JVLA images of W51A

Abstract

We used the Karl G. Jansky Very Large Array (JVLA) in multiple bands and configurations. In project 12B-365, we observed in A-array in S and C bands (3 and 5GHz, approximately) with 2GHz total bandwidth. In project 13A-064, we observed in C-band in C (1h) and A (5h) arrays and in Ku-band in D (1h) and B (5h) arrays. We also include continuum data from project 12A-274 (Goddi et al., 2015A&A...573A.109G, 2016, Cat. J/A+A/589/A44) covering the range 25-36GHz using the JVLA B array configuration. Our spectral coverage included o-H<SUB>2</SUB>CO 1<SUB>1,0</SUB>1-1,1_ at 4.82966GHz and 2<SUB>1,1</SUB>-2<SUB>1,2</SUB> at 14.488GHz with 0.3km/s resolution and the radio recombination lines (RRLs) H77α (14.1286GHz) and H110α (4.8741GHz) at 1km/s resolution. The H110α line had lower S/N than the H77α line but was otherwise similar; it is not discussed further in this paper. (3 data files). We present new JVLA observations of the high-mass cluster-forming region W51A from 2 to 16 GHz with resolution θfwhm~=0.3-0.5''. The data reveal a wealth of observational results: (1) Currently forming, very massive (proto-O) stars are traced by o-H2CO21,1-21,2 emission, suggesting that this line can be used efficiently as a massive protostar tracer; (2) there is a spatially distributed population of ≲mJy continuum sources, including hypercompact HII regions and candidate colliding wind binaries, in and around the W51 proto-clusters; and (3) there are two clearly detected protoclusters, W51e and W51 IRS2, that are gas-rich but may have most of their mass in stars within their inner ≤0.05pc. The majority of the bolometric luminosity in W51 most likely comes from a third population of OB stars between these clusters. The presence of a substantial population of exposed O-stars coincident with a population of still-forming massive stars, together with a direct measurement of the low mass loss rate via ionized gas outflow from W51 IRS2, implies that feedback is ineffective at halting star formation in massive protoclusters. Instead, feedback may shut off the large-scale accretion of diffuse gas onto the W51 protoclusters, implying that they are evolving toward a state of gas exhaustion rather than gas expulsion. Recent theoretical models predict gas exhaustion to be a necessary step in the formation of gravitationally bound stellar clusters, and our results provide an observational validation of this process.