PRE-MAIN-SEQUENCE LITHIUM BURNING .1. WEAK T-TAURI STARS

We report high-resolution spectroscopic observations for a sample of 3 8 T Tauri stars (TTS), complemented by UBVRI photometry of 13 TTS, and CCD VRI photometry for 2 visual binaries. Based on these observations and data taken from the literature, we derive lithium abundances in 5 3 TTS, concentrating on weak-line TTS (WTTS). The sample spans the ran ge in spectral types from K0-M3, approximately corresponding to masses between 1.2 and 0.2 M.. Our study of the statistical distribution of lithium abundances in WTTS gives the following results: (1) At luminos ities greater-than-or-equal-to 0.9 L. the Li abundances are remarkably uniform. The mean value, log N(Li)=3.1, coincides with the ''cosmic'' lithium abundance. (2) We find strong evidence for PMS lithium burnin g. Significant Li depletion appears below 0.5 L. in the mass range 0.9 -0.2 M. and increases towards lower luminosities. Current theoretical evolutionary models do not seem to fit consistently the observed patte rn of Li abundances in the whole mass range. In particular, at the low er mass end (0.4-0.2 M.), the observed luminosity of the Li burning tu rning point is about a factor 4 higher than predicted by the models. A t masses 1.2-1.0 M. the observations imply less PMS Li burning than th eoretically expected. Investigation of a possible relation between lit hium and rotation in TTS shows that: (1) Low Li abundances appear only among stars with low v sini. Fast rotators with masses around 0.8 M. do not show evidence for strong Li depletion towards lower luminositie s as slow rotators do. (2) In a sample restricted to only K5-K7 stars with photometrically measured rotational periods, we find that the ang ular momentum spread before Li burning begins is larger than a factor 10. Lithium depletion associated to angular momentum loss during PMS e volution is not required to explain the observed abundances. The obser vations suggest that the efficiency of PMS Li burning in the mass rang e 0.9-0.7 M. is reduced in the presence of rapid rotation.