Molecular Abundance Enhancements in the Highly Collimated Bipolar Outflow BHR 71

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Abstract
We report observations of the J = 3 → 2 and J = 2 → 1 transitions of SiO and CS, the Jk = 3k → 2k and Jk = 2k → 1k transitions of CH3OH, and the J = 1 → 0 transition of HCO+, made with Swedish-ESO Submillimetre Telescope (SEST), toward the highly collimated bipolar outflow BHR 71. Broad wing emission was detected toward the outflow lobes in all the observed molecular lines. The shapes of the profiles are strikingly different from molecule to molecule. For CS and HCO+ the emission from the outflowing gas appears as a weak broad feature superposed upon a strong narrow emission from the quiescent ambient gas. For CH3OH the intensity of the broad emission feature is considerably stronger than that of the narrow component, whereas for SiO the broad feature completely dominates the emission spectra. The spatial distribution of the integrated wing emission is considerably extended and broadly similar in all the observed molecular transitions, showing well-separated blueshifted and redshifted lobes with FWHM angular sizes of 24 × 13 and 24 × 14, respectively. We find that the abundance of methanol and silicon monoxide in the outflow lobes is enhanced with respect to that of the ambient cloud by factors of up to ~40 and 350, respectively. The large enhancements of methanol and silicon monoxide in the outflow lobes are most likely due to the release from grains of ice mantles and Si-bearing species via shocks produced by the interaction between the outflow and dense ambient gas. On the other hand, we find that the abundance of HCO+ in the outflowing gas is smaller than that in the ambient gas by about a factor of 20, a decrease consistent with theoretical predictions of shock models.