A HST study of the environment of the Herbig Ae/Be star LkHα 233 and its bipolar jet

Abstract

Context: 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.
Aims: 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.
Methods: We present the results of HST/STIS and WFPC2 observations of LkHα 233 and its environment. These are the first observations of this object with a spatial resolution of ≤0.1 arcsec 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 ≈2000 AU from the star in the blueshifted outflow lobe and ≈4000 AU 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.
Results: 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 (2.5 × 10^4 cm^-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 500 AU of the flow and shows two re-ionisation events further away from the origin. The electron temperature T_e varies along the flow between 10^4 K and 3 × 10^4 K. The n_H is between 3 × 10³ and 10^5 cm^-3, and the mass flux dot{M}≈ 10^-8{-}10^-7 {M}_⊙ yr^-1. The (radial) outflow velocities are ≈80{-}160 km s^-1, 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.
Conclusions: 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.