The irradiated Herbig-Haro jets near σ Orionis

We present high-resolution echelle spectra of the irradiated Herbig-Haro jets HH 444, 445, 446, and 447 from the Keck I telescope. The structural, kinematic, and physical properties of the jets near their sources are analyzed in detail. In particular, spatio-kinematic electron density variations and evidence for excited, low-velocity entrained material are found. Despite their proximity to the massive stars in the sigma Orionis system, the inner portions of these jets generally exhibit low-excitation conditions, consistent with shadowing from circumstellar material. The outflow sources show Halpha and forbidden line emission on top of late-type photospheric spectra and strong lithium absorption, indicative of typical classical T Tauri stars. These spectroscopic data are discussed in the context of theoretical models that attempt to account for the generation of outflows. In addition, narrowband Hubble Space Telescope (HST) WFPC2 images of HH 444 and 445 are presented and compared with the spectroscopic data. In addition to the jets, the images of both sources reveal extended emission structures that point directly away from sigma Ori and are therefore interpreted as photoevaporating proplyds.