ON THE AGES OF PRE-MAIN-SEQUENCE STARS IN TAURUS

This paper reports a search for pre-main-sequence stars in a region of about 12.7 deg2 near the L1551 complex in the Taurus molecular cloud. We identify only two new pre-main-sequence stars from optical spectra of 383 proper motion candidates on the basis of Li I 6707 angstrom absorption. These stars have ages of approximately 4 X 10(6) yr; we find no evidence for a large number of pre-main-sequence stars with ages comparable to the crossing time of the cloud (approximately 10(7) yr). The T Tauri stars in the southern region of Taurus appear to be older (ages approximately 4 X 10(6) yr) than the stars in the central molecular cloud region (ages approximately 1 X 10(6) yr), suggesting a modest difference in age among molecular complexes in Taurus. The present census of optical pre-main-sequence stars appears to be fairly complete down to V approximately 15.5, and to this magnitude limit there are roughly equal numbers of weak-emission and strong-emission T Tauri stars. The absence of older stars suggests that the Taurus-Auriga molecular cloud may not be very much older than 10(7) yr.