Volume 566, June 2014
|Number of page(s)||11|
|Section||Interstellar and circumstellar matter|
|Published online||12 June 2014|
Detection of a dense clump in a filament interacting with W51e2 ⋆
1 Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
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
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.
Key words: ISM: molecules / submillimeter: ISM / ISM: lines and bands / line: formation / line: identification / molecular data
© ESO, 2014
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