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

3. Results and discussion

While we suspected the presence of a jet from LkH 233 on the basis of our earlier forbidden line study of this star (Corcoran & Ray 1997), direct narrow-band imaging appeared difficult, especially close to the source, because of the intensity of the associated reflection nebula. Instead several long-slit spectra of LkH 233 were taken at a range of position angles in an attempt to spectroscopically map the immediate region around the star. We do not reproduce the full set of position-velocity (PV) diagrams here for reasons of space. Fig. 1 shows the variation of extent of the jet and counter-jet with position angle. The complete set of position-velocity diagrams indicate the HH-type jet/counter-jet is at a position angle of 240-250^o/60-70^o. This is along the symmetry axis of the bipolar nebula (PA 250^o) and perpendicular to the polarization angle in the vicinity of the source, i.e. 155^o (Vrba et al. 1979; Leinert et al. 1993).

Fig. 2 presents the position-velocity diagram for LkH 233 along its outflow axis as determined from Fig. 1. Clearly shown are the jet and counter-jet emerging from diametrically opposite sides of this young stellar object. The jet has a maximum blue-shifted radial centroid velocity relative to the velocity of the star (vertical lines) of -140 kms^-1 and the counter-jet a maximum red-shifted radial centroid velocity of +110 kms^-1. Also present is blue-shifted LVC emission but this is seen only close () to LkH 233. The electron densities, as measured from the ratio of the [SII]6716/6731 lines for a representative temperature of 10⁴K, are marked in Fig. 1 in square brackets, at various points along the jet and counter-jet, and are accurate to about 20%. Nearer to the star, electron densities are quoted for both the LVC and HVC. Away from the star's position, electron densities can only be measured for the HVC (i.e. the jet and counter-jet). As one would suspect, electron densities are higher closer to the star than further away.

Rather interestingly, the red-shifted counter-jet cannot be traced as close to the stellar photo-centre (horizontal line in Fig. 2) as the blue-shifted jet. Additionally there is a blue-shifted low velocity peak ( -25 kms^-1) only seen close to the star. The fact that the low velocity material is blue-shifted and that the red-shifted counter-jet is not seen to reach all the way to the star suggests there is obscuring circumstellar material, presumably a disk, that occludes the initial few tenths of an arcsecond of the receding outflow. The angular separation from the stellar photo-centre to the edge of the counter-jet is which implies an upper limit for the projected radius of the disk of approximately 600 AU. Leinert et al. (1993) placed an upper limit of 100 AU (FWHM) on the unresolved core they observed using near-IR speckle interferometry and a dimension of about 1" on the "scattering halo" they detected around LkH 233. Clearly the occluding object is not the unresolved core but rather is comparable in size to the scattering halo. The blue-shifted asymmetry of the forbidden emission line region does suggest a disk-like or other flattened geometry, however. Note that the blue-shifted LVC emission is broader than the HVC emission (see also Table 1) and may be modelled (see Kwan & Tademaru 1995) as a rotationally broadened disk wind.

There is a possible over-subtraction of the continuum at the [SII]6716 line which may effect the calculated values of the electron densities, N_e, close to the stellar photo-centre. Fig. 2 shows that both the HVC and LVC emission at the [SII]6716 line is restricted essentially to the western side of the star, unlike the [SII]6731 line. If the photospheric spectrum of LkH 233 is not purely continuum in the region of the 6716Å line, then there may be a problem of over-subtraction during the removal of the continuum. However, no absorption line at that wavelength is visible in the spectra of two spectral standards of types A5II and A7V taken during the same observing run as the LkH 233 data. As no comparable effect is observed at [SII]6731, and the red-shifted counter-jet is observed not to reach the stellar continuum centre in either line, the inference of obscuring circumstellar material is not affected.

Measurements of HH objects and jets associated with lower mass stars (see, for example, Hartigan et al. 1994) show typical [SII]6716+6731/H line ratios close to 1. Such ratios are indicative of low excitation shocks with shock velocities in the range 15-40kms . A comparison of the H (not shown here) and the [SII] line intensities from the LkH 233 jet and counter-jet indicates that low excitation shocks must be responsible for the emission. Interestingly there appears to be a difference between the jet and the counter-jet in that the counter-jet is of somewhat lower excitation on average than the jet itself. We note also that the absolute velocities, with respect to the systemic velocity of the star, in the counter-jet are lower, and the line widths probably smaller, than in the jet (see Table 1). Such intrinsic asymmetries are common amongst bipolar HH jets (e.g. HH 30, Ray et al. 1996) and arise close to the source. Their origin, however, is not fully understood.

In view of the rarity of jets from HAEBES, further studies of this outflow are obviously required including deep direct narrow-band imaging. Although not used here, deep direct imaging could be useful in determining the large-scale morphology, and to some degree the history, of the outflow from LkH 233. Is there, for example, evidence of episodic energetic outbursts in the past? If so then this would indicate that there may be an equivalent of the FU Orionis phenomenon amongst the rarer HAEBES. Spectroscopic studies also of LkH 233 itself, e.g. of the first overtone bands of CO (Chandler et al. 1993) in the near-infrared, might also show evidence for Keplerian rotation by the innermost regions of the disk indirectly detected here.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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