Herschel-HIFI observations of H2O, NH3, and N2H+ toward high-mass starless and protostellar clumps identified by the Hi-GAL survey

摘要

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 ? = 59° and ? = 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 ? = 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 ? = 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 ? = 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 ? = 30° region are in a somewhat later evolutionary phase compared to the ? = 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.