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Register a new userDetection of a large massive circumstellar disk around a high-mass young stellar object in the Carina Nebula
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(abbreviated) We investigate the spatial structure and spectral energy distribution of an edge-on circumstellar disk around an optically invisible young stellar object that is embedded in a dark cloud in the Carina Nebula. Whereas the object was detected as an apparently point-like source in earlier infrared observations, only the superb image quality (FWHM ~0.5) of our VLT / HAWK-I data could reveal, for the first time, its peculiar morphology. It consists of a very red point-like central source that is surrounded by a roughly spherical nebula, which is intersected by a remarkable dark lane through the center. We construct the spectral energy distribution of the object from 1 to 870 microns and perform a detailed radiative transfer modeling of the spectral energy distribution and the source morphology. The observed object morphology in the near-IR images clearly suggests a young stellar object that is embedded in an extended, roughly spherical envelope and surrounded by a large circumstellar disk with a diameter of ~5500 AU that is seen nearly edge-on. The radiative transfer modeling shows that the central object is a massive (10-15 Msun) young stellar object. The circumstellar disk has a mass of about 2 Msun. The disk object in Carina is one of the most massive young stellar objects for which a circumstellar disk has been detected so far, and the size and mass of the disk are very large compared to the corresponding values found for most other similar objects.
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Solar-mass stars form via circumstellar disk accretion (disk-mediated accretion). Recent findings indicate that this process is likely episodic in the form of accretion bursts, possibly caused by disk fragmentation. Although it cannot be ruled out that high-mass young stellar objects (HMYSOs; $M>$8 M$_odot$, $L_{bol}>$5$times$10$^3$ L$_odot$) arise from the coalescence of their low-mass brethren, latest results suggest that they more likely form via disks. Accordingly, disk-mediated accretion bursts should occur. Here we report on the discovery of the first disk-mediated accretion burst from a $sim$20 M$_odot$ HMYSO. Our near-infrared images show the brightening of the central source and its outflow cavities. Near-infrared spectroscopy reveals emission lines typical of accretion bursts in low-mass protostars, but orders of magnitude more luminous. Moreover, the energy released and the inferred mass-accretion rate are also orders of magnitude larger. Our results identify disk accretion as the common mechanism of star formation across the entire stellar mass spectrum.
OTS44 is one of only four free-floating planets known to have a disk. We have previously shown that it is the coolest and least massive known free-floating planet ($sim$12 M$_{rm Jup}$) with a substantial disk that is actively accreting. We have obtained Band 6 (233 GHz) ALMA continuum data of this very young disk-bearing object. The data shows a clear unresolved detection of the source. We obtained disk-mass estimates via empirical correlations derived for young, higher-mass, central (substellar) objects. The range of values obtained are between 0.07 and 0.63 M$_{oplus}$ (dust masses). We compare the properties of this unique disk with those recently reported around higher-mass (brown dwarfs) young objects in order to infer constraints on its mechanism of formation. While extreme assumptions on dust temperature yield disk-mass values that could slightly diverge from the general trends found for more massive brown dwarfs, a range of sensible values provide disk masses compatible with a unique scaling relation between $M_{rm dust}$ and $M_{*}$ through the substellar domain down to planetary masses.
The Young Stellar Object (YSO) W33A is one of the best known examples of a massive star still in the process of forming. Here we present Gemini North ALTAIR/NIFS laser-guide star adaptive-optics assisted K-band integral-field spectroscopy of W33A and its inner reflection nebula. In our data we make the first detections of a rotationally-flattened outer envelope and fast bi-polar jet of a massive YSO at near-infrared wavelengths. The predominant spectral features observed are Br-gamma, H_2, and a combination of emission and absorption from CO gas. We perform a 3-D spectro-astrometric analysis of the line emission, the first study of its kind. We find that the objects Br-gamma emission reveals evidence for a fast bi-polar jet on sub-milliarcsecond scales, which is aligned with the larger-scale outflow. The hybrid CO features can be explained as a combination of hot CO emission arising in a disk close to the central star, while cold CO absorption originates in the cooler outer envelope. Kinematic analysis of these features reveals that both structures are rotating, and consistent with being aligned perpendicularly to both the ionised jet and the large-scale outflow. Assuming Keplerian rotation, we find that the circumstellar disk orbits a central mass of >10Msun, while the outer envelope encloses a mass of ~15Msun. Our results suggest a scenario of a central star accreting material from a circumstellar disk at the centre of a cool extended rotating torus, while driving a fast bi-polar wind. These results therefore provide strong supporting evidence for the hypothesis that the formation mechanism for high-mass stars is qualitatively similar to that of low-mass stars.
Photometric detections of dust circumstellar disks around pre-main sequence (PMS) stars, coupled with estimates of stellar ages, provide constraints on the time available for planet formation. Most previous studies on disk longevity, starting with Haisch, Lada & Lada (2001), use star samples from PMS clusters but do not consider datasets with homogeneous photometric sensitivities and/or ages placed on a uniform timescale. Here we conduct the largest study to date of the longevity of inner dust disks using X-ray and 1--8 micrometre infrared photometry from the MYStIX and SFiNCs projects for 69 young clusters in 32 nearby star-forming regions with ages t<=5 Myr. Cluster ages are derived by combining the empirical AgeJX method with PMS evolutionary models, which treat dynamo-generated magnetic fields in different ways. Leveraging X-ray data to identify disk-free objects, we impose similar stellar mass sensitivity limits for disk-bearing and disk-free YSOs while extending the analysis to stellar masses as low as M~0.1 Mo. We find that the disk longevity estimates are strongly affected by the choice of PMS evolutionary model. Assuming a disk fraction of 100% at zero age, the inferred disk half-life changes significantly, from t1/2 ~ 1.3--2 Myr to t1/2 ~ 3.5 Myr when switching from non-magnetic to magnetic PMS models. In addition, we find no statistically significant evidence that disk fraction varies with stellar mass within the first few Myr of life for stars with masses <2 Mo, but our samples may not be complete for more massive stars. The effects of initial disk fraction and star-forming environment are also explored.
We report the discovery of maser emission in the two lowest rotational transitions of CS toward the high-mass protostar W51 e2e with ALMA and the JVLA. The masers from CS J=1-0 and J=2-1 are neither spatially nor spectrally coincident (they are separated by ~150 AU and ~30 km/s), but both appear to come from the base of the blueshifted outflow from this source. These CS masers join a growing list of rarely-detected maser transitions that may trace a unique phase in the formation of high-mass protostars.
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