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Register a new userAccelerating infall and rotational spin-up in the hot molecular core G31.41+0.31
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M. T. Beltran
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As part of our effort to search for circumstellar disks around high-mass stellar objects, we observed the well-known core G31.41+0.31 with ALMA at 1.4 mm with an angular resolution of~0.22 (~1700 au). The dust continuum emission has been resolved into two cores namely Main and NE. The Main core, which has the stronger emission and is the more chemically rich, has a diameter of ~5300 au, and is associated with two free-free continuum sources. The Main core looks featureless and homogeneous in dust continuum emission and does not present any hint of fragmentation. Each transition of CH3CN and CH3OCHO, both ground and vibrationally excited, as well as those of CH3CN isotopologues, shows a clear velocity gradient along the NE-SW direction, with velocity linearly increasing with distance from the center, consistent with solid-body rotation. However, when comparing the velocity field of transitions with different upper level energies, the rotation velocity increases with increasing energy of the transition, which suggests that the rotation speeds up towards the center. Spectral lines towards the dust continuum peak show an inverse P-Cygni profile that supports the existence of infall in the core. The infall velocity increases with the energy of the transition suggesting that the infall is accelerating towards the center of the core, consistent with gravitational collapse. Despite the monolithic appearance of the Main core, the presence of red-shifted absorption, the existence of two embedded free-free sources at the center, and the rotational spin-up are consistent with an unstable core undergoing fragmentation with infall and differential rotation due to conservation of angular momentum. Therefore, the most likely explanation for the monolithic morphology is that the large opacity of the dust emission prevents the detection of any inhomogeneity in the core.
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G31.41+0.31 is a well known chemically rich hot molecular core (HMC). Using Band 3 observations of Atacama Large Millimeter Array (ALMA), we have analyzed the chemical and physical properties of the source. We have identified methyl isocyanate (CH3NCO), a precursor of prebiotic molecules, towards the source. In addition to this, we have reported complex organic molecules (COMs) like methanol (CH3OH), methanethiol (CH3SH), and methyl formate (CH3OCHO). Additionally, we have used transitions from molecules like HCN, HCO+, SiO to trace the presence of infall and outflow signatures around the star-forming region. For the COMs, we have estimated the column densities and kinetic temperatures, assuming molecular excitation under local thermodynamic equilibrium (LTE) conditions. From the estimated kinetic temperatures of certain COMs, we found that multiple temperature components may be present in the HMC environment. Comparing the obtained molecular column densities between the existing observational results toward other HMCs, it seems that the COMs are favourably produced in the hot-core environment ($sim 100$ K or higher). Though the spectral emissions towards G31.41+0.31 are not fully resolved, we find that CH$_3$NCO and other COMs are possibly formed on the grain/ice phase and populate the gas environment similar to other hot cores like Sgr B2, Orion KL, and G10.47+0.03, etc.
We present a model aimed to reproduce the observed spectral energy distribution (SED) as well as the ammonia line emission of the G31.41+0.31 hot core. The core is modeled as an infalling envelope onto a massive star that is undergoing an intense accretion phase. We assume an envelope with a density and velocity structure resulting from the dynamical collapse of a singular logatropic sphere. The stellar and envelope physical properties are determined by fitting the SED. From these physical conditions, the ammonia line emission is calculated and compared with subarcsecond resolution VLA data of the (4,4) transition. The only free parameter in this line fitting is the ammonia abundance. The observed properties of the NH3(4,4) lines and their spatial distribution can be well reproduced provided it is taken into account the steep increase of the abundance in the hotter (> 100 K), inner regions of the core produced by the sublimation of icy mantles where ammonia molecules are trapped. The model predictions for the (2,2), (4,4), and (5,5) transitions are also in reasonably agreement with the single-dish spectra available in the literature. The best fit is obtained for a model with a star of 25 Msun, a mass accretion rate of 0.003 Msun/yr, and a total luminosity of 200,000 Lsun. The gas-phase ammonia abundance ranges from 2 times 10^{-8} in the outer region to 3 times 10^{-6} in the inner region. To our knowledge, this is the first time that the dust and molecular line data of a hot molecular core, including subarcsecond resolution data that spatially resolve the structure of the core, have been simultaneously explained by a physically self-consistent model. This modeling shows that massive protostars are able to excite high excitation ammonia transitions up to the outer edge (30,000 AU) of the large scale envelope.
