Aims: We present Herschel-HIFI observations of 14 water lines in W43-MM1, a massive protostellar object in the luminous star-cluster-forming region W43. We place our study in the more general context of high-mass star formation. The dynamics of these regions may be represented by either the monolithic collapse of a turbulent core, or competitive accretion. Water turns out to be a particularly good tracer of the structure and kinematics of the inner regions, allowing an improved description of the physical structure of the massive protostar W43-MM1 and an estimation of the amount of water around it.
Methods: We analyze the gas dynamics from the line profiles using Herschel-HIFI observations acquired as part of the Water In Star-forming regions with Herschel project of 14 far-IR water lines (H216O, H217O, H218O), CS(11-10), and C18O(9-8) lines, using our modeling of the continuum spectral energy distribution. The spectral modeling tools allow us to estimate outflow, infall, and turbulent velocities and molecular abundances. We compare our results to previous studies of low-, intermediate-, and other high-mass objects.
Results: As for lower-mass protostellar objects, the molecular line profiles are a mix of emission and absorption, and can be decomposed into “medium” (full width at half maximum FWHM ≃ 5-10 km s-1), and “broad” velocity components (FWHM ≃ 20-35 km s-1). The broad component is the outflow associated with protostars of all masses. Our modeling shows that the remainder of the water profiles can be well-fitted by an infalling and passively heated envelope, with highly supersonic turbulence varying from 2.2 km s-1 in the inner region to 3.5 km s-1 in the outer envelope. In addition, W43-MM1 has a high accretion rate of between 4.0 × 10-4 and 4.0 × 10-2 M⊙ yr-1, as derived from the fast (0.4-2.9 km s-1) infall observed. We estimate a lower mass limit for gaseous water of 0.11 M⊙ and total water luminosity of 1.5 L⊙ (in the 14 lines presented here). The central hot core is detected with a water abundance of 1.4 × 10-4, while the water abundance for the outer envelope is 8 × 10-8. The latter value is higher than in other sources, and most likely related to the high turbulence and the micro-shocks created by its dissipation.
Conclusions: Examining the water lines of various energies, we find that the turbulent velocity increases with the distance from the center. While not in clear disagreement with the competitive accretion scenario, this behavior is predicted by the turbulent core model. Moreover, the estimated accretion rate is high enough to overcome the expected radiation pressure.
