Probing the initial conditions of high-mass star formation

II. Fragmentation, stability, and chemistry towards high-mass star-forming regions G29.96−0.02 and G35.20−1.74 ^⋆,⋆⋆

¹ California Institute Of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
e-mail: tpillai@astro.caltech.edu
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³ Initiative in Innovative Computing (IIC), Harvard University, 60 Oxford Street, Cambridge, MA 02138, USA
⁴ Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ University of Exeter, UK
⁷ Department of Physics & Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
⁸ Centre for Astrophysics Research, Science & Technology Research Institute, University of Hertfordshire, College Lane, Hatfield, AL10 9AB, UK

Received: 7 October 2010
Accepted: 31 March 2011

Abstract

Most work on high-mass star formation has focused on observations of young massive stars in protoclusters. Very little is known about the preceding stage. Here, we present a new high-resolution study of pre-protocluster regions in tracers exclusively probing the coldest and dense gas (NH₂D). The two target regions G29.96−0.02 and G35.20−1.74 (W48) are drawn from the SCAMPS project, which searches for pre-protoclusters near known ultracompact Hii regions. We used our data to constrain the chemical, thermal, kinematic, and physical conditions (i.e., densities) in G29.96e and G35.20w. NH₃, NH₂D, HCO ⁺ , and continuum emission were mapped using the VLA, PdBI, and BIMA. In particular, NH₂D is a unique tracer of cold, precluster gas at high densities, while NH₃ 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). Dust and line observations reveal fragmentation into multiple massive cores strung along filamentary structures. Most of these are cold (<20   K), dense (>10⁵   cm^-3) and highly deuterated ([NH₂D/NH₃]  > 6%). In particular, we observe very low line widths in NH₂D (FWHM ≲ 1   km   s^-1). These are very narrow lines that are unexpected towards a region forming massive stars. Only one core in the center of each filament appears to be forming massive stars (identified by the presence of masers and massive outflows); however, it appears that only a few such stars are currently forming (i.e., just a single Spitzer source per region). 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 ([NH₂D/NH₃]  > 6%), which make the above discoveries possible; (vi) and the destruction of NH₂D toward embedded stars.