Herschel-HIFI observations of H2O, NH3, and N2H+ toward high-mass starless and protostellar clumps identified by the Hi-GAL survey⋆,⋆⋆
1 INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
2 University of Puerto Rico, Rio Piedras Campus, Physics Dept., Box 23343, UPR station, San Juan, Puerto Rico, USA
3 Chalmers University of Technology, Department of Earth and Space Sciences, Onsala Space Observatory, 439 92 Onsala, Sweden
Received: 6 July 2015
Accepted: 25 August 2015
Context. Our present understanding of high-mass star formation still remains very schematic. In particular, it is not yet clear how much of the difference between low-mass and high-mass star formation occurs during the earliest star formation phases.
Aims. The chemical characteristics of massive cold clumps, and the comparison with those of their low-mass counterparts, could provide crucial clues about the exact role that chemistry plays in differentiating the early phases of low-mass and high-mass star formation. Water, in particular, is a unique probe of physical and chemical conditions in star-forming regions.
Methods. Using the HIFI instrument of Herschel, we have observed the ortho−NH3 (10−00) (572 GHz), ortho−H2O (110−101) (557 GHz), and N2H+ (6−5) (559 GHz) lines toward a sample of high-mass starless and protostellar clumps selected from the Herschel Infrared Galactic Plane Survey (Hi-GAL). We compare our results to previous studies of low-mass and high-mass protostellar objects.
Results. At least one of the three molecular lines was detected in 4 (out of 35) and 7 (out of 17) objects in the ℓ = 59° and ℓ = 30° galactic regions, respectively. All detected sources are protostellar. The water spectra are complex and consist of several kinematic components, identified through a Gaussian decomposition, and we detected inverse and regular P-Cygni profiles in a few sources. All water line profiles of the ℓ = 59° region are dominated by a broad Gaussian emission feature, indicating that the bulk of the water emission arises in outflows. No such broad emission is detected toward the ℓ = 30° objects. The ammonia line in some cases also shows line wings and an inverse P-Cygni profile, thus confirming that NH3 rotational transitions can be used to probe the dynamics of high-mass, star-forming regions. Both bolometric and water line luminosity increase with the continuum temperature.
Conclusions. The higher water abundance toward the ℓ = 59° sources, characterized by the presence of outflows and shocks, supports a scenario in which the abundance of this molecule is linked to the shocked gas. Various indicators suggest that the detected sources toward the ℓ = 30° region are in a somewhat later evolutionary phase compared to the ℓ = 59° field, although a firm conclusion is limited by the small number of observed sources. We find many similarities with studies carried out toward low-mass protostellar objects, but there are indications that the level of infall and turbulence in the high-mass protostars studied here could be significantly higher.
Key words: line: formation / stars: formation / ISM: molecules / ISM: abundances
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
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© ESO, 2015