The young triple system HV Tau consists of a tight binary, HV Tau AB, with a projected separation of 0.074" (Simon et al. 1996, S96), and a third component, HV Tau C, located away from the primary. The JHK photometry of HV Tau AB and HV Tau C measured on our images is listed in Table 1. HV Tau AB has similar JHK magnitudes as those previously reported by Woitas & Leinert (1998), but HV Tau C has dimmed by about 1 mag between their observations and ours. In this section we discuss in turn the high angular resolution images obtained for the various components of the system.
3.1. HV Tau C: a nearly edge-on disk
The JHK images of HV Tau C are shown in Fig. 1. The central dark lane, best seen in the K image and oriented at a P.A. of , delineates the disk's midplane. On each side, bright lobes correspond to photons from the central star scattered back to the observer in the disk's upper layers and in optically thin bipolar cavities. The central star itself is not seen, being heavily extincted by the disk midplane. Comparing these images with synthetic ones computed from a single scattering disk model (Lazareff et al 1990, LPM90), we derive an inclination of for HV Tau C's circumstellar disk.
Intensity contours shown in Fig. 2 provide further details on the circumstellar structure around HV Tau C.
In all the images, the disk appears to have the same radial extension of AU. That the disk radius does not depend upon wavelength suggests a sharp outer disk boundary, possibly truncated by the the tidal influence of HV Tau AB. The flux ratio between the northern and southern scattering cavities varies with wavelength. The northern lobe is located on the frontside of the disk and the southern one on the backside so that stellar photons scattered in the latter are obscured by the disk outer regions. Hence, the northern lobe is brighter than the southern one at JHK and the southern one is better seen at longer wavelengths as the disk's optical depth decreases. Finally, the extension of the northern lobe in a direction perpendicular to the disk midplane seems larger at J than at K. This may result from the higher scattering efficiency at shorter wavelengths that allows the observer to detect a more extended part of the scattering medium above the disk midplane at J. All these features and their variations with wavelength are in qualitative agreement with model images of nearly edge-on disks and associated bipolar scattering cavities (LPM90).
3.2. HV Tau AB
Contour plots of HV Tau AB in the JHK bands are shown in Fig. 3. The contours are clearly elongated in all 3 images with a PA of . This PA differs by 180o of derived by Simon et al. (1996) for this system at visible wavelengths. It might thus be that the eastern component is the brightest one in the near-IR but becomes fainter than the western component at visible wavelengths. However, since the observations were not simultaneous, it may also be that the 180o flip of the PA between S96's observations and ours is merely the result of instrinsic photometric variability of one of the components or both. HV Tau AB is a weak-line T Tauri star and photometric variations of a few tenths of a magnitude would not be surprising. In fact, S96 found the V-band flux ratio to vary at different epochs, and at least one of their measurement yields a flux ratio of unity.
We have performed a cut along the PA of the binary in the J image and fitted the resulting profile with three gaussian curves (see Fig. 3), two of which correspond to the FWHM profiles of HV Tau A and B (FWHM=), and the third one has a much larger FWHM in order to remove the low-intensity pedestal that arises from incomplete adaptive correction. From this fit, we derive a separation of mas between HV Tau A and B, fully consistent with the mas V-band measurement of S96. Given that HV Tau AB semi-major axis is AU, it is not surprising to find the same PA and separation in our 1996 data than in S96's ones obtained in 1994. On a longer time basis, however, HV Tau AB is a good candidate to detect actual orbital motion. We also derive a flux ratio A/B ().
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000