Probing the initial conditions of high-mass star formation

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 (NH 2 D). 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. NH 3 , NH 2 D, HCO + , and continuum emission were mapped using the VLA, PdBI, and BIMA. In particular, NH 2 D 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). Dust and line observations reveal fragmentation into multiple massive cores strung along filamentary structures. Most of these are cold (<20 K), dense (>10 5 cm −3 ) and highly deuterated ([NH 2 D/NH 3 ] > 6%). In particular, we observe very low line widths in NH 2 D (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 ([NH 2 D/NH 3 ] > 6%), which make the above discoveries possible; (vi) and the destruction of NH 2 D toward embedded stars.

A Hi-GAL study of the high-mass star-forming region G29.96–0.02

“…Only the brightest millimeter continuum core shows signs of high-mass star-formation activity, as indicated by the point source already visible at 24 μm that is driving a molecular outflow. That no active star formation has been detected in other parts of this IRDC (Pillai et al 2011) supports the idea of this extincted filament being in a very early evolutionary phase.…”

“…6,4.5,5.8,8.0,and 24 μm and Herschel 70,160,250,350, and 500 μm images in linear scale of the G29-SFR cloud. The white arrow in the 8.0 μm image points to the filamentary IRDC (Pillai et al 2011). in a very early stage of evolution, and could be in a pre-cluster phase. Only the brightest millimeter continuum core shows signs of high-mass star-formation activity, as indicated by the point source already visible at 24 μm that is driving a molecular outflow.…”

Dense molecular cocoons in the massive protocluster W3 IRS5: a test case for models of massive star formation

“…Both theories are supported by observations (e.g. Cesaroni et al 1997;Pillai et al 2011), suggesting that there may be two different modes of massive star formation (Krumholz & Bonnell 2009). However, it is challenging to observe massive star-forming regions since they are at large distances (a few kpc), requiring high-angular resolution to resolve the highly embedded complex structures in which gravitational fragmentation, powerful outflows and stellar winds, and ionizing radiation fields play influence the starformation processes (Beuther et al 2007).…”

Deuteration and evolution in the massive star formation process

“…However, while by neglecting the beam dilution the absolute values of the column densities can certainly be affected, the evolutionary trend of the column density ratio should not be affected by this assumption because the beam dilution is expected to be almost constant, and it should thus introduce only a systematic correction (see also Paper I). Also, observations at high angular resolution towards massive star forming regions Pillai et al 2011) indicate that the emission of NH 2 D can be as extended as that of NH 3 , despite the different critical density. For example, the emitting regions of NH 2 D(1 1,1 −1 0,1 ) and NH 3 (1,1) in I20293-WC and I20293-MM1, both included in our survey, are approximately the same .…”

“…Growing observational evidence suggests that high values of D frac are also typical in high-mass star-forming cores (e.g. Fontani et al 2006;Pillai et al 2007Pillai et al , 2011Miettinen et al 2011), and that D frac of some species could be also an evolutionary indicator in the intermediate-and high-mass regime (e.g. Fontani et al 2011;Sakai et al 2012).…”