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Register a new userALMA View of the Infalling Envelope around a Massive Protostar in S255IR SMA1
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The massive young stellar object S255IR NIRS3 embedded in the star forming core SMA1 has been recently observed with a luminosity burst, which is conjectured as a disc-mediated variable accretion event. In this context, it is imperative to characterize the gas properties around the massive young stellar object. With this in mind, we carried out high angular resolution observations with the Atacama Large Millimeter and submillimeter Array and imaged the 900 $mu m$ dust continuum and the CH$_3$CN $J$=19$-$18 $K$=0$-$10 transitions of S255IR SMA1. The integrated CH$_3$CN emission exhibits an elongated feature with an extent of 1800 au in the northwest-southeast direction at a position angle of 165 degree, which is nearly perpendicular to the bipolar outflow. We confirm the presence of dense (a few $times 10^{9}$ cm$^{-3}$) and hot ($sim$ 400 K) gas immediately surrounding the central protostar. The CH$_3$CN emission features a velocity gradient along the elongated ridge and by modelling the gas kinematics based on features in the position-velocity diagram, we infer that the gas is best described by a flattened rotating infalling envelope (or pseudo-disc). A mass infall rate of a few $times$ 10$^{-4}$ solar-mass per year is derived. If there exists a putative Keplerian disc directly involved in the mass accretion onto the star and jet/outflow launching, it is likely smaller than 125 au and unresolved by our observations. We show qualitative resemblances between the gas properties (such as density and kinematics) in 255IR SMA1 inferred from our observations and those in a numerical simulation particularly tailored for studying the burst mode of massive star formation.
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Sub-arcsecond images of the rotational line emission of CS and SO have been obtained toward the Class I protostar IRAS 04365$+$2535 in TMC-1A with ALMA. A compact component around the protostar is clearly detected in the CS and SO emission. The velocity structure of the compact component of CS reveals infalling-rotating motion conserving the angular momentum. It is well explained by a ballistic model of an infalling-rotating envelope with the radius of the centrifugal barrier (a half of the centrifugal radius) of 50 AU, although the distribution of the infalling gas is asymmetric around the protostar. The distribution of SO is mostly concentrated around the radius of the centrifugal barrier of the simple model. Thus a drastic change in chemical composition of the gas infalling onto the protostar is found to occur at a 50 AU scale probably due to accretion shocks, demonstrating that the infalling material is significantly processed before being delivered into the disk.
Temporal photometric variations at near infrared to submillimeter wavelengths have been found in low-mass young stellar objects. These phenomena are generally interpreted as accretion events of star-disk systems with varying accretion rates. There is growing evidence suggesting that similar luminosity flaring also occurs in high-mass star/cluster-forming regions. We report in this Letter the rise and fall of the 900 ${mu}$m continuum emission and the newly found 349.1 GHz methanol maser emission in the massive star forming region S255IR~SMA1 observed with the Submillimeter Array and the Atacama Large Millimeter/submillimeter Array. The level of flux variation at a factor of $sim$ 2 at the submillimeter band and the relatively short 2-year duration of this burst suggest that the event is probably similar to those milder and more frequent minor bursts seen in 3D numerical simulations.
Context: The protostellar envelopes, outflow and large-scale chemistry of Class~0 and Class~I objects have been well-studied, but while previous works have hinted at or found a few Keplerian disks at the Class~0 stage, it remains to be seen if their presence in this early stage is the norm. Likewise, while complex organics have been detected toward some Class~0 objects, their distribution is unknown as they could reside in the hottest parts of the envelope, in the emerging disk itself or in other components of the protostellar system, such as shocked regions related to outflows. Aims: In this work, we aim to address two related issues regarding protostars: when rotationally supported disks form around deeply embedded protostars and where complex organic molecules reside in such objects. Methods: We observed the deeply embedded protostar, L483, using Atacama Large Millimeter/submillimeter Array (ALMA) Band~7 data from Cycles~1 and 3 with a high angular resolution down to $sim$~0.1$^{primeprime}$ (20~au) scales. Results: We find that the kinematics of CS~$J=7$--$6$ and H$^{13}$CN~$J=4$--$3$ are best fitted by the velocity profile from infall under conservation of angular momentum and not by a Keplerian profile. The spatial extents of the observed complex organics are consistent with an estimated ice sublimation radius of the envelope at $sim$~50~au, suggesting that the complex organics exist in the hot corino of L483. Conclusions: We find that L483 does not harbor a Keplerian disk down to at least $15$~au in radius. Instead, the innermost regions of L483 are undergoing a rotating collapse. This result highlights that some Class~0 objects contain only very small disks, or none at all, with the complex organic chemistry taking place on scales inside the hot corino of the envelope, in a region larger than the emerging disk.
We present new spectral line observations of the CH3CN molecule in the accretion disk around the massive protostar IRAS 20126+4104 with the Submillimeter Array that for the first time measure the disk density, temperature, and rotational velocity with sufficient resolution (0.37, equivalent to ~600 AU) to assess the gravitational stability of the disk through the Toomre-Q parameter. Our observations resolve the central 2000 AU region that shows steeper velocity gradients with increasing upper state energy, indicating an increase in the rotational velocity of the hotter gas nearer the star. Such spin-up motions are characteristics of an accretion flow in a rotationally supported disk. We compare the observed data with synthetic image cubes produced by three-dimensional radiative transfer models describing a thin flared disk in Keplerian motion enveloped within the centrifugal radius of an angular-momentum-conserving accretion flow. Given a luminosity of 1.3x10^4 Lsun, the optimized model gives a disk mass of 1.5 Msun and a radius of 858 AU rotating about a 12.0 Msun protostar with a disk mass accretion rate of 3.9x10^{-5} Msun/yr. Our study finds that, in contrast to some theoretical expectations, the disk is hot and stable to fragmentation with Q > 2.8 at all radii which permits a smooth accretion flow. These results put forward the first constraints on gravitational instabilities in massive protostellar disks, which are closely connected to the formation of companion stars and planetary systems by fragmentation.
We present preliminary results of the high resolution $ (0.10^{primeprime} times 0.15^{primeprime}) $ observations of the high mass star forming region S255IR with ALMA in several spectral windows from ~ 335 GHz to ~ 350 GHz. The main target lines were C$^{34}$S(7-6), CH$_3$CN($19_K-18_K$), CO(3-2) and SiO(8-7), however many other lines of various molecules have been detected, too. We present sample spectra and maps, discuss briefly the source structure and kinematics. A new, never predicted methanol maser line has been discovered.
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