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A CATALOG OF LOW-MASS STAR-FORMING CORES OBSERVED WITH SHARC-II AT 350 <i>μ</i> m
摘要： We present a catalog of low-mass dense cores observed with the SHARC-II instrument at 350 μm. Our observations have an effective angular resolution of 10″, approximately 2.5 times higher than observations at the same wavelength obtained with the Herschel Space Observatory, albeit with lower sensitivity, especially to extended emission. The catalog includes 81 maps covering a total of 164 detected sources. For each detected source, we tabulate basic source properties including position, peak intensity, flux density in fixed apertures, and radius. We examine the uncertainties in the pointing model applied to all SHARC-II data and conservatively find that the model corrections are good to within ～3″, approximately 1/3 of the SHARC-II beam. We examine the differences between two array scan modes and find that the instrument calibration, beam size, and beam shape are similar between the two modes. We also show that the same flux densities are measured when sources are observed in the two different modes, indicating that there are no systematic effects introduced into our catalog by utilizing two different scan patterns during the course of taking observations. We find a detection rate of 95% for protostellar cores but only 45% for starless cores, and demonstrate the existence of a SHARC-II detection bias against all but the most massive and compact starless cores. Finally, we discuss the improvements in protostellar classification enabled by these 350 μm observations.
关键词： stars: formation,ISM: clouds,submillimeter: ISM,stars: low-mass
更新于2025-09-23 15:23:52
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A library of near-infrared integral field spectra of young M–L dwarfs
摘要： Context. At young ages, low surface gravity affects the atmospheric properties of ultracool dwarfs. The impact on medium-resolution near-infrared (NIR) spectra has only been slightly investigated at the M–L transition so far. Aims. We present a library of NIR (1.1–2.45 μm) medium-resolution (R ～ 1500–2000) integral field spectra of 15 young M6?L0 dwarfs. We aim at deriving updated NIR spectral type, luminosity, and physical parameters (Teff, log g, M, L/L☉) for each source. This work also aims at testing the latest generation of BT-SETTL atmospheric models. Methods. We estimated spectral types using spectral indices and the spectra of young objects classified in the optical. We used the 2010 and 2012 releases of the BT-SETTL synthetic spectral grid and cross-checked the results with the DRIFT-PHOENIX models to derive the atmospheric properties of the sources. Results. We do not find significant differences between the spectra of young companions and those of young isolated brown dwarfs in the same spectral type range. We derive infrared spectral types L0 ± 1, L0 ± 1, M9.5 ± 0.5, M9.5 ± 0.5, M9.25 ± 0.25, M8+0.5?0.75, and M8.5 ± 0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B, respectively. The BT-SETTL and DRIFT-PHOENIX models yield close Teff and log g estimates for each source. The models seem to show a 600+600?300 K drop in the effective temperature at the M–L transition. Assuming the former temperatures are correct, we then derive new mass estimates that confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lie inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5–L0 sources to the Teff to derive semi-empirical radii estimates that do not match 'hot-start' evolutionary models predictions at 1–3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the NIR spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue raises doubts on the ability of these models to predict effective temperatures from NIR spectra alone. Conclusions. The library provides templates for characterizing other young and late type objects. The study advocates the use of photometric and spectroscopic information over a broad range of wavelengths to study the properties of very low-mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).
