First spectrally-resolved H₂ observations towards HH 54 ^⋆

Low H₂O abundance in shocks

¹ Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy
e-mail: gina@arcetri.astro.it
² Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone, Italy
³ Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø., Denmark
⁴ Centre for Star and Planet Formation and Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K., Denmark
⁵ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁶ European Southern Observatory, Karl Schwarzschild str.2, 85748 Garching bei Muenchen, Germany
⁷ LERMA, Observatoire de Paris, UMR 8112 of the CNRS, 61 Av. de l’Observatoire, 75014 Paris, France
⁸ ASDC, 00044 Frascati, Roma, Italy
⁹ The Johns Hopkins University, Baltimore, MD 21218, USA
¹⁰ Observatorio Astronómico Nacional (IGN), Alfonso XII 3, 28014 Madrid, Spain
¹¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹² Max Planck Institut für Extraterrestrische Physik (MPE), Giessenbachstr.1, 85748 Garching, Germany

Received: 4 August 2014
Accepted: 7 September 2014

Abstract

Context. Herschel observations suggest that the H₂O distribution in outflows from low-mass stars resembles the H₂ emission. It is still unclear which of the different excitation components that characterise the mid- and near-IR H₂ distribution is associated with H₂O.

Aims. The aim is to spectrally resolve the different excitation components observed in the H₂ emission. This will allow us to identify the H₂ counterpart associated with H₂O and finally derive directly an H₂O abundance estimate with respect to H₂.

Methods. We present new high spectral resolution observations of H₂ 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 H₂O (2₁₂ − 1₀₁) observations at 1670 GHz are presented.

Results. Our observations show for the first time a clear separation in velocity of the different H₂ 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. H₂O and high-J CO appear to be associated with the H₂ 0−0 S(4) emission, which traces a gas component with a temperature of 700−1000 K. The H₂O abundance with respect to H₂ 0−0 S(4) is estimated to be X(H₂O) < 1.4 × 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 H₂O emitting gas (≤1000 K) and to derive correct estimates of H₂O abundance in the shocked gas, which is lower than what is expected from shock model predictions.