* Astronomy

Title: Herschel observations of the Herbig-Haro objects HH52-54
Authors: P. Bjerkeli, R. Liseau, B. Nisini, M. Tafalla, M. Benedettini, P. Bergman, O. Dionatos, T. Giannini, G. Herczeg, K. Justtanont, B. Larsson, C. McCoey, M. Olberg, A.O.H Olofsson

We are aiming at the observational estimation of the relative contribution to the cooling by CO and H2O, as this provides decisive information for the understanding of the oxygen chemistry behind interstellar shock waves. Methods. The high sensitivity of HIFI, in combination with its high spectral resolution capability, allows us to trace the H2O outflow wings at unprecedented signal-to-noise. From the observation of spectrally resolved H2O and CO lines in the HH52-54 system, both from space and from ground, we arrive 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 ALI code, where we use the fact that variable shock strengths, distributed along the front, are naturally implied by a curved surface. 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. From the best-fit model to the CO emission, we arrive at an H2O abundance close to 1e-5. The line profiles exhibit two components, one of which is triangular and another, which is a superposed, additional feature. This additional feature likely originates from a region smaller than the beam where the ortho-water abundance is smaller than in the quiescent gas. Comparison with recent shock models indicate that a planar shock can not 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 than predicted by these type of shocks are derived.

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