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

¹ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany e-mail: melnikov@tls-tautenburg.de
² Ulugh Beg Astronomical Institute, Astronomical Str. 33, 700052 Tashkent, Uzbekistan
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ Dipartimento di Matematica, Università degli Studi di Roma “Tor Vergata”, via della Ricerca Scientifica 1, 00133 Roma, Italy
⁵ Dublin Institute for Advanced Studies, 5 Merrion Square, Dublin 2, Ireland

Received: 9 August 2007
Accepted: 26 October 2007

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 , hydrogen ionisation fraction , 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 ≤ 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 , , T_e, and mass density 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 ) in the first arcsecond and decreases with distance from the source. The ionisation gently rises for the first 500 AU of the flow and shows two re-ionisation events further away from the origin. The electron temperature varies along the flow between and 3 . The is between 3 10³ and , and the mass flux . The (radial) outflow velocities are ≈, and they appear to increase with distance from the source. In the redshifted lobe, the excitation conditions are quite different: , , , and 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.