Classical TTauri stars usually display irregular light variability on many time-scales, but sometimes also show a periodic or quasi-periodic modulation of their light curve on a time scale of a few days (Bouvier et al. 1986, 1993; Vrba et al. 1986, 1989). This quasi-periodic variation is linked to the rotation of the star and can be reproduced by models of inhomogeneous surface temperature distribution. In several cases one finds that spots responsible for the light curve variations are hot; they are interpreted as the shock regions of an infalling flow of matter. In other CTTSs, however, the signatures are typical of dark, magnetic spots. It can be shown that hot spots dominate the light curve when they are present on the stellar surface, even in the presence of dark magnetic spots (cf. Bertout et al. 1996)
Irregular variability is more difficult to interpret and
various mechanism have been proposed in the
literature (Gahm et al. 1993; Worden et al. 1981;
Guenther & Hessman 1993;
Herbig & Soderblom 1980).
Because it is likely ubiquitous in CTTSs, accretion is the most probable process causing the irregular variations in CTTSs.
Vrba et al. (1985)
showed that the colors of stars with periodic and non-periodic
variations of their data set could be reproduced by variations of hot
plage-like regions on the stellar surface. Furthermore there seems to be
a strong correlation between the overall amplitude of light variations and
the variation of H
equilavent width (Vrba et al. 1993). This suggests that both phenomenon are caused by the same
mechanism. Further arguments are discussed by Herbst et al. (1994) who showed that hot spots responsible for the photometric
variability and the hot components producing the veiling are likely to be
the same.
As it can be seen in Fig. 2 (click here), all stars get redder in the color V-I and V-R when the V magnitude becomes fainter. There is more scatter in B-V but the same trend seems to be present in at least some stars, although one has to caution that the measurement errors is quite large in B. The behavior of the V-I and V-R colors of the YYOrionis stars in the observed sample thus resembles that observed in CTTSs, and can be explained by the presence on the stellar surface of an additional variable blackbody-like source with a temperature different from the stellar effective temperature.
The photometric behavior of our program stars however does not give any further insight in the characteristics of hot spots found on YYOrionis stars, since we did not observe a periodic modulation of their light curves. Like CTTSs, they display most of the time an irregular variability. This means that hot spots may be distributed more or less homogeneously on the stellar surface most of the time. It is only when one group of spots becomes dominant and survives at least one rotation period that a periodic or quasi-periodic variation can be observed.
In order to gain insight into the physical connection between photometric variability and accretion it is necessary to perform spectroscopic observations on our program stars with simultaneous photometry. We shall then be able to study possible correlations of the red-displaced absorption components with photometry. This will be done in a future observation campaign.
Acknowledgements
We are indebted to the director and staff of Wise Observatory for time allocation and logistic help. We would like to thank Stéphanie Allain and Hervé Geoffray for providing us additional observations of YYOri obtained at the OHP. This research has made use of the Simbad data base, operated at CDS, Strasbourg, France.