Herschel observations of the Herbig-Haro objects HH 52-54^⋆

¹ Department of Earth and Space SciencesChalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
e-mail: per.bjerkeli@chalmers.se
² INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
³ Observatorio Astronómico Nacional (IGN), Calle Alfonso XII, 3. 28014, Madrid, Spain
⁴ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5 − 7, 1350 Copenhagen, Denmark
⁶ Max Planck Institut for Extraterestrische Physik, Garching, Germany
⁷ Department of Astronomy, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
⁸ University of Waterloo, Department of Physics and Astronomy, Waterloo, Ontario, Canada

Received: 8 March 2011
Accepted: 14 June 2011

Abstract

Context. The emission from Herbig-Haro objects and supersonic molecular outflows is understood as cooling radiation behind shocks, which are initiated by a (proto-)stellar wind or jet. Within a given object, one often observes both dissociative (J-type) and non-dissociative (C-type) shocks, owing to the collective effects of internally varying shock velocities.

Aims. We aim at the observational estimation of the relative contribution to the cooling by CO and H₂O, as this provides decisive information for understanding the oxygen chemistry behind interstellar shock waves.

Methods. The high sensitivity of HIFI, in combination with its high spectral resolution capability, allowed us to trace the H₂O outflow wings at an unprecedented signal-to-noise ratio. From the observation of spectrally resolved H₂O and CO lines in the HH52-54 system, both from space and from the ground, we arrived at the spatial and velocity distribution of the molecular outflow gas. Solving the statistical equilibrium and non-LTE radiative transfer equations provides us with estimates of the physical parameters of this gas, including the cooling rate ratios of the species. The radiative transfer is based on an accelerated lambda iteration code, where we use the fact that variable shock strengths, distributed along the front, are naturally implied by a curved surface.

Results. Based on observations of CO and H₂O spectral lines, we conclude that the emission is confined to the HH54 region. The quantitative analysis of our observations favours a ratio of the CO-to-H₂O-cooling-rate  ≫1. Formally, we derived the ratio Λ(CO)/Λ(o-H₂O)  = 10, which is in good agreement with earlier determination of 7 based on ISO-LWS observations. From the best-fit model to the CO emission, we arrive at an H₂O abundance close to 1 × 10^-5. The line profiles exhibit two components, one that is triangular and another that is a superposed, additional feature. This additional feature is likely to find its origin in a region that is smaller than the beam where the ortho-water abundance is smaller than in the quiescent gas.

Conclusions. Comparison with recent shock models indicate that a planar shock cannot easily explain the observed line strengths and triangular line profiles. We conclude that the geometry can play an important role. Although abundances support a scenario where J-type shocks are present, higher cooling rate ratios are derived than predicted by these types of shocks.