Detection of a dense clump in a filament interacting with W51e2 ^⋆

¹ Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
e-mail: bhaswati@tifr.res.in
² Université de Toulouse, UPS-OMP, IRAP, Toulouse, France
³ CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁴ Department of Physics and Astronomy, Siena College, Loudonville NY 12211, USA
⁵ LERMA, Observatoire de Paris ENS, and UMR8112 du CNRS, 24 rue Lhomond, 75231 Paris Cedex 05, France
⁶ IRAM, 300 rue de la Piscine, 38406 St. Martin d’Heres, France
⁷ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Boulevard, Pasadena, CA 91109, USA
⁸ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
⁹ Institute of Physics, University of Kassel, 34132 Kassel, Germany
¹⁰ I. Physikalisches Institut, University of Cologne, 50937 Cologne, Germany

Received: 26 November 2013
Accepted: 19 March 2014

Abstract

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.