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Astron. Astrophys. 321, 492-496 (1997)

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3. Post T Tauri stars

Herbig (1978) remarked that the T Tauri phase represents only about 10% of the PMS evolution of solar-type stars. He used the term PTTS for a PMS star which has lost its T Tauri properties. However, with the recognition of the WTTSs it has become unclear how to characterize observationally the PTTSs. Some authors have suggested that PTTSs are only distinguished from WTTSs because they lie closer to the ZAMS in the H-R diagram (Martín, Magazzù & Rebolo 1992; Pallavicini, Pasquini & Randich 1992). The caveat of this criterion is that it relies on an asssumption of the distance to the star and on comparison with theoretical isochrones. It would be desirable to define the PTTS from phenomenological properties rather than from comparison with models.

Stars with masses [FORMULA][FORMULA]   do not deplete Li during PMS evolution (Martín & Montes 1996), and therefore this element cannot be used as a PMS age indicator for those masses. Activity and rotation can neither be used because they do not behave fundamentally different in PTTSs than in WTTSs and YMS stars (Bouvier et al. 1996, Jeffries et al. 1996). A distinguishing quality of PTTSs is that they evolve towards the ZAMS on partially radiative evolutive tracks, and thereby they increase their [FORMULA]  . Stars in the mass range 2-1  [FORMULA]   evolve from [FORMULA]   5100-4500 K (K1-K5) at the bottom of their fully convective PMS paths to 8600-5600 K (A3-G6), respectively, on the ZAMS. Therefore, the population of PTTSs with masses [FORMULA][FORMULA]   consists of A, F and G-type stars. Possibly, the only way of discriminating such PTTSs from field YMS stars is to very accurately place them in an H-R diagram by measuring their trigonometric parallax.

For stellar masses [FORMULA][FORMULA]  , Li is depleted during PMS evolution and it could be used as an age indicator. This is illustrated in Fig. 1, where the Li I   equivalent widths of TTSs and members of young open clusters are plotted as a function of [FORMULA]   or spectral type. 64 equivalent widths coming from Basri et al. (1991), Magazzù et al. (1992) and Martín et al. (1994) of both CTTSs (corrected for veiling) and WTTSs are shown. The X-ray discovered stars (Table 8 of Martín et al. 1994) have not been included. 89% of the TTSs fall on or above the lithium isoabundance line which represents the locus of "minimum" [FORMULA] values. The few TTSs with smaller [FORMULA] could be PTTSs or 1- [FORMULA] statistical deviations. The open cluster stars include 214 members of the Pleiades with equivalent widths (upper limits for 9 of them) measured by Soderblom et al. (1993) and García López, Rebolo & Martín (1994); 19 stars of IC 2602 (Randich et al. 1996); 9 members of IC 2391 observed by Stauffer et al. (1989); and 14 members of IC 4665 (Martín & Montes 1996). According to Mermilliod (1981), the age of these three IC open clusters are younger (36 Myr) than that of the Pleiades (78 Myr). Uncertainties in equivalent width vary from star to star, but in general they are in the range 10-80 mÅ . For [FORMULA]   in the range 5250-4800 K, the TTSs are not clearly separated from the cluster stars, but for decreasing temperature the separation becomes larger. PTTSs can be identified unambigously if they fill the empty space in Fig. 1 between TTSs and K,M-type young cluster stars. This region is what I call hereafter PTT-gap. PMS calculations predict that low-mass stars should spend a few Myr in the PTT-gap, but unfortunately the models cannot yet be used quantitatively because they fail to reproduce the pattern of Li abundances in young open clusters (Martín & Montes 1996 and references therein). The number of stars located in the PTT-gap that are found in a given survey can be used as a lower limit to the total number of PTTSs present in the surveyed area.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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