Kinematics and stability of high-mass protostellar disk candidates at sub-arcsecond resolution

Insights from the IRAM NOEMA large programme CORE^★,★★

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: aahmadi@strw.leidenuniv.nl
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Max Planck Institute for Extraterrestrial Physics, Gießenbachstraße 1, 85749 Garching bei München, Germany
⁴ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17 (Sternwarte) 1180 Wien, Austria
⁵ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
⁶ Institut de Ciències de l’Espai (ICE, CSIC), Can Magrans s/n, 08193, Bellaterra, Barcelona, Spain
⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Barcelona, Spain
⁸ Observatorio Astronómico Nacional (OAN, IGN), Calle Alfonso XII 3, 28014, Madrid, Spain
⁹ I. Physikalisches Institut der Universität zu Köln, Zülpicher Str. 77, 50937, Köln, Germany
¹⁰ Dept. of Physics and Astronomy, McMaster University, 1280 Main Street West, Hamilton, ON L8S 4K1, Canada
¹¹ Department of Chemistry, Ludwig Maximilian University, Butenandtstr. 5-13, 81377 Munich, Germany
¹² IRAM, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint-Martin-d’Hères, France
¹³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁴ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
¹⁵ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701, Ex-Hda. San José de la Huerta, 58089, Morelia, Michoacán, Mexico
¹⁶ School of Physics and Astronomy, The University of Leeds, Sir William Henry Bragg Building, Woodhouse Lane, Leeds, LS2 9JT, UK
¹⁷ Center for Astrophysics, Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02420, USA
¹⁸ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinbugh EH9 3HJ, UK
¹⁹ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047, Selargius (CA), Italy
²⁰ Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK
²¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching bei München, Germany
²² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
²³ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, UK
²⁴ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, CT2 7NH, UK
²⁵ Universidad Autonoma de Chile, Nucleo de Astroquimica y Astrofisica, 425 Avda Pedro de Valdivia, Providencia, Santiago de Chile, Chile

Received: 29 November 2022
Accepted: 24 April 2023

Abstract

Context. The fragmentation mode of high-mass molecular clumps and the accretion processes that form the most massive stars (M ≳ 8 M_⊙) are still not well understood. A growing number of case studies have found massive young stellar objects (MYSOs) to harbour disk-like structures, painting a picture that the formation of high-mass stars may proceed through disk accretion, similar to that of lower-mass stars. However, the properties of such structures have yet to be uniformly and systematically characterised.

Aims. The aim of this work is to uniformly study the kinematic properties of a large sample of MYSOs and characterise the stability of possible circumstellar disks against gravitational fragmentation.

Methods. We have undertaken a large observational programme (CORE) making use of interferometric observations from the Northern Extended Millimetre Array (NOEMA) for a sample of 20 luminous (L > 10⁴ L_⊙) protostellar objects in the 1.37 mm wavelength regime in both continuum and spectral line emission, reaching 0.4″ resolution (800 au at 2 kpc).

Results. We present the gas kinematics of the full sample and detect dense gas emission surrounding 15 regions within the CORE sample. Using the dense gas tracer CH₃CN, we find velocity gradients across 13 cores perpendicular to the directions of bipolar molecular outflows, making them excellent disk candidates. The extent of the CH₃CN emission tracing the disk candidates varies from 1800 to 8500 au. Analysing the free-fall to rotational timescales, we find that the sources are rotationally supported. The rotation profiles of some disk candidates are well described by differential rotation while for others the profiles are poorly resolved. Fitting the velocity profiles with a Keplerian model, we find protostellar masses in the range of ~ 10–25 M_⊙. Modelling the level population of CH₃CN (12_K–11_K) K = 0–6 lines, we present temperature maps and find median temperature in the range 70–210 K with a diversity in distributions. Radial profiles of the specific angular momentum (j) for the best disk candidates span a range of 1–2 orders of magnitude, on average ~10⁻³ km s⁻¹ pc, and they follow j ∝ r^1.7, which is consistent with a poorly resolved rotating and infalling envelope-disk model. Studying the Toomre stability of the disk candidates, we find almost all (11 out of 13) disk candidates to be prone to fragmentation due to gravitational instabilities at the scales probed by our observations, as a result of their high disk to stellar mass ratio. In particular, disks with masses greater than ~ 10–20% of the mass of their host (proto)stars are Toomre unstable, and more luminous YSOs tend to have disks that are more massive compared to their host star and hence more prone to fragmentation.

Conclusions. In this work, we show that most disk structures around high-mass YSOs are prone to disk fragmentation early in their formation due to their high disk to stellar mass ratio. This impacts the accretion evolution of high-mass protostars which will have significant implications for the formation of the most massive stars.