The impact of the Herbig Haro object HH2 on local dust and gas

We present results from a study of molecular gas and dust in the vicinity of the Herbig Haro object HH2. Emission from the sub-mm continuum, 12CO and HCO+ was mapped with angular resolutions ranging from 14 arcsec to 5 arcsec (or 0.01pc at the distance of HH2). The continuum shows an extended dust clump of mass 3.8Msun and temperature 22K, located downstream of the bright optical HH knots. However, a compact emission peak lies within ~0.01pc of the low-excitation H2-prominent shocks, with a luminosity consistent with local heating by the outflow. The HCO+ emission shows two velocity components: firstly, ambient-velocity gas lying in a region roughly corresponding to the dust clump, with abundance enhanced 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 high-velocity jet, the line wings show an abundance ratio HCO+/CO proportional to v^2, with an HCO+ enhancement compared with ambient gas of up to 10^3 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. A UV precursor may not, therefore be necessary to explain the elevated HCO+ abundance in this gas.