2008A&A...483..199M

LkHα 233 is a Herbig Ae/Be star with a collimated bipolar jet. As such, it may be a high-mass analogue to the classical T Tauri stars and their outflows. We investigate optical forbidden lines along the LkHα 233 jet to determine physical parameters of this jet (electron density n_e, hydrogen ionisation fraction x_e, electron temperature T_e). The knowledge of these parameters allows us a direct comparison of a jet from a Herbig star with those from T Tauri stars. We present the results of HST/STIS and WFPC2 observations of LkH{alpha} 233 and its environment. These are the first observations of this object with a spatial resolution of ≤0.1" at optical wavelengths. Our STIS data provide spectroscopic maps that allow us to reconstruct high angular resolution images of the bipolar jet from LkHα 233 covering the first ≃2000AU from the star in the blueshifted outflow lobe and ≃4000AU in the redshifted lobe. These maps are analysed with a diagnostic code that yields n_e, x_e, T_e, and mass density n_H within the jet. The WFPC2 images in broad-band filters clearly show a dark lane caused either by a circumstellar disk or a dust torus. The circumstellar environment of LkHα 233 can be interpreted as a conical cavity that was cleared by a bipolar jet. In this interpretation, the maximum of the optical and near-infrared brightness distribution does not coincide with the star itself which is, in fact, deeply extincted. In the blueshifted lobe, n_e is close to or above the critical density for [SII] lines (2.5x10⁴cm^–3) in the first arcsecond and decreases with distance from the source. The ionisation x_e≃0.2-0.6 gently rises for the first 500AU of the flow and shows two re-ionisation events further away from the origin. The electron temperature T_e varies along the flow between 10⁴K and 3x10⁴K. The n_H is between 3x10³ and 10⁵cm^–3, and the mass flux {dot}(M)≃10^–8-10^–7M_☉/yr. The (radial) outflow velocities are ≃80-160km/s, and they appear to increase with distance from the source. In the redshifted lobe, the excitation conditions are quite different: T_e, n_e, x_e, and n_H 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.