Detection of a dense clump in a filament interacting with W51e2

In the framework of the Herschel/PRISMAS guaranteed time key program, the line of sight to the distant ultracompact H ii 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 ~70 km s-1 in the observed spectra of HDO, NH3 and C3. The 70 km s-1 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 70 km s-1 component. Using a non-LTE analysis of multiple transitions of NH3 and CN, we estimated the density (n(H2) ~ (1-5) × 105 cm-3) and temperature (10-30 K) 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 70 km s-1 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 H ii 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. Based on data acquired with Herschel and IRAM observatories. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.