C. Alves de Oliveira

YSOVAR: THE FIRST SENSITIVE, WIDE-AREA, MID-INFRARED PHOTOMETRIC MONITORING OF THE ORION NEBULA CLUSTER

We present initial results from time-series imaging at infrared wavelengths of 0.9 deg^2 in the Orion Nebula Cluster (ONC). During Fall 2009 we obtained 81 epochs of Spitzer 3.6 and 4.5 μm data over 40 consecutive days. We extracted light curves with ~3% photometric accuracy for ~2000 ONC members ranging from several solar masses down to well below the hydrogen-burning mass limit. For many of the stars, we also have time-series photometry obtained at optical (I_c) and/or near-infrared (JK_s ) wavelengths. Our data set can be mined to determine stellar rotation periods, identify new pre-main-sequence eclipsing binaries, search for new substellar Orion members, and help better determine the frequency of circumstellar disks as a function of stellar mass in the ONC. Our primary focus is the unique ability of 3.6 and 4.5 μm variability information to improve our understanding of inner disk processes and structure in the Class I and II young stellar objects (YSOs). In this paper, we provide a brief overview of the YSOVAR Orion data obtained in Fall 2009 and highlight our light curves for AA-Tau analogs—YSOs with narrow dips in flux, most probably due to disk density structures passing through our line of sight. Detailed follow-up observations are needed in order to better quantify the nature of the obscuring bodies and what this implies for the structure of the inner disks of YSOs.

HAWK-I: the high-acuity wide-field K-band imager for the ESO Very Large Telescope

We describe the design, development, and performance of HAWK-I, the new High-Acuity Wide-field K-band Imager for ESO’s Very Large Telescope, which is equipped with a mosaic of four 2 k × 2 k arrays and operates from 0.9−2.4 μm over 7.5 � × 7.5 � with 0.1 �� pixels. A novel feature is the use of all reflective optics that, together with filters of excellent throughput and detectors of high quantum efficiency, has yielded an extremely high throughput. Commissioning and science verification observations have already delivered a variety of excellent and deep images that demonstrate its high scientific potential for addressing important astrophysical questions of current interest.

Deep near-IR variability survey of pre-main-sequence stars in ρ Ophiuchi

Context. Variability is a common characteristic of pre-main-sequence stars (PMS). Near-IR variability surveys of young stellar objects (YSOs) can probe stellar and circumstellar environments and provide information about the dynamics of the ongoing magnetic and accretion processes. Furthermore, variability can be used as a tool to uncover new cluster members in star formation regions. Aims. We hope to achieve the deepest near-IR variability study of YSOs targeting the ρ Ophiuchi cluster. Methods. Fourteen epochs of observations were obtained with the Wide Field Camera (WFCAM) at the UKIRT telescope scheduled in a manner that allowed the study of variability on timescales of days, months, and years. Statistical tools, such as the multi-band cross correlation index and the reduced chi-square, were used to disentangle signals of variability from noise. Variability characteristics are compared to existing models of YSOs in order to relate them to physical processes, and then used to select new candidate members of this star-forming region. Results. Variability in the near-IR is found to be present in 41% of the known population of ρ Ophiuchi recovered in our sample. The behaviours shown are several and can be associated with the existence of spots on the stellar surface, variations in circumstellar extinction, or changes in the geometry of an accretion disc. Using variability, a new population of objects has been uncovered that is believed to be part of the ρ Ophiuchi cluster.

The low-mass population of the ρ Ophiuchi molecular cloud ,

Context. Star formation theories are currently divergent regarding the fundamental physical processes that dominate the substellar regime. Observations of nearby young open clusters allow the brown dwarf (BD) population to be characterised down to the planetary mass regime, which ultimately must be accommodated by a successful theory. Aims. We hope to uncover the low-mass population of the p Ophiuchi molecular cloud and investigate the properties of the newly found brown dwarfs. Methods. We used near-IR deep images (reaching completeness limits of approximately 20.5 mag in J and 18.9 mag in H and K s ) taken with the Wide Field IR Camera (WIRCam) at the Canada France Hawaii Telescope (CFHT) to identify candidate members of p Oph in the substellar regime. A spectroscopic follow-up of a small sample of the candidates allows us to assess their spectral type and subsequently their temperature and membership. Results. We select 110 candidate members of the p Ophiuchi molecular cloud, from which 80 have not previously been associated with the cloud. We observed a small sample of these and spectroscopically confirm six new brown dwarfs with spectral types ranging from M6.5 to M8.25.

