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Astron. Astrophys. 356, L75-L78 (2000)

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4. Discussion

The nature of HV Tau C has been subject to controversy. On the basis of a low-resolution spectrum that exhibits strong H[FORMULA] and forbidden lines, Magazzú & Martín (1994, MM94) suggested that HV Tau C is a Herbig-Haro object formed in an outflow emanating from HV Tau AB. This interpretation has been disputed by Woitas & Leinert (1998, WL98) as they failed to measure any significant proper motion of HV Tau C between MM94 observations and theirs, in apparent contradiction to expectations for a fast-moving HH object. Moreover, they noted that the low-resolution spectrum of HV Tau C obtained by MM94 also exhibits a strong continuum ressembling that of a late-type photosphere. They thus concluded that HV Tau C is a low-mass stellar companion to HV Tau AB and further hypothesized that the star is seen through a nearly edge-on circumstellar disk. This peculiar viewing angle would qualitatively account for the much lower apparent luminosity of HV Tau C compared to HV Tau AB in spite of its spectral energy distribution indicative of a hotter black-body temperature.

The images reported here directly confirm Woitas & Leinert's conjecture. HV Tau C thus appears to be a low-mass star of spectral type about M0 (or slightly later if veiling is significant) according to the low-resolution spectrum shown in MM94. The spectrum exhibits strong forbidden emission lines and EW(H[FORMULA]) [FORMULA] 15Å, which indicate that HV Tau C is a classical T Tauri star which actively accretes from its disk. The forbidden line emission most likely arises from a jet originating from HV Tau C itself. As already pointed out by WL98, the small Doppler shift of forbidden line profiles is consistent with a fast outflow moving nearly in the plane of the sky, i.e. perpendicular to the disk. In fact, the [SII] image obtained by MM94 seems to show an elongation of the HV Tau C contours toward the south, which might correspond to a jet emanating from the central object. Higher angular resolution images are required to confirm this possibility.

HV Tau C spectral energy distribution also exhibits a strong excess flux at 10 [FORMULA] (WL98) and the unresolved HV Tau system has been detected as a mild continuum source at 1.3mm, with a flux density of 40 mJy (Osterloh & Beckwith 1995). The source of continuum emission in the triple system is most probably HV Tau C's disk since HV Tau AB is a weak-line binary with EW(H[FORMULA]) = 8.5Å (Kenyon et al. 1998).

Assuming HV Tau C and HV Tau AB are coeval, the 3 components of the system appear to have a similar mass of about 0.5[FORMULA] (similar spectral type for HV Tau A and C and a V-band flux ratio close to unity for HV Tau A and B) and an age of about 1.5 106 yr, according to Siess et al. (2000) evolutionary tracks. A major difference, however, is that HV Tau C is still actively accreting from its disk while HV Tau AB shows no sign of active accretion, nor even of a passive disk since its spectral energy distribution is well fitted by a black-body curve (WL98). It is tempting to speculate that tidal interaction between the various components of the system and their circumstellar environments is responsible for this situation.

With a projected separation of 10 AU, primordial circumstellar disks in the HV Tau AB binary have been severely truncated leading to a fast evolution on a short viscous timescale. The lack of IR excess in HV Tau AB further suggests that the system is not surrounded by a circumbinary disk or ring, unlike e.g. GG Tau or UY Aur. While such a disk/ring might have been expected to survive on a longer timescale, it is possible that its absence is due to the tidal influence of HV Tau C, located at a projected distance of 560 AU, i.e., a distance not very different from the radius of the circumbinary disks observed around GG Tau and UY Aur.

Conversely, we argued above that HV Tau C's circumstellar disk might have been tidally truncated by the gravitational influence of HV Tau AB since its 50 AU radius does not seem to strongly depend upon wavelength. However, the ratio of outer disk radius to the projected separation between HV Tau C and AB is about 0.1, which is significantly less than expected for a tidally truncated disk in a system with [FORMULA] (Armitage et al. 1999). By comparison, this ratio amounts to 0.3 for the circumsecondary disk of the HK Tau system (Stappelfeldt et al. 1998). Hence, unless the orbit of HV Tau C is highly eccentric, its disk might well have evolved up to an age of [FORMULA]1.5 106 yr without having been much perturbed by the presence of HV Tau AB.

Finally, we note that the position angle of the disk around HV Tau C is nearly the same as the position angle of the tight binary HV Tau AB ([FORMULA]110o and [FORMULA]130o, respectively). Although it may be a mere coincidence since HV Tau AB has been observed at only one phase of its orbit, it could also indicate that the orbital plane of HV Tau AB is parallel to the plane of the circumstellar disk of HV Tau C. Coplanarity of HV Tau AB orbit and HV Tau C disk is, however, unlikely since it would then imply an actual separation between AB and C of [FORMULA] given a disk inclination of [FORMULA]. The formation of parallel but non coplanar systems has been predicted to result from the collapse of elongated clouds with rotation about an arbitrary axis (Bonnell et al. 1992). The collapse produces two fragments surrounded by parallel but non coplanar disks, each of which may further fragment to form a close binary system. The appearance of the HV Tau triple system supports this formation mechanism.

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Online publication: April 17, 2000