Abstract
Abstract The initial physical conditions of high-mass stars and protoclusters remain poorly characterized. To this end, we present the first targeted ALMA Band 6 1.3 mm continuum and spectral line survey toward high-mass starless clump candidates, selecting a sample of 12 of the most massive candidates ( ) within . The joint array maps have a high spatial resolution of ( , θ syn ≈ 0.″8) and have high point-source mass-completeness down to at (or column density sensitivity of ). We discover previously undetected signposts of low-luminosity star formation from and bipolar outflows and other signatures toward 11 out of 12 clumps, showing that current MIR/FIR Galactic plane surveys are incomplete to low- and intermediate-mass protostars ( ), and emphasizing the necessity of high-resolution follow-up. We compare a subset of the observed cores with a suite of radiative transfer models of starless cores. We find a high-mass starless core candidate with a model-derived mass consistent with when integrated over size scales of . Unresolved cores are poorly fit by radiative transfer models of externally heated Plummer density profiles, supporting the interpretation that they are protostellar even without detection of outflows. A high degree of fragmentation with rich substructure is observed toward 10 out of 12 clumps. We extract sources from the maps using a dendrogram to study the characteristic fragmentation length scale. Nearest neighbor separations, when corrected for projection with Monte Carlo random sampling, are consistent with being equal to the clump average thermal Jeans length ( ; i.e., separations equal to ). In the context of previous observations that, on larger scales, see separations consistent with the turbulent Jeans length or the cylindrical thermal Jeans scale ( ), our findings support a hierarchical fragmentation process, where the highest-density regions are not strongly supported against thermal gravitational fragmentation by turbulence or magnetic fields.
Highlights
High-mass stars (M* > 8 M ) strongly influence the evolution of galaxies and the ISM, yet many fundamental questions remain to be answered concerning the incipient phases of highmass star formation (e.g., Beuther et al 2007; Tan et al 2014; Motte et al 2018)
We have identified SCCs through the combined catalogs and images of primarily two dust continuum Galactic plane surveys: (1) an evolutionary analysis of Bolocam Galactic Plane Survey13 (BGPS) 1.1 mm (Svoboda et al 2016, hereafter S16), and (2) comparison of the Peretto & Fuller (2009) infrared dark cloud (IRDC) catalog with Hi-GAL images (Traficante et al 2015)
We present the first systematic observations of a large sample of well-vetted starless clump candidates with Atacama Large Millimeter/submillimeter Array (ALMA) at high resolution (~3000 au) capable of resolving the thermal Jeans length and sensitivity (50 mJy beam-1) necessary for detecting point sources down to ~0.3 M and moderately compact starless cores down to ~1.0 M )
Summary
High-mass stars (M* > 8 M ) strongly influence the evolution of galaxies and the ISM, yet many fundamental questions remain to be answered concerning the incipient phases of highmass star formation (e.g., Beuther et al 2007; Tan et al 2014; Motte et al 2018). Observational constraints on the initial physical conditions of protocluster evolution are a necessary prerequisite to improved understanding of high-mass star and cluster formation. Of particular importance are observations of the quiescent environments before the initial conditions are disrupted by the extreme radiative and mechanical feedback of high-mass stars. Our understanding of both how cluster formation is initiated and the ensuing protocluster evolution. Jansky Fellow of the National Radio Astronomy Observatory. Adjunct Astronomer of the National Radio Astronomy Observatory.
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