Herschel view of the large-scale structure in the Chamaeleon dark clouds

The Chamaeleon molecular cloud complex is one of the nearest star-forming sites encompassing three molecular clouds with a different star-formation history, from quiescent (Cha III) to actively forming stars (Cha II), and reaching the end of star-formation (Cha I). To charactize its large-scale structure, we derived column density and temperature maps using PACS and SPIRE observations from the Herschel Gould Belt Survey, and applied several tools, such as filament tracing, power-spectra, \Delta-variance, and probability distribution functions of column density (PDFs), to derive physical properties. The column density maps reveal a different morphological appearance for the three clouds, with a ridge-like structure for Cha I, a clump-dominated regime for Cha II, and an intricate filamentary network for Cha III. The filament width is measured to be around 0.12\pm0.04 pc in the three clouds, and the filaments found to be gravitationally unstable in Cha I and II, but mostly subcritical in Cha III. Faint filaments (striations) are prominent in Cha I showing a preferred alignment with the large-scale magnetic field. The PDFs of all regions show a lognormal distribution at low column densities. For higher densities, the PDF of Cha I shows a turnover indicative of an extended higher density component, culminating with a power-law tail. Cha II shows a power-law tail with a slope characteristic of gravity. The PDF of Cha III can be best fit by a single lognormal. The turbulence properties of the three regions are found to be similar, pointing towards a scenario where the clouds are impacted by large-scale processes. The magnetic field could possibly play an important role for the star-formation efficiency in the Chamaeleon clouds if proven that it can effectively channel material on Cha I, and possibly Cha II, but probably less efficiently on the quiescent Cha III cloud.

Spectroscopy of brown dwarf candidates in IC 348 and the determination of its substellar IMF down to planetary masses

Context. Brown dwarfs represent a sizable fraction of the stellar content of our Galaxy and populate the transition between the stellar and planetary mass regime. There is however no agreement on the processes responsible for their formation. Aims. We have conducted a large survey of the young, nearby cluster IC 348, to uncover its low-mass brown dwarf population and study the cluster properties in the substellar regime. Methods. Deep optical and near-IR images taken with MegaCam and WIRCam at the Canada-France-Hawaii Telescope (CFHT) were used to select photometric candidate members. A spectroscopic follow-up of a large fraction of the candidates was conducted to assess their youth and membership. Results. We confirmed spectroscopically 16 new members of the IC 348 cl uster, including 13 brown dwarfs, contributing significant ly to the substellar census of the cluster, where only 30 brown dwarfs were previously known. Five of the new members have a L0spectral type, the latest-type objects found to date in this cluster. At 3 Myr, evolutionary models estimate these brown dwarfs to have a mass of∼13 M Jup . Combining the new members with previous census of the cluster, we constructed the IMF complete down to 13 M Jup . Conclusions. The IMF of IC 348 is well fitted by a log-normal function, and we do not see evidence for variations of the mass function down to planetary masses when compared to other young clusters.

Herschel far-IR observations of the Chamaeleon molecular cloud complex . Chamaeleon I: A first view of young stellar objects in the cloud

Context. The Herschel Gould Belt survey of nearby star forming regions is providing great insights into the early stages of the formation and the evolution of stars and their circumstellar disks. The Chamaeleon I dark cloud is an elongated region of dense dust and gas where star formation is ongoing in two centres, a northern region centred on Ced 112 and a southern cluster subdivided into the two regions Ced 110 and 111. Aims. In this initial study we present Herschel data of previously identified young stellar objects (YSOs) in the cluster, focusing on the spatial distribution of the YSOs and the determination of the relative colours of the protostars and the disk-bearing stars in Chamaeleon I. Methods. Chamaeleon I has been observed as part of the Herschel Gould Belt Survey, using the PACS and SPIRE parallel mode imaging at 70, 160, 250, 350, and 500 μm. Source extraction was performed using the getsources software. Results. We have detected 397 sources over the five available PACS and SPIRE bands, and through comparison with previously identified objects in the cluster we have identified 49 YSOs, 4 bright nebular emission features, five CO clumps, and twenty-eight candidate prestellar or starless cores in the Herschel sample. The remaining sample consists of candidate prestellar cores, condensations within the cloud, or background galaxies. The Herschel detected YSOs are highly clustered and mainly associated with the three known Cederblad groups. The observations clearly show that the young stars are forming coincident with the denser regions of cold dust and gas, visible at longer Herschel wavelengths. Those YSOs detected with Herschel were found to have an IRAC m3.6 μm < 10. No difference was found in the Herschel colours between the class I and class II young stars, however the class I sources were brighter than the class II at most Herschel wavelengths. One class III star and three transition disks are detected.

