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Astron. Astrophys. 326, 647-654 (1997)

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4. Lithium equivalent width, age, and T-Tauri nature

The attribution of a precise age to individual stars is difficult, specially when individual accurate distances are not available, which, specially for young stars, allow the placement on evolutionary tracks. The only available indicator of WTTS status for individual low-mass stars of unknown distance is the presence of a very large lithium abundance: low-mass stars are supposed to burn lithium at the basis of their convective zones, with lower mass stars having proportionally deeper convection zones and thus shorter lithium depletion characteristics times. The available observational evidence, however, is not so simple, and all the currently available data point toward several factors, in addition to age, influencing the lithium abundance of a low-mass star.

Old dwarfs earlier than G5 show a large range of lithium abundance, with lithium-rich stars being found at essentially any age ( Pasquini et al. 1994, Favata et al. 1996), and the same stars still have essentially undepleted lithium when they are well in the main sequence stage (as in the Pleiades, Soderblom et al. 1993), showing that the lithium criterium is of little relevance for their being classified as PMS sources. For low-mass stars cooler than [FORMULA] G5, the evidence so far available points toward lithium being depleted with age, although young stars have a wide range of lithium abundance at any given age, as shown, for example, by Soderblom et al. (1993) for the solar-type stars in the Pleiades, and by Stauffer et al. (1993) for the [FORMULA] Persei cluster.

Thus, while high levels of lithium (i.e. comparable to the "cosmic" abundance, [FORMULA] on the usual scale where [FORMULA] ) are characteristic of G- and K-type PMS stars (and are a "necessary" condition for being classified as a PMS), a lithium abundance of order [FORMULA] is by no means sufficient for classifying a star as PMS, given than many stars down to mid-K spectral type in the Pleiades have a lithium abundance close to 3.0, yet they clearly are on (or very close to) the main sequence.

Given that cooler stars are expected (and generally observed) to be depleting their lithium more rapidly than higher mass solar-type stars, it is not possible to adopt a single, mass-independent threshold of the lithium abundance as discriminating between PMS and main-sequence stars, with the same lithium abundance having quite different implications on the evolutionary status in G, K or M stars.

In Fig. 8, we have plotted the lithium abundance implied by different (true) observed equivalent widths of the Li I doublet, for an equivalent width of 100, 200 and 300 mÅ. The region to the left of the thick vertical line in Fig. 8 is the region occupied by the early- to mid-G dwarfs, for which lithium cannot be used to separate PMS and main-sequence objects. The boundary of the dashed region is the approximate upper boundary of the lithium abundances measured in the Pleiades by Soderblom et al. (1993), so that any star lying outside the dashed region has lithium abundances compatible with its being a main sequence star. The region in which PMS stars can be discriminated from the lithium abundance is thus the dashed area in Fig. 8, although, given the spread of lithium abundance observed at any given age, the actual boundary has to be assumed to be quite fuzzy. Taking a mass-independent threshold of 100 mÅ for the equivalent width of the Li I doublet will select (as already remarked by Brice"no et al. 1997), in addition to whatever true WTTS there may be in the sample, many stars which are simply young main sequence stars, thus inflating the detected number of WTTS with several spurious sources. A threshold of [FORMULA] mÅ would approximately follow the sloping boundary of the shaded region, but at the same time it would miss many true WTTS among the hotter stars, as well as a few at the cooler end.

[FIGURE] Fig. 8. The lithium abundance is shown, as a function of effective temperature, for equivalent width values of the Li I doublet of 100, 200 and 300 mA , following the curves of growth of Soderblom et al. (1993). The heavy vertical line represents the cool [FORMULA] bound for [FORMULA] G5 stars, and the shaded region is the region which is expected to be occupied by bona fide PMS stars.
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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997