  Herbig-Haro objects are regions of shocked gas and dust which are produced
when collimated outflows from a protostar interact with the surrounding dense
gas. They have many similarities to supernova remnants which are interacting
with molecular clouds. 1720-MHz OH masers have been identified towards a number
of interacting supernova remnants. Observations and models indicate that these
masers are shock excited and are produced behind C-type shocks. If conditions
behind the shock fronts of Herbig-Haro objects are similarly able to support
1720-MHz OH masers they could be a useful diagnostic tool for star formation.
We therefore searched for 1720-MHz OH maser emission towards a sample of 97
Herbig-Haro objects using the Green Bank radio telescope. We detected 1720-MHz
OH lines in emission in 17 of them, but neither their spectral signature nor
follow-up observations with the Very Large Array showed any conclusive evidence
of maser emission. We conclude that the emission detected from our single-dish
observations must be extended and most likely originates from thermal or
quasi-thermal excitation processes. We also investigated the properties of
Herbig-Haro shocks more closely and conclude that despite the overall
similarities to supernova remnants, the conditions required for maser emission,
in particular, a sufficient velocity-coherent column density, are not likely to
occur in Herbig-Haro objects.
