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Astron. Astrophys. 360, 637-641 (2000)

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4. Discussion

We have shown that the brightness variations in V 1080 Tau are periodic and are present in all data, spanning about 9 years and covering the region longward from the U band. The proximity effects (mainly the ellipsoidal variations) can plausibly reproduce the main features of the double-wave light curve of V 1080 Tau with the orbital period of 17.69 days while no match of the 8.8 day single-wave curve is possible. We therefore argue that 17.69 days is the true orbital period. The proximity effects in the binary can explain especially the smooth course of the modulation, the decrease of the amplitude from the red to the blue spectral region and bluer color at minimum brightness. The brightness variations can be interpreted in terms of modulation in a typical Algol-type binary with [FORMULA], comprising the evolved cool star ([FORMULA] K, consistent with filling its lobe) and the detached main-sequence or slightly evolved primary ([FORMULA] K).

The double-wave light curve of V 1080 Tau shows that the reflection effect is very small and that the observed variations are caused almost exclusively by the ellipticity of the large cool secondary. Model 2 in Table 2 implies that the fractional luminosities of the primary and the secondary are comparable in the R band while the secondary contributes just about 33% in the B filter. This is consistent with the early spectral type in the blue region of the low-resolution spectrum, such as that of Walter et al. (1990), and, on the other hand, with the presence of the lines of the secondary near H[FORMULA] (Martín 1993). We can conclude that these parameters are in accordance with the spectroscopic classification of the components by Walter et al. (1990) and Martín (1993).

The validity of the above-mentioned parameters can be also checked using the total luminosity of the binary. The distance to V 1080 Tau, measured by Hipparcos, is 400 pc. Assuming the mass of the primary [FORMULA] (Harmanec 1988), appropriate for [FORMULA] in model 2, the absolute radii of the components can be calculated from the fractional radii. The combined magnitude for the reddening [FORMULA] (Walter et al. 1990) and [FORMULA] pc is then approx. 11 mag(V). This is slightly fainter than the mean observed brightness 10.45 mag(V) (Fig. 1). This difference can be explained in several ways. The reddening was determined by Walter et al. (1990) under the assumption that V 1080 Tau is a single A star. Taking the secondary star into account therefore could diminish [FORMULA] and hence increase the total brightness. Also the mass of the primary may be higher because its radius for model 2 corresponds to a star evolved off the main sequence. Increase of [FORMULA] would lead to increase of the absolute radii of both stars and hence to their larger luminosities. We can therefore conclude that the agreement is plausible and probably the best that can be obtained with the data at hand.

Assuming an early A primary, we obtain the expected full amplitude of RV variations of the secondary about 170 km s-1, in agreement with the lower limit of 160 km s-1 determined from Martín's (1993) data.

There are several lines of evidence of activity in V 1080 Tau. Martín (1993) found H[FORMULA] in strong double-peaked emission with variable strength and profile. The U-band data appear to be affected by the activity, too, the modulation in U is largely divergent from those in the B and R bands, especially near the phase 0.5. Slight distortions, not explicable by the pure proximity effects, can also be resolved in the light curve in the R band, although they are much smaller than in the U-filter. Comparison of set A1 and set B suggests that the latter, which displays smaller scatter, was obtained at epoch of weaker activity. The course of the light curve in the R filter slightly deviates from the course of the synthetic curve mainly at phases 0.25 and 0.5. The dip in the color index [FORMULA], centered on the phase 0.5, shows that V 1080 Tau is bluer here than can be explained purely by the proximity effects. In the framework of the proximity effects, the bluer color at minima is caused by smaller contribution of the cool distorted secondary. The "blue excess" at phase 0.5 means that the contribution of the secondary is even smaller here. We offer an interpretation of this effect in terms of the mass stream. The effects of such streams are well-known from the distortions of the light curves of the eclipsing Algols (e.g. Olson & Bell (1989), Olson & Etzel (1994)). The gas leaving the L1 point is cooler than the photosphere of the secondary. The stream projects on the disk of this star within phases 0.4-0.7. Stream with sufficiently large optical depth will therefore block part of the light from the secondary and will give rise to a decrease of brightness and bluer color index near phase 0.5. On the other hand, the side of the stream irradiated by the primary will be exposed to the observer near phase 0.0 and can give rise to excess light seen in some data of B93a.

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© European Southern Observatory (ESO) 2000

Online publication: August 17, 2000
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