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

¹ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: mbeltran@arcetri.astro.it
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁴ Kapteyn Astronomical Institute, University of Groningen, 9700 AV Groningen, The Netherlands
⁵ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD Groningen, The Netherlands
⁶ Jodrell Bank Centre for Astrophysics, The University of Manchester, Alan Turing Building, Manchester M13 9PL, UK
⁷ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Apdo. Postal 72-3 (Xangari), Morelia, Michoacán 58089, Mexico
⁸ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁹ ALLEGRO/Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ School of Physics & Astronomy, E. C. Stoner Building, The University of Leeds, Leeds LS2 9JT, UK
¹¹ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹² Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany
¹³ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762, Porto, Portugal
¹⁴ Centre for Astrophysics, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK
¹⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
¹⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany

Received: 12 February 2018
Accepted: 14 March 2018

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 toward the center. Spectral lines towardtoward 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 toward 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.