1 micron). Since the cold dust emission arises from deeper layers in the inner (< 3 AU) disk as well as from the surface layers of the outer (3-5 AU) disk, this provides direct evidence for combined grain growth and settling in the disk. The inferred crystalline mass fractions in the two components are comparable. Since only the inner 0.02 AU of the disk is warm enough to anneal the amorphous silicate grains, even the lowest fraction of 15% of crystalline material requires either very efficient mixing or other formation mechanisms., Accepted for publication in ApJ. 19 pages and 4 figures. V2 with correct references

Recent results indicate that the grain size and crystallinity inferred from observations of silicate features may be correlated with spectral type of the central star and/or disk geometry. In this paper, we show that grain size, as probed by the 10 um silicate feature peak-to-continuum and 11.3-to-9.8 um flux ratios, is inversely proportional to log L_star. These trends can be understood using a simple two-layer disk model for passive irradiated flaring disks, CGPLUS. We find that the radius, R_10, of the 10 um silicate emission zone in the disk goes as (L_star/L_sun)^0.56, with slight variations depending on disk geometry (flaring angle, inner disk radius). The observed correlations, combined with simulated emission spectra of olivine and pyroxene mixtures, imply a grain size dependence on luminosity. Combined with the fact that R_10 is smaller for less luminous stars, this implies that the apparent grain size of the emitting dust is larger for low-luminositysources. In contrast, our models suggest that the crystallinity is only marginally affected, because for increasing luminosity, the zone for thermal annealing (assumed to be at T>800 K) is enlarged by roughly the same factor as the silicate emission zone. The observed crystallinity is affected by disk geometry, however, with increased crystallinity in flat disks. The apparent crystallinity may also increase with grain growth due to a corresponding increase in contrast between crystalline and amorphous silicate emission bands., 6 pages, 3 figures, scheduled to appear in ApJ, vol 658 (April 1, 2007)

Aims. The chemistry of, and infrared (IR) emission from, polycyclic aromatic hydrocarbons (PAHs) in disks around Herbig Ae/Be and T Tauri stars are investigated. The equilibrium distribution of the PAHs over all accessible charge/hydrogenation states depends on the size and shape of the PAHs and on the physical properties of the star and surrounding disk. Methods. A chemistry model is created to calculate this equilibrium distribution. Destruction of PAHs by ultraviolet (UV) photons, possibly in multi-photon absorption events, is taken into account. The chemistry model is coupled to a radiative transfer code to provide the physical parameters and to combine the PAH emission with the spectral energy distribution (SED) from the star+disk system. Results. Normally hydrogenated PAHs in Herbig Ae/Be disks account for most of the observed PAH emission, with neutral and positively ionized species contributing in roughly equal amounts. Close to the midplane, the PAHs are more strongly hydrogenated and negatively ionized, but these species do not contribute to the overall emission because of the low UV/optical flux deep inside the disk. PAHs of 50 carbon atoms are destroyed out to 100 AU in the disk's surface layer, and the resulting spatial extent of the emission does not agree well with observations. Rather, PAHs of about 100 carbon atoms or more are predicted to cause most of the observed emission. The emission is extended on a scale similar to that of the size of the disk. Furthermore, the emission from T Tauri disks is much weaker and concentrated more towards the central star than that from Herbig Ae/Be disks. Positively ionized PAHs are predicted to be largely absent in T Tauri disks because of the weaker radiation field., 13 pages, 8 figures, accepted for publication in A&A