关键词： stars: low-mass,techniques: spectroscopic,planetary systems,brown dwarfs
更新于2025-09-23 15:23:52
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Deep, multiband photometry of low-mass stars to reveal young clusters: A blind study of the NGC2264 region
摘要： Context. Thanks to their extensive and homogeneous sky coverage, deep, large-scale, multiwavelength surveys are uniquely suited to statistically identify and map young star clusters in our Galaxy. Such studies are crucial to address issues like the initial mass function, or the modes and dynamics of star cluster formation and evolution. Aims. We aim to test a purely photometric approach to statistically identify a young clustered population embedded in a large population of field stars, with no prior knowledge of the nature of stars in the field. We conducted our blind test study on the NGC 2264 region, which hosts a well-known, richly populated young cluster (～3 Myr-old) and several active star-forming sites. Methods. We selected a large (4 deg2) area around the NGC 2264 cluster, and assembled an extensive r, i, J catalog of the field from pre-existing large-scale surveys, notably Pan-STARRS1 and UKIDSS. We then mapped the stellar color locus on the (i ? J, r ? i) diagram to select M-type stars, which offer the following observational advantages with respect to more massive stars: (i) they comprise a significant fraction of the Galactic stellar population; (ii) their pre-main sequence phase lasts significantly longer than for higher mass stars; (iii) they exhibit the strongest luminosity evolution from the pre-main sequence to the main sequence; (iv) their observed r, i, J colors provide a direct and empirical estimate of AV . A comparative analysis of the photometric and spatial properties of M-type stars as a function of AV enabled us to probe the structure and stellar content of our field. Results. Using only r, i, J photometry, we could identify two distinct populations in our field: a diffuse field population and a clustered population in the center of the field. The presence of a concentration of occulting material, spatially associated with the clustered population, allowed us to derive an estimate of its distance (800–900 pc) and age (～0.5–5 Myr); these values are overall consistent with the literature parameters for the NGC 2264 star-forming region. The extracted clustered population exhibits a hierarchical structure, with two main clumps and peaks in number density of objects around the most reddened locations within the field. An excellent agreement is found between the observed substructures for the clustered population and a map of the NGC 2264 subregions reported in the literature. Our selection of clustered members is coherent with the literature census of the NGC 2264 cluster for about 95% of the objects located in the inner regions of the field, where the estimated contamination rate by field stars in our sample is only 2%. In addition, the availability of a uniform dataset for a large area around the NGC 2264 region enabled us to discover a population of about a hundred stars with indications of statistical membership to the cluster, therefore extending the low-mass population census of NGC 2264 to distances of 10–15 pc from the cluster cores. Conclusions. By making use solely of deep, multiband (r, i, J) photometry, without assuming any further knowledge of the stellar population of our field, we were able to statistically identify and reconstruct the structure of a very young cluster that has been a prime target for star formation studies over several decades. The method tested here can be readily applied to surveys such as Pan-STARRS and the future LSST to undertake a first complete census of low-mass, young stellar populations down to distances of several kiloparsecs across the Galactic plane.
关键词： techniques: photometric,stars: low-mass,methods: statistical,surveys,open clusters and associations: individual: NGC 2264,methods: observational
更新于2025-09-23 15:22:29
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The VISTA Carina Nebula Survey
摘要： We performed a deep wide-field (6.76 sq. deg) near-infrared survey with the VISTA telescope that covers the entire extent of the Carina nebula complex (CNC). The point-source catalog created from these data contains around four million individual objects down to masses of 0.1 M(cid:3). We present a statistical study of the large-scale spatial distribution and an investigation of the clustering properties of infrared-excesses objects, which are used to trace disk-bearing young stellar objects (YSOs). A selection based on a near-infrared (J ? H) versus (H ? Ks) color–color diagram shows an almost uniform distribution over the entire observed area. We interpret this as a result of the very high degree of background contamination that arises from the Carina Nebula’s location close to the Galactic plane. Complementing the VISTA near-infrared catalog with Spitzer IRAC mid-infrared photometry improves the situation of the background contamination considerably. We find that a (J ? H) versus (Ks ? [4.5]) color–color diagram is well suited to tracing the population of YSO-candidates (cYSOs) by their infrared excess. We identify 8781 sources with strong infrared excess, which we consider as cYSOs. This sample is used to investigate the spatial distribution of the cYSOs with a nearest-neighbor analysis. The surface density distribution of cYSOs agrees well with the shape of the clouds as seen in our Herschel far-infrared survey. The strong decline in the surface density of excess sources outside the area of the clouds supports the hypothesis that our excess-selected sample consists predominantly of cYSOs with a low level of background contamination. This analysis allows us to identify 14 groups of cYSOs outside the central area.Our results suggest that the total population of cYSOs in the CNC comprises about 164 000 objects, with a substantial fraction (～35%) located in the northern, still not well studied parts. Our cluster analysis suggests that roughly half of the cYSOs constitute a non-clustered, dispersed population.
关键词： stars: formation,infrared: stars,stars: low-mass,stars: pre-main sequence,ISM: individual objects: NGC 3372
更新于2025-09-23 15:19:57
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Rotational modulation of the linear polarimetric variability of the cool dwarf TVLM 513?46546
摘要： Aims. We aim to monitor the optical linear polarimetric signal of the magnetized, rapidly rotating M8.5 dwarf TVLM 513?46546. Methods. R- and I-band linear polarimetry images were collected with the Andalucía Faint Object Spectrograph and Camera (ALFOSC) instrument of the 2.56-m Nordic Optical Telescope (NOT) on two consecutive nights, covering about 0.5 and four rotation cycles in the R and I ?lters, respectively. We also obtained simultaneous intensity curves by means of di?erential photometry. The typical precision of the data is ±0.46% (R), ±0.35% (I) in the linear polarization degree and ±9 mmag (R), ±1.6 mmag (I) in the di?erential intensity curves. Results. Strong and variable linear polarization is detected in the R and I ?lters, with values of maximum polarization (p? = 1.30 ± 0.35 %), that are similar for both bands. The intensity and the polarimetric curves present a sinusoid-like pattern with a periodicity of ～1.98 h, which we ascribe to structures in the surface of TVLM 513?46546 synchronized with its rotation. We found that the peaks of intensity and polarimetric curves occur with a phase di?erence of 0.18 ± 0.01 and that the maximum of the linear polarization occurs nearly half a period (0.59 ± 0.03) after the radio pulse. We discuss di?erent scenarios to account for the observed properties of the light curves.
