Astron. Astrophys. 324, 155-160 (1997)

5. Conclusions

We have observed the classical T Tauri star BP Tau simultaneously with optical UBVRI high-speed photometry and with the ROSAT PSPC detector during five nights. The results of this investigation can be summarized as:

1. BP Tau reveals brightness changes in X-ray on a night-to-night basis. It is not possible from this investigation to tell whether the X-ray variability is connected to the rotation of the star or if the X-ray emission is intrinsically variable.
2. The optical brightness level did also change on a night-to-night basis in all photometric bands. However, the variation was not correlated to the variation in X-ray flux. We thus conclude that the sources responsible for the nightly X-ray and optical variability of BP Tau are not related.
3. During one night we observed a fast low amplitude optical event on BP Tau with amplitudes of in U and in B lasting for 1.2 hours. The simultaneous X-ray measurements did not reveal any increase in count rate during the optical event. This strongly suggests that even the rare fast events observed on BP Tau (see for instance Paper I) are produced in different regions than what is the case for the X-ray emission which differs from what is observed on flare stars.
4. On one night a short duration X-ray flare was observed with a rise time of 5 minutes and a decay time (to the approximate -level) of 40 minutes. Unfortunately no simultaneous optical observations are available, however the similarity of this event to the flares observed on dMe flare stars strengthens the idea that the X-ray variability is related to magnetically active regions on the stellar surface.

We propose that the mechanisms behind the observed optical and X-ray variability are different. Presumably the major part of the optical variability is produced by variable accretion while the X-ray variability is related to magnetically active regions which only cause a low level optical activity that is overwhelmed by the accretion-induced variations. This strengthens the idea that the optical variability, even on shorter time-scales, is produced by accretion phenomena. Similar multi-wavelengths observations of WTTS should give other relations between the optical and X-ray activities.

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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