Accelerating infall and rotational spin-up in the hot molecular core G31.41+0.31

Abstract

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.

From clump to disc scales in W3 IRS4

“…13 CN results in a slightly worse fit, but the difference is not large enough to be conclusive as to whether this is true for all carbon-bearing species in the hot core. Low values of C/ 13 C compared to those expected for the host cloud have recently been observed in complex species in two other hot core regions observed at high resolutions, IRAS 20126+4104 (∼30 in CH 3 OH and CH 3 CN compared to an expected ratio of ∼60-70, Palau et al 2017) and G31.41+0.31 ( 10 in CH 3 CN and CH 3 OCHO compared to an expected ratio of ∼47, Beltrán et al 2018). Such low values may therefore be a feature of hot cores.…”

“…For example, Palau et al (2017) found similar abundances for SO, HNCO, H 13 2 CO, HCOOH, and CH 3 CN towards IRAS 20126+4104. However, the values for H 2 CCO, CH 3 OH, CH 3 OCHO, and CH 3 COCH 3 are approximately an order of magnitude higher in IRAS 20126 (Palau et al 2017), and both CH 3 CN and CH 3 OCHO are more than one order of magnitude higher in G31.41+0.31 (Beltrán et al 2018) than in IRS4, suggesting some difference in conditions or age between these sources.…”

“…The results for the fits for the inner, outer and whole region are given in Table 7, with the power-law exponent q for the whole region being 0.91±0.05. Values of 0.34−0.40 have been found for massive dense cores (for example Palau et al 2014), 0.4-0.5 is typically found from hydrostatic radiative transfer modelling of the envelopes and discs of embedded YSOs (Chiang & Goldreich 1997;Beuther et al 2007;Persson et al 2016), observations of more evolved T-Tauri discs result in q = 0.4−0.75 (Andrews & Williams 2005), while recent ALMA results for the hot core source G31.41 found a value of 0.77 (Beltrán et al 2018). Notes.…”

The evolution of temperature and density structures of OB cluster-forming molecular clumps

“…Although we have verified the high level of dense gas of this source compared with the rest of the sample by full radiative transfer modeling of CH 3 OH lines (Appendix F), we caution that in this density regime the CH 3 OH (5-4) lines become heavily optically thick and the critical density for the considered line transitions is reached (e.g., n > n crit , Table 3), such that the relative differences between the level populations do not serve as ideal densitometers anymore. Nonetheless, we can safely argue that the hot massive core G31 has much higher gas densities in its inner region than other sources, which is also reflected by the very monolithic nature of its central core from higher angular resolution observations (∼2000 au, see Beltrán et al 2018). For source G10, there are prominently higher gas densities at extended radii of 0.1-0.4 pc than for other sources, which is related to the presence of a large disk-like flattened structure.…”

ALMA observations of the Extended Green Object G19.01−0.03 – I. A Keplerian disc in a massive protostellar system

“…An enclosed stellar mass of ∼33−65 M corresponding to a single MYSO would place G19.01−0.03 MM1 among the most massive proto-O star candidates discovered to date. Of the handful of sources with reported central stellar masses >30 M (AFGL 2591-VLA3, G11.92−0.61 MM1, G17.64+0.16, G31.41+0.31: Jiménez-Serra et al 2012;Ilee et al 2018;Maud et al 2019;Beltrán et al 2018, respectively; see also Johnston et al 2020), only G17.64+0.16 (AFGL 2136) and AFGL 2591-VLA3 have central stellar masses ≥40 M and both have luminosities ≥10 5 L . Theoretical models of accreting protostars also predict L>10 5 L for M * >30 M (e.g.…”