Context. ALMA observations at 1.4 mm and 0.2 (750au) angular resolution of the Main core in the high-mass star forming region G31.41+0.31 have revealed a puzzling scenario: on the one hand, the continuum emission looks very homogeneous and the core appears to undergo solid-body rotation, suggesting a monolithic core stabilized by the magnetic field; on the other hand, rotation and infall speed up toward the core center, where two massive embedded free-free continuum sources have been detected, pointing to an unstable core having undergone fragmentation. Aims. To establish whether the Main core is indeed monolithic or its homogeneous appearance is due to a combination of large dust opacity and low angular resolution, we carried out millimeter observations at higher angular resolution and different wavelengths. Methods. We carried out ALMA observations at 1.4 mm and 3.5 mm that achieved angular resolutions of 0.1(375 au) and 0.075 (280 au), respectively. VLA observations at 7 mm and 1.3 cm at even higher angular resolution, 0.05 (190 au) and 0.07 (260 au), respectively, were also carried out to better study the nature of the free-free continuum sources detected in the core. Results. The millimeter continuum emission of the Main core has been clearly resolved into at least four sources, A, B, C, and D, within 1, indicating that the core is not monolithic. The deconvolved radii of the dust emission of the sources, estimated at 3.5 mm, are 400-500au, their masses range from 15 to 26 Msun, and their number densities are several 1E9 cm-3. Sources A and B, located closer to the center of the core and separated by 750 au, are clearly associated with two free-free continuum sources, likely thermal radio jets, and are the brightest in the core. The spectral energy distribution of these two sources and their masses and sizes are similar and suggest a common origin.
Context. Submillimeter Array (SMA) 870 micron polarization observations of the hot molecular core G31.41+0.31 revealed one of the clearest examples up to date of an hourglass-shaped magnetic field morphology in a high-mass star-forming region. Aims. To better establish the role that the magnetic field plays in the collapse of G31.41+0.31, we carried out Atacama Large Millimeter/submillimeter Array (ALMA) observations of the polarized dust continuum emission at 1.3 mm with an angular resolution four times higher than that of the previous (sub)millimeter observations to achieve an unprecedented image of the magnetic field morphology. Methods. We used ALMA to perform full polarization observations at 233 GHz (Band 6). The resulting synthesized beam is 0.28x020 which, at the distance of the source, corresponds to a spatial resolution of ~875 au. Results. The observations resolve the structure of the magnetic field in G31.41+0.31 and allow us to study the field in detail. The polarized emission in the Main core of G31.41+0.41is successfully fit with a semi-analytical magnetostatic model of a toroid supported by magnetic fields. The best fit model suggests that the magnetic field is well represented by a poloidal field with a possible contribution of a toroidal component of ~10% of the poloidal component, oriented southeast to northwest at ~ -44 deg and with an inclination of ~-45 degr. The magnetic field is oriented perpendicular to the northeast to southwest velocity gradient detected in this core on scales from 1E3-1E4 au. This supports the hypothesis that the velocity gradient is due to rotation and suggests that such a rotation has little effect on the magnetic field. The strength of the magnetic field estimated in the central region of the core with the Davis-Chandrasekhar-Fermi method is ~8-13 mG and implies that the mass-to-flux ratio in this region is slightly supercritical ...
Understanding the degree of chemical complexity that can be reached in star-forming regions, together with the identification of precursors of the building blocks of life in the interstellar medium, is one of the goals of astrochemistry. Unbiased spectral surveys with large bandwidth and high spectral resolution are thus needed, to resolve line blending in chemically rich sources and identify complex organic molecules. This kind of observations has been successfully carried out, mainly towards the Galactic Center, a region that shows peculiar environmental conditions. We present an unbiased spectral survey at 3mm of one of the most chemically rich hot molecular cores located outside the Galactic Center, in the high-mass star-forming region G31.41+0.31. In this first paper, we present the survey and discuss the detection of the 3 isomers of C$_{2}$H$_{4}$O$_{2}$: methyl formate, glycolaldehyde and acetic acid. Observations were carried out with ALMA and cover the entire Band 3 from 84 to 116 GHz with an angular resolution of $1.2^{}$x$1.2^{}$ and a spectral resolution of $sim0.488$ MHz. The transitions of the 3 molecules have been analyzed with the software XCLASS. All three isomers were detected and methyl formate and acetic acid abundances in G31 are the highest detected up to now, if compared to sources in literature. The size of the emission varies among the three isomers with acetic acid showing the most compact emission while methyl formate the most extended. The comparison with chemical models suggests the necessity of grain-surface routes for the formation of methyl formate in G31, while for glycolaldehyde both scenarios could be feasible. Proposed grain-surface reaction for acetic acid is not able to reproduce the observed abundance in this work, while gas-phase scenario should be further tested due to large uncertainties.
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