Probing the initial conditions of high-mass star formation. II. Fragmentation, stability, and chemistry

TL;DR

This study uses high-resolution observations of dense, cold gas tracers in high-mass star-forming regions to investigate fragmentation, stability, and chemical conditions, challenging some cloud collapse models and highlighting accretion processes.

Contribution

It provides new insights into the physical and chemical conditions of pre-protocluster regions, emphasizing turbulence, core stability, and high deuteration ratios in high-mass star formation.

Findings

Turbulent Jeans fragmentation of massive clumps into cores.

Cores are 'over-bound/subvirial', indicating collapse.

High deuteration ratios (>6%) observed in NH_2D/NH_3].

Abstract

We present a new high-resolution study of pre-protocluster regions in tracers exclusively probing the coldest and dense gas (NH_2D). The data are used to constrain the chemical, thermal, kinematic, and physical conditions (i.e., densities) in G29.96e and G35.20w. NH_3, NH_2D, and continuum emission were mapped using the VLA, and PdBI. In particular, NH_2D is a unique tracer of cold, precluster gas at high densities, while NH_3 traces both the cold and warm gas of modest-to-high densities. In G29.96e, Spitzer images reveal two massive filaments, one of them in extinction (infrared dark cloud). We observe very low line widths in NH_3 (FWHM <1km/s). These multi-wavelength, high-resolution observations of high-mass pre-protocluster regions show that the target regions are characterized by (i) turbulent Jeans fragmentation of massive clumps into cores (from a Jeans analysis); (ii) cores and clumps that are "over-bound/subvirial", i.e. turbulence is too weak to support them against collapse, meaning that (iii) some models of monolithic cloud collapse are quantitatively inconsistent with data; (iv) accretion from the core onto a massive star, which can (for observed core sizes and velocities) be sustained by accretion of envelope material onto the core, suggesting that (similar to competitive accretion scenarios) the mass reservoir for star formation is not necessarily limited to the natal core; (v) high deuteration ratios ([NH_2D/NH_3]>6%), which make the above discoveries possible; (vi) and the destruction of NH_2D toward embedded stars. [abridged]