First spectrally-resolved H$_2$ observations towards HH 54 / Low H$_2$O abundance in shocks

G. Santangelo; S. Antoniucci; B. Nisini; C. Codella; P. Bjerkeli; T. Giannini; A. Lorenzani; L. K. Lundin; S. Cabrit; L. Calzoletti; R. Liseau; D. Neufeld; M. Tafalla; E. F. van Dishoeck

doi:10.1051/0004-6361/201424748

paper

First spectrally-resolved H $_{2}$ observations towards HH 54 / Low H $_{2}$ O abundance in shocks

arXiv:1409.2779 · doi:10.1051/0004-6361/201424748

Abstract

Context: Herschel observations suggest that the H $_{2}$ O distribution in outflows from low-mass stars resembles the H $_{2}$ emission. It is still unclear which of the different excitation components that characterise the mid- and near-IR H $_{2}$ distribution is associated with H $_{2}$ O. Aim: The aim is to spectrally resolve the different excitation components observed in the H $_{2}$ emission. This will allow us to identify the H $_{2}$ counterpart associated with H $_{2}$ O and finally derive directly an H $_{2}$ O abundance estimate with respect to H $_{2}$ . Methods: We present new high spectral resolution observations of H $_{2}$ 0-0 S(4), 0-0 S(9), and 1-0 S(1) towards HH 54, a bright nearby shock region in the southern sky. In addition, new Herschel-HIFI H $_{2}$ O (2 $_{12}$ $-$ 1 $_{01}$ ) observations at 1670~GHz are presented. Results: Our observations show for the first time a clear separation in velocity of the different H $_{2}$ lines: the 0-0 S(4) line at the lowest excitation peaks at $-$ 7~km~s $^{- 1}$ , while the more excited 0-0 S(9) and 1-0 S(1) lines peak at $-$ 15~km~s $^{- 1}$ . H $_{2}$ O and high- $J$ CO appear to be associated with the H $_{2}$ 0-0 S(4) emission, which traces a gas component with a temperature of 700 $-$ 1000 K. The H $_{2}$ O abundance with respect to H $_{2}$ 0-0 S(4) is estimated to be $X$ (H $_{2}$ O) $<$ 1.4 $\times$ 10 $^{- 5}$ in the shocked gas over an area of 13 $^{''}$ . Conclusions: We resolve two distinct gas components associated with the HH 54 shock region at different velocities and excitations. This allows us to constrain the temperature of the H $_{2}$ O emitting gas ( $\leq$ 1000 K) and to derive correct estimates of H $_{2}$ O abundance in the shocked gas, which is lower than what is expected from shock model predictions.

Astronomy & Astrophysics Letter, in press

First spectrally-resolved H $_{2}$ observations towards HH 54 / Low H $_{2}$ O abundance in shocks

Abstract

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First spectrally-resolved H2​ observations towards HH 54 / Low H2​O abundance in shocks

Abstract

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First spectrally-resolved H $_{2}$ observations towards HH 54 / Low H $_{2}$ O abundance in shocks