A HST study of the environment of the Herbig Ae/Be star LkHa 233 and its bipolar jet
Abstract
We present the results of HST/STIS and WFPC2 observations of LkHa 233 and its environment. LkHa233 is a Herbig Ae/Be star with a collimated bipolar jet. We investigate optical forbidden lines along the LkHa 233 jet to determine physical parameters of this jet (electron density ne, hydrogen ionisation fraction xe, electron temperature Te, and mass density nH). The knowledge of these parameters allows us a direct comparison of a jet from a Herbig star with those from T Tauri stars. The WFPC2 images in broad-band filters clearly show a dark lane caused either by a circumstellar disk or a dust torus. In the blueshifted lobe, ne is close to or above the critical density for [SII] lines (2.5x104 cm-3) in the first arcsecond and decreases with distance from the source. The ionisation xe~0.2-0.6 gently rises for the first 500 AU of the flow and shows two re-ionisation events further away from the origin. The Te varies along the flow between 104 K and 3x104 K. The (radial) outflow velocities are ~ 80-160 km s(-1), and they appear to increase with distance from the source. In the redshifted lobe, Te, ne, xe, and nH are all lower than in the blueshifted lobe, but have the same order of magnitude. All these derived parameters are just beyond or at the upper limits of those observed for classical T Tauri star jets. This may indicate that the flows from the higher mass Herbig stars are indeed scaled-up examples of the same phenomenon as in T Tauri stars.