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

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${}^{\prime \prime }$. 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 ($\le$1000 K) and to derive correct estimates of H${}_2$O abundance in the shocked gas, which is lower than what is expected from shock model predictions.

Astronomy & Astrophysics Letter, in press