COYOTES-I - THE PHOTOMETRIC VARIABILITY AND ROTATIONAL EVOLUTION OF T-TAURI STARS

In a multi-site photometric campaign to monitor T Tauri stars (TTS) in the Taurus-Auriga cloud over more than two months - dubbed Coordinate d Observations of Young ObjecTs from Earthbound Sites (COYOTES) -, we find all 24 of our target stars show evidence for periodic light varia tions with periods between 1.2 and 24.0 days. This more than doubles t he number of periods published for Tau-Aur TTS. The variations of 20 o f these stars can be interpreted as rotational modulation of the stell ar flux by surface spots. The periods of the four remaining stars (RY Tau, BD+24-degrees-676, TAP 26, and LkCa-21) may correspond to orbital periods of binary systems. Models of the UBVRI light curves lead to t he properties of the spots, both hotter and cooler than the photospher ic temperature, which appear to be the principal source of the photome tric variability of TTS on timescales of days and weeks. The longest r otational period we measure (12 d, for GM Aur) leads to an equatorial velocity of 8km s-1, which con firms the paucity of extremely slow rot ators (V(eq) much less than 10 km s-1) among TTS younger than 5 10(6) yrs. Combining our 20 rotational periods with those published for 17 o ther Tau-Aur TTS, we find that the Weak-line TTS (EW(H(alpha)) < 10 an gstrom) rotate faster than Classical TTS (EW(H(alpha)) greater-than-or -equal-to 10 angstrom) at the 99.9% confidence level (according to a K -S test). The mean rotational period for the 11 WTTS is 4.1 +/- 1.7 d; for the 15 CTTS, 7.6 +/- 2.1 d. We interpret this difference as evide nce that WTTS spin-up as they contract on their convective tracks, whi le CTTS are prevented from doing so by either: (a) their strong winds carrying away excess angular momentum and/or (b) a magnetic coupling b etween the stars and their inner accretion disks, as suggested by rece nt models. We discuss the implications of this interpretation for the subsequent evolution of TTS toward the main sequence. In particular, w e propose that the different rotational histories of WTTS and CTTS on their convective tracks may account for the large range of rotational velocities observed among low-mass dwarfs in young clusters.