THE EVOLUTIONARY STATUS OF ACTIVITY-SELECTED SOLAR-TYPE STARS AND OF T-TAURI STARS AS DERIVED FROM HIPPARCOS PARALLAXES - EVIDENCE FOR LONG-LIVED T-TAURI DISKS

We have used the Hipparcos parallaxes to study the evolutionary status of a sample of stars with spectral types from late F to M0 (hereafter ''solar-type stars''), selected on the basis of their activity, mainl y from Einstein-based surveys. The parallaxes have been used to place the objects in the H-R diagram, determining their age by comparison wi th theoretical evolutionary tracks and observational main sequences. T his age is compared with age estimates derived from the lithium abunda nce, the activity level and the presence of circumstellar disks. To co mplement our sample at the young end we have also studied the Hipparco s-determined distances of a sample of optically-selected pre-main sequ ence stars, mostly classical T Tauri stars (CTTS). Some CTTS appear to be much nearer to us than previously determined, and far away from th eir putative parent cloud. This implies a significantly larger age pro viding observational evidence for the existence of long-lived T Tauri disks which could produce slow rotators on the Zero-Age Main Sequence (ZAMS). None of the above-mentioned age proxies appears to reliably an d unambiguously select very young stars in the range of spectral types considered hen, with some apparently very young objects effectively l ying onto or very close to the main sequence. The attribution of ages to young solar-type stars on the basis of any of the standard proxies may thus significantly under- or over-estimate the evolutionary age of the object. Caution must therefore be exercised when attributing ages to individual stars, and claims about the large number of PMS stars f ound in X-ray based surveys may need to be at least in part reconsider ed in this light.