2003MNRAS.339..633D

We present results from a study of molecular gas and dust in the vicinity of the Herbig-Haro object HH2. Emission from the sub-millimetre continuum, ¹²CO and HCO⁺ was mapped with angular resolutions ranging from 14 to 5 arcsec (or 0.01 pc at the distance of HH2). The continuum shows an extended dust clump of mass 3.8 M_☉and temperature 22 K, located downstream of the bright optical HH knots. However, a compact emission peak lies within 0.01 pc of the low-excitation H₂-prominent shocks, with a luminosity consistent with local heating by the outflow. The HCO⁺ emission shows two velocity components: first, ambient-velocity gas lying in a region roughly corresponding to the dust clump, with abundance enhancement by a factor of a few close to the H₂-prominent knots; secondly, a component of high-velocity emission (20 km/s linewidth), found mainly in a collimated jet linking the low-excitation HH objects. In this latter high-velocity jet, the line wings show an abundance ratio χ_HCO+/χ_CO∝v², with an HCO⁺ enhancement compared with ambient gas of up to ∼10³ at the most extreme velocities. Such high abundances are consistent with models of shock chemistry in turbulent mixing layers at the interaction boundaries of jets. Extrapolating this effect to low velocities suggests that the more modest HCO⁺ enhancement in the clump gas could be caused by low-velocity shocks. An ultraviolet precursor may therefore not be necessary to explain the elevated HCO⁺ abundance in this gas.