Structure and kinematics of the clouds surrounding the Galactic mini-starburst W43 MM1

¹ Laboratoire d’Astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, Allée Geoffroy Saint-Hilaire, 33615 Pessac, France
e-mail: thierry.jacq@u-bordeaux.fr
² SRON Netherlands Institute for Space Research, PO Box 800, 9700AV Groningen, The Netherlands
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Kapteyn Astronomical Institute, University of Groningen, 9712 CP Groningen, The Netherlands

Received: 6 April 2016
Accepted: 28 July 2016

Abstract

Massive stars have a major influence on their environment, yet their formation is difficult to study as they form quickly in highly obscured regions and are rare, hence more distant than lower mass stars. Westerhout 43 (W43) is a highly luminous galactic massive star-forming region at a distance of 5.5 kpc and the MM1 part hosts a particularly massive dense core (1000 M_⊙ within 0.05 pc). We present new Herschel HIFI maps of the W43 MM1 region covering the main low-energy water lines at 557, 987, and 1113 GHz; their H₂¹⁸O counterparts; and other lines such as ¹³CO (10–9) and C¹⁸O (9–8), which trace warm gas. These water lines are, with the exception of line wings, observed in absorption. Herschel SPIRE and JCMT 450 μm data have been used to make a model of the continuum emission at the HIFI wavelengths. Analysis of the maps, and in particular the optical depth maps of each line and feature, shows that a velocity gradient, possibly due to rotation, is present in both the envelope (r ≳ 0.5 pc) and the protostellar core (r ≲ 0.2 pc). Velocities increase in both components from SW to NE, following the general source orientation. While the H₂O lines trace essentially the cool envelope, we show that the envelope cannot account for the H₂¹⁸O absorption, which traces motions close to the protostar. The core has rapid infall, 2.9 km s^-1, as manifested by the H₂¹⁸O absorption features which are systematically redshifted with respect to the ¹³CO (10–9) emission line which also traces the inner material with the same angular resolution. Some H₂¹⁸O absorption is detected outside the central core and thus outside the regions expected (from a spherical model) to be above 100 K; we attribute this to warm gas associated with the other massive dense cores in W43 MM1. Using the maps to identify absorption from cool gas on large scales, we subtract this component to model spectra for the inner envelope. Modeling the new, presumably corrected, spectra results in a lower water abundance, decreased from 8 × 10^-8 to 8 × 10^-9, with no change in infall rate.