关键词： polarization,scattering,stars: individual: TVLM 513?46546,stars: late-type,stars: low-mass
更新于2025-09-19 17:15:36
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The CARMENES search for exoplanets around M dwarfs
摘要： We announce the discovery of two planetary companions orbiting around the low-mass stars Ross 1020 (GJ 3779, M4.0V) and LP 819-052 (GJ 1265, M4.5V). The discovery is based on the analysis of CARMENES radial velocity (RV) observations in the visual channel as part of its survey for exoplanets around M dwarfs. In the case of GJ 1265, CARMENES observations were complemented with publicly available Doppler measurements from HARPS. The datasets reveal two planetary companions, one for each star, that share very similar properties: minimum masses of 8.0 ± 0.5 M⊕ and 7.4 ± 0.5 M⊕ in low-eccentricity orbits with periods of 3.023 ± 0.001 d and 3.651 ± 0.001 d for GJ 3779 b and GJ 1265 b, respectively. The periodic signals around 3 d found in the RV data have no counterpart in any spectral activity indicator. Furthermore, we collected available photometric data for the two host stars, which confirm that the additional Doppler variations found at periods of approximately 95 d can be attributed to the rotation of the stars. The addition of these planets to a mass-period diagram of known planets around M dwarfs suggests a bimodal distribution with a lack of short-period low-mass planets in the range of 2–5 M⊕. It also indicates that super-Earths (>5 M⊕) currently detected by RV and transit techniques around M stars are usually found in systems dominated by a single planet.
关键词： techniques: radial velocities,stars: late-type,planetary systems,stars: low-mass
更新于2025-09-19 17:15:36
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Mapping accretion and its variability in the young open cluster NGC 2264: a study based on <i>u</i> -band photometry
摘要： Context. The accretion process has a central role in the formation of stars and planets. Aims. We aim at characterizing the accretion properties of several hundred members of the star-forming cluster NGC 2264 (3 Myr). Methods. We performed a deep ugri mapping as well as a simultaneous u-band+r-band monitoring of the star-forming region with CFHT/MegaCam in order to directly probe the accretion process onto the star from UV excess measurements. Photometric properties and stellar parameters are determined homogeneously for about 750 monitored young objects, spanning the mass range ～0.1–2 M(cid:4). About 40% of the sample are classical (accreting) T Tauri stars, based on various diagnostics (Hα, UV and IR excesses). The remaining non-accreting members de?ne the (photospheric + chromospheric) reference UV emission level over which ?ux excess is detected and measured. Results. We revise the membership status of cluster members based on UV accretion signatures, and report a new population of 50 classical T Tauri star (CTTS) candidates. A large range of UV excess is measured for the CTTS population, varying from a few times 0.1 to ～3 mag. We convert these values to accretion luminosities and accretion rates, via a phenomenological description of the accretion shock emission. We thus obtain mass accretion rates ranging from a few 10?10 to ～10?7 M(cid:4)/yr. Taking into account a mass-dependent detection threshold for weakly accreting objects, we ?nd a >6σ correlation between mass accretion rate and stellar mass. A power-law ?t, properly accounting for censored data (upper limits), yields ˙Macc ∝ M1.4±0.3. At any given stellar mass, we ?nd a large spread of accretion rates, extending over about 2 orders of magnitude. The monitoring of the UV excess on a timescale of a couple of weeks indicates that its variability typically amounts to 0.5 dex, i.e., much smaller than the observed spread in accretion rates. We suggest that a non-negligible age spread across the star-forming region may e?ectively contribute to the observed spread in accretion rates at a given mass. In addition, di?erent accretion mechanisms (like, e.g., short-lived accretion bursts vs. more stable funnel-?ow accretion) may be associated to di?erent ˙Macc regimes. Conclusions. A huge variety of accretion properties is observed for young stellar objects in the NGC 2264 cluster. While a de?nite correlation seems to hold between mass accretion rate and stellar mass over the mass range probed here, the origin of the large intrinsic spread observed in mass accretion rates at any given mass remains to be explored.
关键词： accretion,stars: pre-main sequence,stars: formation,stars: low-mass,ultraviolet: stars,open clusters and associations: individual: NGC 2264,accretion disks
更新于2025-09-04 15:30:14