We present the photometric variability of 26 T Tauri Stars
in an effort to increase the statistics of the photometric
data of pre-Main Sequence Stars and to deepen
the information regarding TTS observed in the Southern hemisphere.
More than half of our objects were initially identified using IRAS
data and subsequently classified using medium resolution spectra
revealing the LiI 6707 line in absorption together with the H
line in emission or filled in (GHETAL92). The other objects in our sample are
listed in the HBC catalogue.
We derive photometric periods for PDS 59, Hen 892, PDS 70, V896 Sco, VZ Cha, WY Cha, AS 216, AS 218 and FK Ser. We suggest possible periodicity during the observing season for PDS 77, CT Cha, SZ 45, SZ 108 and confirm previous values for SY Cha. Irregular variability is found for SZ 77, SZ 82 and TW Cha.
TW Cha shows no periodicity during the observing season, in agreement with the previous work of Bouvier et al. (1986). It is worth noting that Kappelman & Mauder (1981) and Mauder & Sosna (1975) do find periodic patterns in the TW Cha light curve. Therefore, TW Cha should be regarded as an example of a cTTS whose light curve may be either rotationally modulated or dominated by irregular variations. Likewise, our exercise on the archived data of T Cha and our analysis of AS 216 produce similar results - results which suggest these stars have rotational modulations which are occasionally dominated by irregular flux variations.
The rich time series of AS 216 distributed over 5 years allows us to test the stability the measured rotational period. The light curve modulates with a period of 3.12d between 1985 and 1987. This result is obtained whether the entire data set is taken together or each year is considered separately. During the first three years, the amplitude is larger in the bluer bands. The light curve amplitudes as a function of wavelength reveal that a hot spot is likely controlling the periodic modulation. Based on the persistence of the 3.12d period, the spot is presumably stable. In subsequent observing seasons (1989-1990), we are not able to measure any periodicity. Instead, we find larger amplitude, irregular variations.
Acknowledgements
We would like to express our appreciation to Dr. C. Akerlof for helping us to install the cubic B-spline code and for showing us how to use it. We thank the referee Dr. E. Covino, for a careful and critical reading of the paper, leading to an improved final version.