Toward gas exhaustion in the W51 high-mass protoclusters^⋆,⋆⋆

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
e-mail: Adam.Ginsburg@eso.org
² National Radio Astronomy Observatory, Socorro, NM 87801, USA
³ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁴ ALLEGRO/Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Instituto de Radioastronomía y Astrofísica, UNAM, A.P. 3-72, Xangari, 58089 Morelia, Mexico
⁶ University Observatory/Excellence Cluster “Universe” Scheinerstraße 1, 81679 München, Germany
⁷ CASA, University of Colorado, 389-UCB, Boulder, CO 80309, USA
⁸ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁹ LASP, University of Colorado, 600 UCB, Boulder, CO 80309, USA
¹⁰ SOFIA Science Center, NASA Ames Research Center, M/S 232-12, Moffett Field, CA 94035, USA
¹¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹² Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
¹³ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, 69120 Heidelberg, Germany

Received: 15 February 2016
Accepted: 22 June 2016

Abstract

We present new JVLA observations of the high-mass cluster-forming region W51A from 2 to 16 GHz with resolution θ_fwhm ≈ 0.3−0.5″. The data reveal a wealth of observational results: (1) Currently forming, very massive (proto-O) stars are traced by o - H₂CO2_1,1−2_1,2 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 H ii 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 ≲0.05 pc. 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.