HH 1158: The lowest luminosity externally irradiated Herbig-Haro jet
We have identified a new externally irradiated Herbig-Haro (HH) jet, HH 1158, within ~2 pc of the massive OB type stars in the sigma Orionis cluster. At an Lbol ~ 0.1 Lsun, HH 1158 is the lowest luminosity irradiated HH jet source identified to date in any cluster/region. The external radiation field has improved the visibility both close to and far from the driving source, thus enabling unambiguous determination of the jet physical parameters. Results from spectro-astrometric analysis of high-resolution VLT/UVES spectra indicate a highly asymmetric jet morphology, where the jet beam facing away from the OB stars is significantly brighter and denser than the counter jet beam. The presence of an extended collimated jet that is bipolar and the evidence of shocked emission knots make HH 1158 the first unique case of irradiated HH jets at the very low-luminosity end, and provide an opportunity to learn empirically the physical properties of very faint HH jet sources.