Activity on the classical T Tauri star BP Tauri.

Details Activity on the classical T Tauri star BP Tauri. Activity on the classical T Tauri star BP Tauri.

Mar 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...307..791g&link_type=abstract

Astronomy and Astrophysics, v.307, p.791-802

Astronomy and Astrophysics

Astrophysics

26

Stars: Activity, Flare: Pre-Main-Sequence, Stars: Individual: Bp Tauri

Scientific paper

We have made a detailed investigation of the short-term variability in optical light (UBVRI) of the classical T Tauri star BP Tauri. Photometric data (in UBVRI) were collected from Wendelstein Observatory, Germany in 1991, 1992 and 1993 with time-resolutions down to 1sec and, from binning, fluctuations with total amplitudes down to a few milli-magnitudes could be resolved. Additional observations (in UBV) were collected in China. The total time of monitoring amounts to 135 hours. The normal state of BP Tau is that it stays completely constant in brightness in all bands, or shows only very slow and smooth changes during a night, to the limit of detection. Brightenings, events, occurred on time-scales from 0.6hours to a few hours but none of these reached a total amplitude >0.3mag in U. As a rule these events do not have the characteristic flare profile as in the lightcurves of stellar surface flares. The total optical energies of the events are a few times 10^35^erg, with a relatively small spread. The energy distributions at peak flux can be represented by black-body radiation. However, the inferred temperature is very low, 7000-8000K, and not significantly different from that derived for the background veiling. Hence, the events on BP Tau are very different from normal stellar flares. From power analysis of the time series, we conclude that there is no power indicating frequent and short lasting phenomena, like surface flares. In particular there is no signal in the U band. Such flares would have been expected to be numerous in this high-sensitivity survey, however, if BP Tau had a magnetic surface activity comparable to that of ordinary flare stars. Also, there is no tail in the distribution of events towards smaller amplitudes and shorter durations. We show that the events of BP Tau are consistent with inhomogeneous mass infall from magnetically controlled accretion between a circumstellar disk and a hot spot at the star. To account for the constancy in temperature and the distribution of events over frequency and amplitude, a model is proposed where the steady accretion of BP Tau is composed of small fragments which arrive close to the star in a random fashion. By simulations we show that such a flow can explain the smooth variations in the veiling and also the sudden occurence of events. In this "fractal" model, each event is composed of several superimposed fragments, which produce the sometimes complex light profiles. Finally, we find regular long-term variations with a period of 6.6days, which is in line with (but not exactly) the periods found by others.

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