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1. Introduction

The work of Joy (1942) identified T Tauri Stars (TTS) as a new class of stars characterized by irregular photometric variability. Several TTS became targets of constant photometric observation - some even subject to systematic daily monitoring during the 80's. Herbst et al. (1994) combined photometry from the literature and established periodicity for a number of these low mass pre-main sequence stars. Several stars show irregular variations superimposed on the periodic patterns. TTS with large accretion rates present, in general, no detectable periodic behavior (see Gahm et al. 1993; Bouvier et al. 1993; Covino et al. 1992).

It is often assumed that a classical TTS (cTTS) possesses a dipole magnetic field that disrupts the geometrically thin circumstellar disk, funneling the incoming gas onto high latitude regions of the central star, thereby generating hot spots. Theory (Königl 1991; Shu et al. 1994), observations (Johns & Basri 1995; Kenyon et al. 1994; Edwards et al. 1994) and semi-empirical models (Hartmann et al. 1994) support this scenario in which hot spots covering a total projected area of less than 3%, are sitting at the base of the magnetic column. If the field is axially symmetric over the stellar surface and not aligned with the geometric poles, then the light curve will modulate with the same period of the stellar rotation. In fact, the comprehensive catalogue of Herbst et al. (1994) indicates that high latitude hot spots account for the light modulation of classical TTS (cTTS), whereas cool spots are responsible for that of weak TTS (wTTS).

Bouvier et al. (1993) highlight two important findings in their photometric campaign of the Taurus - Aurigae complexes. First, they trace periodic light curves for all of the objects in their sample (24 stars). After comparing the equatorial velocity inferred from the computed periods with the projected vsini, they conclude that the transit of surface spots are the most likely source of light modulation observed in 20 of their stars (88% of the sample). A second important finding emerges after including an additional sample of 17 TTS with previously established periods in their data set. They conclude that the TTS rotational periods segregate themselves into two distributions with a mean value of 4.1 days for wTTS and 7.6d for cTTS. Almost simultaneously, the analysis of previously published rotational periods leads Edwards et al. (1993) to similar conclusions, with a somewhat larger spread in the distribution of periods for the wTTS. The recent campaigns of Bouvier et al. (1995 and 1997) confirm these findings.

Why, on average, do wTTS rotate faster than the cTTS? Both weak and cTTS undergo gravitational contraction with the resulting increase in their rotation rates. Furthermore, classical TTS have a surplus of angular momentum resulting from the torque applied to the stellar surface by the accretion disk which should therefore lower the periods. In addition to a magnetized stellar wind which would steadily deplete the stellar angular momentum, Edwards et al. (1993) propose an active coupling between the stellar magnetosphere and the slowly rotating outer regions of the disk as a possible solution to this dilemma. Thus, the stellar angular velocities of the cTTS would be kept at values that are, on average, lower than those found in wTTS.

Another view is championed by Smith (1994) and stems from the fact that differential rotation is established in the Sun but is conveniently ignored throughout the calculations of TTS periods which are computed assuming that the star rotates as a rigid body. Smith proposes that the bimodal distribution of rotational periods among classical and weak TTS is an artifact of the differential rotation. The classical TTS apparently rotate slower than the weak TTS because the spots that define their light curves are thought to be at high latitudes. On the contrary, those of the weak TTS are hypothesized to be at low latitude regions as in the solar analogy, although recent Doppler images of weak TTS consistently show high latitude features (Joncour et al. 1994; Strassmeier et al. 1994). The question remains whether the bimodal distribution of periods is an indication that steep differential rotation prevails among PMS stars or whether the stellar initial angular momentum is controlled by the presence of an accretion disk.

These competing interpretations can only be settled with systematic photometric and/or high resolution spectroscopic monitoring campaigns. It is crucial to devote observational efforts in order to increase the statistics of rotational periods among TTS. Monitoring selected targets over several consecutive seasons, tracking possible changes from periodic to aperiodic patterns, and ideally combining with spectroscopic timeseries and Doppler imaging, may shed some light on the nature of these pre main sequence objects - their level of stellar activity, their initial angular momentum and their angular momentum evolution.

The main goal of this work is to increase the information about the photometric variability of T Tauri Stars. It is based on photometric data collected by different researchers during the past 10 years at Pico dos Dias Observatory (OPD), including extensive photoelectric monitoring of AS 216, AS 218 and FK Ser. In Sect. 2 we describe the observations and data reduction. The periodogram methods employed in this investigation are described in Sect. 3 and results are summarized in Sect. 4.


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