International audience; Aims.Our aim is to determine the presence and location of the emission from polycyclic aromatic hydrocarbons (PAHs) towards low and intermediate mass young stars with disks using large aperture telescopes. Methods: VLT-VISIR N-band spectra and VLT-ISAAC and VLT-NACO L-band spectra of 29 sources are presented, spectrally resolving the 3.3, 8.6, 11.2, and 12.6 μm PAH features. Spatial-extent profiles of the features and the continuum emission have been derived and used to associate the PAH emission with the disks. The results are discussed in the context of recent PAH-emission disk models. Results: The 3.3, 8.6, and 11.2 μm PAH features are detected toward a small fraction of the T Tauri stars, with typical upper limits between 1 × 10-15 and 5 × 10-17 W m-2. All 11.2 μm detections from a previous Spitzer survey are confirmed with (tentative) 3.3 μm detections, and both the 8.6 and the 11.2 μm features are detected in all PAH sources. For 6 detections, the spatial extent of the PAH features is confined to scales typically smaller than 0.12-0.34'', consistent with the radii of 12-60 AU disks at their distances (typically 150 pc). For 3 additional sources, WL 16, HD 100546, and TY CrA, one or more of the PAH features are more extended than the hot dust continuum of the disk, whereas for Oph IRS 48, the size of the resolved PAH emission is confirmed as smaller than for the large grains. For HD 100546, the 3.3 μm emission is confined to a small radial extent of 12±3 AU, most likely associated with the outer rim of the gap in this disk. Gaps with radii out to 10-30 AU may also affect the observed PAH extent for other sources. For both Herbig Ae and T Tauri stars, the small measured extents of the 8.6 and 11.2 μm features are consistent with larger (≥100 carbon atoms) PAHs. Based on observations obtained at the European Southern Observatory, Paranal, Chile, within the observing programs 164.I-0605 (ISAAC May 2002), 074.C-0413 (NACO, March/April 2005), 075.C-0420 (ISAAC August 2005), 077.C-0668 (VISIR/ISAAC April/May 2006). Appendix A is only available in electronic form at http://www.aanda.org

We have identified four circumstellar disks with a deficit of dust emission from their inner 15-50 AU. All four stars have F-G spectral type, and were uncovered as part of the Spitzer Space Telescope ``Cores to Disks'' Legacy Program Infrared Spectrograph (IRS) first look survey of ~100 pre-main sequence stars. Modeling of the spectral energy distributions indicates a reduction in dust density by factors of 100-1000 from disk radii between ~0.4 and 15-50 AU, but with massive gas-rich disks at larger radii. This large contrast between the inner and outer disk has led us to use the term `cold disks' to distinguish these unusual systems. However, hot dust [0.02-0.2 Mmoon] is still present close to the central star (R ~0.8 AU). We introduce the 30/13 micron, flux density ratio as a new diagnostic for identifying cold disks. The mechanisms for dust clearing over such large gaps are discussed. Though rare, cold disks are likely in transition from an optically thick to an optically thin state, and so offer excellent laboratories for the study of planet formation., Comment: 13 pages, 3 figures, accepted to ApJL

SONYC - Substellar Objects in Nearby Young Clusters - is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. For the ~1Myr old rho Ophiuchi cluster, in our earlier paper we reported deep, wide-field optical and near-infrared imaging using Subaru, combined with 2MASS and Spitzer photometry, as well as follow-up spectroscopy confirming three likely cluster members, including a new brown dwarf with a mass close to the deuterium-burning limit. Here we present the results of extensive new spectroscopy targeting a total of ~100 candidates in rho Oph, with FMOS at the Subaru Telescope and SINFONI at the ESO's Very Large Telescope. We identify 19 objects with effective temperatures at or below 3200 K, 8 of which are newly identified very-low-mass probable members of rho Oph. Among these eight, six objects have Teff, Accepted for publication in Astrophysical Journal

SONYC - Substellar Objects in Nearby Young Clusters - is a survey program to investigate the frequency and properties of substellar objects with masses down to a few times that of Jupiter in nearby star-forming regions. In this third paper, we present our recent results in the Chamaeleon-I star forming region. We have carried out deep optical and near-infrared imaging in four bands (I, z, J, Ks) using VIMOS on the ESO Very Large Telescope and SOFI on the New Technology Telescope, and combined our data with mid-infrared data from the Spitzer Space Telescope. The survey covers ~0.25 sqdeg on the sky, and reaches completeness limits of 23.0 in the I-band, 18.3 in the J-band, and 16.7 in Ks-band. Follow-up spectroscopy of the candidates selected from the optical photometry (I, Comment: Accepted by Astrophysical Journal