ALMA Observations of Fragmentation, Substructure, and Protostars in High-mass Starless Clump Candidates

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 towards high-mass starless clump candidates, selecting a sample of 12 of the most massive candidates (4 × 10 2 M M cl 4 × 10 3 M ) within d < 5 kpc. The joint 12 + 7 m array maps have a high spatial resolution of 3000 au (0.015 pc, θ syn ≈ 0. 8) and have high point source mass-completeness down to M ≈ 0.3 M at 6σ rms (or 1σ rms column density sensitivity of N = 1.1 × 10 22 cm −2 ). We discover previously undetected signposts of low-luminosity star formation from CO J = 2 → 1 and SiO J = 5 → 4 bipolar outflows towards 11 out of 12 clumps, showing that current MIR/FIR Galactic Plane surveys are incomplete to low-and intermediate-mass protostars (L bol 50 L ), and emphasizing the necessity of high-resolution followup. 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 29 52 15 M when integrated over size scales of R < 2 × 10 4 au. Unresolved cores are poorly fit by radiative transfer models of externally heated Plummer density profiles, supporting the interpretation they are protostellar even without detection of outflows. A high degree of fragmentation with rich sub-structure is observed towards 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 (λ j,th ; i.e., separations equal to 0.4 − 1.6 × λ j,th ). In context of previous observations that on larger scales see separations consistent with the turbulent Jeans length or the cylindrical thermal Jeans scale (≈ 3 − 4 × λ j,th ), 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.Corresponding author: Brian E. Svoboda bsvoboda@nrao.edu * Jansky Fellow of the National Radio Astronomy Observatory † Adjunct Astronomer of the National Radio Astronomy Observatory essary 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. Thus our understanding of both how cluster formation is initiated and the ensuing protocluster evolution depend on identifying and constraining

The accretion history of high-mass stars: an ArTéMiS pilot study of infrared dark clouds

“…Also, when observed with ALMA at high-angular resolution, singledish starless sources observed in high-mass star-forming regions systematically fragment into a set of low-mass protostellar cores MNRAS 496, 3482-3501 (2020) (e.g. Svoboda et al 2019). This shows that the classification of sources as starless based on single-dish continuum observations (e.g.…”

“…Also, nearly all high-angular resolution (≤1 ) observations of similar sources show sub-fragmentation (e.g. Louvet et al 2019;Sanhueza et al 2019;Svoboda et al 2019) casting doubts on the true physical meaning of the identified ArTéMiS compact sources. Our approach here is more generic: we compute the mass within the ArTéMiS beam solid angle at the location of the peak flux density of every identified leaf.…”

“…High-angular resolution observations on 1000 AU scale of massive 0.1-pc-sized sources, both starless and protostellar, have indeed revealed sub-fragmentation (e.g. Bontemps et al 2010;Palau et al 2013;Sanhueza et al 2019;Svoboda et al 2019). There is, however, no evidence that starless sources are less fragmented than protostellar ones, and if anything, these studies show the opposite.…”

“…In fact, the search for compact high-mass prestellar cores with ALMA systematically reveals that reasonable candidates identified with single-dish telescopes are systematically sub-fragmented into low-mass cores (e.g. Louvet et al 2019;Sanhueza et al 2019;Svoboda et al 2019). Therefore, the formation of massive stars requires additional physics.…”

Fragmentation and kinematics in high-mass star formation

“…However, at least the latter two examples are at comparably large distances of 3.4 and 4.9 kpc, respectively, reducing the ability to investigate structures at small physical spatial scales. Furthermore, recent investigations with the Atacama Large Millimeter Array (ALMA) and the Submillimeter Array (SMA) also started to further dissect samples of very young high-mass star-forming regions (e.g., Sanhueza et al 2019;Svoboda et al 2019;Li et al 2019Li et al , 2020a.…”