Hierarchical fragmentation and collapse signatures in a high-mass starless region^⋆,⋆⋆

¹ Max-Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: name@mpia.de
² University of Leeds, Leeds, LS2 9JT, UK
³ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
⁴ Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, 182-8585 Tokyo, Japan
⁵ Institute of Astronomy and Astrophysics, University of Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany

Received: 16 June 2015
Accepted: 27 July 2015

Abstract

Aims. We study the fragmentation and collapse properties of the dense gas during the onset of high-mass star formation.

Methods. We observed the massive (~800 M_⊙) starless gas clump IRDC 18310-4 with the Plateau de Bure Interferometer (PdBI) at subarcsecond resolution in the 1.07 mm continuum and N₂H⁺(3–2) line emission.

Results. Zooming from a single-dish low-resolution map to previous 3 mm PdBI data, and now the new 1.07 mm continuum observations, the substructures hierarchically fragment on the increasingly smaller spatial scales. While the fragment separations may still be roughly consistent with pure thermal Jeans fragmentation, the derived core masses are almost two orders of magnitude larger than the typical Jeans mass at the given densities and temperatures. However, the data can be reconciled with models using non-homogeneous initial density structures, turbulence, and/or magnetic fields. While most subcores remain (far-)infrared dark even at 70 μm, we identify weak 70 μm emission toward one core with a comparably low luminosity of ~16 L_⊙, supporting the notion of the general youth of the region. The spectral line data always exhibit multiple spectral components toward each core with comparably small line widths for the individual components (in the 0.3 to 1.0 km s^-1 regime). Based on single-dish C¹⁸O(2–1) data we estimate a low virial-to-gas-mass ratio ≤ 0.25. We propose that the likely origin of these spectral properties may be the global collapse of the original gas clump that results in multiple spectral components along each line of sight. Even within this dynamic picture the individual collapsing gas cores appear to have very low levels of internal turbulence.