The physical conditions in IRDC clumps from Herschel HIFI observations of H2O

Abstract

Context. The earliest phases of high-mass star formation are poorly understood. Aims. Our goal is to determine the physical conditions and kinematic structure of massive star-forming cloud clumps. Methods. We analyze H${}_2$O 557 GHz line profiles observed with HIFI toward four positions in two infrared-dark cloud clumps . By comparison with ground-based C${}^{17}$O, N${}_2$H${}^{+}$, CH${}_3$OH and NH${}_3$ line observations, we constrain the volume density and kinetic temperature of the gas and estimate the column density and abundance of H${}_2$O and N${}_2$H${}^{+}$. Results. The observed water lines are complex with emission and absorption components. The absorption is red shifted and consistent with a cold envelope, while the emission is interpreted as resulting from protostellar outflows. The gas density in the clumps is $\sim$ 10${}^7$ cm${}^{-3}$. The o-H2O outflow column density is 0.3 to 3.0 10${}^{14}$ cm${}^{-2}$, the o-H2O absorption column density is between 1.5 10${}^{14}$ and 2.6 10${}^{15}$ cm${}^{-}2$ with cold o-H2O abundances between 1.5 10${}^{-9}$ and 3.1 10${}^{-8}$. Conclusions. All clumps have high gas densities ($\sim$ 10${}^7$ cm${}^{-3}$) and display infalling gas. Three of the four clumps have outflows. The clumps form an evolutionary sequence as probed by H${}_2$O N${}_2$H${}^{+}$, NH${}_3$ and CH${}_3$OH. We find that G28-MM is the most evolved, followed by G11-MM then G28-NH3. The least evolved clump is G11-NH3 which shows no sign-posts of star-formation. G11-NH3 is a high-mass pre-stellar core.

arXiv comment15 pages, 10 figures

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