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

Abstract

Massive stars have a major influence on their environment yet their formation is difficult to study. W43 is a highly luminous galactic massive star forming region at a distance of 5.5 kpc and the MM1 part hosts a very massive dense core (1000 M$_{\odot}$ 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 H$_2^{18}$O counterparts, and other lines such as $^{13}$CO(10-9) and C$^{18}$O(9-8) which trace warm gas. These water lines are, with the exception of line wings, observed in absorption. Herschel SPIRE and JCMT 450 $\mu$m data have been used to make a model of the continuum emission at these 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 and the protostellar core. Velocities increase in both components from SW to NE, following the general source orientation. While the H$_2$O lines trace essentially the cool envelope, we show that the envelope cannot account for the H$_2^{18}$O absorption, which traces motions close to the protostar. The core has rapid infall, 2.9 kms, as manifested by the H$_2^{18}$O absorption features which are systematically red-shifted with respect to the $^{13}$CO(10-9) emission line which also traces the inner material with the same angular resolution. Some H$_2^{18}$O absorption is detected outside the central core and thus outside the regions expected 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 spectra results in a lower water abundance, decreased from $8 10^{-8}$ to $8 10^{-9}$ , with no change in infall rate.