Astron. Astrophys. 336, 535-538 (1998)

1. Introduction

Optical forbidden emission, such as the [OI]6300 line and the [SII]6716/6731 doublet, has long been used to obtain details of outflows from T Tauri stars (TTS) and Herbig Ae/Be stars (HAEBES). Indirectly they can also be used to infer the distribution of optically thick material around such stars. Appenzeller et al. (1984) and Edwards et al. (1987) were the first to notice that the profiles of the forbidden lines were often asymmetric with the red wing either missing or diminished. They proposed the simple but elegant idea that this asymmetry was due to the receding flow being occulted by an optically thick circumstellar disk. A somewhat naive analysis (see, e.g., Basri & Bertout 1993), based on the emission measure of the forbidden lines, suggested disk sizes, in the case of TTS, of around 100 AU.

Actual forbidden line profiles of young stars can be complex: often two blue-shifted components are present, a high velocity velocity component (or HVC), which can reach velocities of a few hundred kms in TTS, and a low velocity component (or LVC), with velocities of perhaps a few kms with respect to the systemic velocity of the star (see, e.g., Hartigan et al. 1995). Kwan & Tademaru (1988, 1995) developed a model to explain these double-peaked profiles in which they attributed the high velocity peak to a jet, presumably related to the high velocity jets sometimes observed at greater distances from young stars (e.g. Edwards et al. 1993 or Ray 1996). The low velocity emission is then taken to originate from either a disk wind or disk corona, with the width of the line determined by rotational broadening.

While a sizable number of TTS have double-peaked forbidden emission line profiles (Edwards et al. 1987, Hartigan et al. 1995) the number of HAEBES known with similar characteristics is relatively small. One such object is LkH 233 (Corcoran & Ray 1997) an A5e pre-main sequence star (d = 880 pc), associated with a bipolar nebula (Herbig 1960; Calvet & Cohen 1978; Staude & Elsässer 1993) having an optical size of about 0.1 pc. The nebula has a distinct X-like morphology with bright reflection limbs at 50^o/230^o and 90^o/270^o. Although molecular lines are observed towards the star (e.g. Cantó et al. 1984), no molecular outflow has been detected (Leverault 1988). It is a Hillenbrand Group II star (Hillenbrand et al. 1992), and thus has a spectral energy distribution that rises with increasing wavelength out to beyond 100µm. Broad band polarimetry carried out on the star and nebula by Aspin et al. (1985) show deviations from a centro-symmetric pattern in keeping with the presence of a large "polarization disk" having an estimated radius of about 15000 AU. The orientation of the "disk" and the position angle of the intrinsic polarization close to the source is about 155^o (Vardanyan 1979; Vrba et al. 1979; Leinert et al. 1993). Speckle interferometry, performed in the near-infrared, (Leinert et al. 1993) shows that the source is surrounded by a light scattering "halo" of about 1" in size. The polarization of the halo is greater than 10% (at approximately the same position angle as mentioned above) and its discovery prompted Leinert et al. (1993) to suggest that LkH 233 is highly embedded and optically visible mostly in scattered light. Hamann (1994) found the [FeII] lines in the spectrum of LkH 233 to be blue-shifted (at about -150 kms) with respect to the permitted CaII lines, suggesting the presence of a high velocity outflow from this star. Observations by us (Corcoran & Ray 1997) with a randomly oriented slit showed a clear double-peaked profile for the [OI]6300 and [SII]6716/6731 lines. Here we present long-slit spectroscopic observations of LkH 233 and its immediate environment and observe for the first time an extended optical jet from the star.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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