Time scales of Li evolution: a homogeneous analysis of open clusters from ZAMS to late-MS

Abstract

We have performed a new and homogeneous analysis of all the Li data available in the literature for main sequence stars (spectral-types from late F to K) in open clusters. In the present paper we focus on a detailed investigation of MS Li depletion and its time scales for stars in the 6350−5500 K effective temperature range. For the first time, we were able to constrain the age at which non-standard mixing processes, driving MS Li depletion, appear. We have also shown that MS Li depletion is not a continuous process and cannot be simply described by a t −α law. We confirm that depletion becomes ineffective beyond an age of 1−2 Gyr for the majority of the stars, leading to a Li plateau at old ages. We compared the empirical scenario of Li as a function of age with the predictions of three non-standard models. We found that models including only gravity waves as main mixing process are not able to fit the Li vs. age pattern and thus this kind of mixing can be excluded as the predominant mechanism responsible for Li depletion. On the other hand, models including slow mixing induced by rotation and angular momentum loss, and in particular those including also diffusive processes not related to rotation, can explain to some extent the empirical evidence. However, none of the currently proposed models can fit the plateau at old ages.

Transiting exoplanets from the CoRoT space mission

“…Hence, another index of stellar youth is provided by the Li I absorption line of 75 mÅ equivalent width that we detected in the spectrum of CoRoT-6, indicating a 7 Li abundance (log N/N tot ) of +1.91 (Table 5). Based on the Li abundance for the appropriate type of star and the results of Sestito & Randich (2005), we estimate the age of CoRoT-6 to be between 2.5 and 4.0 Gyr. It is however worth noting that solar-type stars reach the zero-age main sequence (ZAMS) rotating at a variety of rates, from roughly solar on up to 100 times that velocity, as seen, for example, in the Pleiades (Stauffer & Hartmann 1987;Soderblom et al 1993b,a).…”

Effect of magnetic fields on Lithium depletion of solar-type stars

“…We compare the theoretical results with the time scales of lithium depletion suggested by Sestito & Randich (2005) and obtain very good agreements. …”

“…The error bars represent the average log N (Li) in different clusters as a function of age given by Sestito & Randich (2005), namely, the time scale of lithium evolution. The dashed lines represent lithium depletion reproduced by magnetic models; solid and dotted lines represent lithium evolution of diffusing and rotating models, respectively.…”

“…Furthermore, Sestito & Randich (2005) performed a new homogeneous analysis of all the lithium data available in the literature for main sequence stars. More detailed features of lithium depletion was obtained in their work, which is the so-called time scale of lithium evolution.…”

“…Since lithium is an extraordinarily sensitive diagnostic of extra-mixing process in stellar interior, to fitting the observed features we need improve our knowledge about extra-mixing in stellar interior. Sestito & Randich (2005), which is the so-called time scales. In all three figures, the solid lines represent models with diffusion; the dotted lines represent rotating models; models with magnetic fields are showed by dashed lines.…”