The Herschel Gould Belt Survey in Chamaeleon II - Properties of cold dust in disks around young stellar objects

Context. We report on the Herschel Gould Belt survey (HGBS) of the Chamaeleon II (Cha II) star-forming region, focusing on the detection of Class I to III young stellar objects (YSOs). Aims. We aim at characterizing the circumstellar material around these YSOs and at understanding which disk parameters are most likely constrained by the new HGBS data, which are expected to be crucial for studying the transition from optically thick disks to evolved debris-type disks. Methods. We recovered 29 of the 63 known YSOs in Cha II with a detection in at least one of the PACS/SPIRE pass-bands: 3 Class I YSOs (i.e.,100%), 1 flat source (i.e., 50%), 21 Class II objects (i.e., 55%), 3 Class III objects (i.e, 16%), and the unclassified far-infrared source IRAS 12522-7640. We explored PACS/SPIRE colors of this sample and modeled their spectral energy distributions (SEDs) from the optical to Herschel’s wavelengths with the RADMC-2D radiative transfer code. Results. We find that YSO colors are typically confined to the following ranges: −0.7 ≲ log (F_(70)/F_(160)) ≲ 0.5, −0.5 ≲ log (F_(160)/F_(250)) ≲ 0.6, 0.05 ≲ log (F_(250)/F_(350)) ≲ 0.25 and −0.1 ≲ log (F_(350)/F_(500)) ≲ 0.5. These color ranges are expected to be only marginally contaminated by extragalactic sources and field stars and, hence, provide a useful YSO selection tool when applied together. We were able to model the SED of 26 of the 29 detected YSOs. We discuss the degeneracy/limitations of our SED fitting results and adopted the Bayesian method to estimate the probability of different values for the derived disk parameters. The Cha II YSOs present typical disk inner radii ≲0.1 AU, as previously estimated in the literature on the basis of Spitzer data. Our probability analysis shows that, thanks to the new Herschel data, the lower limits to the disk mass (M_(disk)) and characteristic radius (R_C) are well constrained, while the flaring angle (1 + φ) is only marginally constrained. The lower limit to R_C is typically around 50 AU. The lower limits to M_(disk) are proportional to the stellar masses with a typical 0.3% ratio, i.e., in the range estimated in the literature for young Class II stars and brown dwarfs across a broad range of stellar masses. The estimated flaring angles, although very uncertain, point toward very flat disks (1 + φ ≲ 1.2), as found for low-mass M-type YSO samples in other star-forming regions. Thus, our results support the idea that disk properties show a dependence on stellar properties.

A model-based approach to the spatial and spectral calibration of NIRSpec onboard JWST

Context. The NIRSpec instrument for the James Webb Space Telescope (JWST) can be operated in multiobject spectroscopy (MOS), long-slit, and integral field unit (IFU) mode with spectral resolutions from 100 to 2700. Its MOS mode uses about a quarter of a million individually addressable minislits for object selection, covering a field of view of ~9 arcmin 2 . Aims. The pipeline used to extract wavelength-calibrated spectra from NIRSpec detector images relies heavily on a model of NIRSpec optical geometry. We demonstrate how dedicated calibration data from a small subset of NIRSpec modes and apertures can be used to optimize this parametric model to the necessary levels of fidelity. Methods. Following an iterative procedure, the initial fiducial values of the model parameters are manually adjusted and then automatically optimized, so that the model predicted location of the images and spectral lines from the fixed slits, the IFU, and a small subset of the MOS apertures matches their measured location in the main optical planes of the instrument. Results. The NIRSpec parametric model is able to reproduce the spatial and spectral position of the input spectra with high fidelity. The intrinsic accuracy (1-sigma, rms) of the model, as measured from the extracted calibration spectra, is better than 1/10 of a pixel along the spatial direction and better than 1/20 of a resolution element in the spectral direction for all of the grating-based spectral modes. This is fully consistent with the corresponding allocation in the spatial and spectral calibration budgets of NIRSpec.

A molecular outflow driven by the brown dwarf binary FU Tauri

We report the detection of a molecular outflow driven by the brown dwarf binary FU Tau. Using the IRAM 30 m telescope we observed the 12 CO(2 1) (CO) emission in the vicinity of FU Tau and detected a bipolar outflow by examining the wings of the CO(2 1) line as we moved away from the source position. An integrated map of the wing emission between 3 km s 1 and 5 km s 1 reveals a blueshifted lobe at a position of 20 00 from the FU Tau system and at a position angle of 20 . The beam size of the observations is 11 00 , so it is not possible to distinguish between the two components of the FU Tau binary. Since optical forbidden emission, a strong tracer of the shocks caused by outflow activity, has been detected in the spectrum of FU Tau A, we assume that this component is the driving source of the molecular outflow. We estimate the mass of the outflow at 4 10 6 M and the mass outflow rate at 6 10 10 M /yr. These results agree well with previous estimates for BD molecular outflows. FU Tau A is now the third BD found to be associated with molecular outflow activity, and this discovery adds to the already extensive list of the interesting properties of FU Tau.