2014A&A...566A..61M

In the framework of the Herschel/PRISMAS guaranteed time key program, the line of sight to the distant ultracompact HII region W51e2 has been observed using several selected molecular species. Most of the detected absorption features are not associated with the background high-mass star-forming region and probe the diffuse matter along the line of sight. We present here the detection of an additional narrow absorption feature at ∼70km/s in the observed spectra of HDO, NH₃ and C₃. The 70km/s feature is not uniquely identifiable with the dynamic components (the main cloud and the large-scale foreground filament) so-far identified toward this region. The narrow absorption feature is similar to the one found toward low-mass protostars, which is characteristic of the presence of a cold external envelope. The far-infrared spectroscopic data were combined with existing ground-based observations of ¹²CO, ¹³CO, CCH, CN, and C₃H₂ to characterize the 70km/s component. Using a non-LTE analysis of multiple transitions of NH₃ and CN, we estimated the density (n(H₂)~(1-5)x10⁵cm^–3) and temperature (10-30K) for this narrow feature. We used a gas-grain warm-up based chemical model with physical parameters derived from the NH₃ data to explain the observed abundances of the different chemical species. We propose that the 70km/s narrow feature arises in a dense and cold clump that probably undergoes collapse to form a low-mass protostar, formed on the trailing side of the high-velocity filament, which is thought to be interacting with the W51 main cloud. While the fortuitous coincidence of the dense clump along the line of sight with the continuum-bright W51e2 compact Hii region has contributed to its nondetection in the continuum images, this same attribute makes it an appropriate source for absorption studies and in particular for ice studies of star-forming regions.