On the ages of pre-main-sequence stars in Taurus

A search for premain-sequence stars in a region of about 1.27 sq deg near the L1551 complex in the Taurus molecular cloud is reported. Two new premain-sequence stars from optical spectra of 383 proper motion candidates are identified on the basis of Li I 6707 A absorption. These stars have ages of about 4,000,000 yr; no evidence is found for a large number of premain-sequence stars with ages comparable to the crossing time of the cloud (about 10 exp 6 yr). The T Tauri stars in the southern region of Taurus appear to be older (about 4,000,000 yr) than the stars in the central molecular cloud region (1,000,000 yr), suggesting a modest difference in age among molecular complexes in Taurus. It is inferred from the absence of older stars that the Taurus-Auriga molecular cloud may not be much older than 10 exp 7 yr.