Volume 407, Number 1, August III 2003
|Page(s)||289 - 301|
|Section||Stellar structure and evolution|
|Published online||17 November 2003|
The evolution of lithium depletion in young open clusters: NGC 6475*
Dipartimento di Astronomia, Università di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
2 INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
3 Institut d' Astronomie, Université de Lausanne, 1290, Chavannes–des–Bois, Switzerland
4 INAF/Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
Corresponding author: P. Sestito, email@example.com
Accepted: 12 May 2003
We have carried out a high resolution spectroscopic survey of the 220–250 Myr old cluster NGC 6475: our main purpose is to investigate Li evolution during the early stages of the Main Sequence. We have determined Li abundances for 33 late F to K–type X–ray selected cluster candidates, extending the samples already available in the literature; for part of the stars we obtained radial and rotational velocities, allowing us to confirm the membership and to check for binarity. We also estimated the cluster metallicity which turned out to be over–solar (). Our Li analysis evidenced that (i) late F–type stars (Teff 6000 K) undergo a very small amount of Li depletion during the early phases on the ZAMS; (ii) G–type stars (6000 Teff 5500 K) instead do deplete lithium soon after arrival on the ZAMS. Whereas this result is not new, we show that the time scale for Li depletion in these stars is almost constant between 100 and 600 Myr; (iii) we confirm that the spread observed in early K–type stars in younger clusters has converged by 220 Myr. No constraints can be put on later–type stars. (iv) Finally, we investigate the effect of metallicity on Li depletion by comparing NGC 6475 with the similar age cluster M 34, but we show that the issue remains open, given the uncertain metallicity of the latter cluster. By using the combined NGC 6475+M 34 sample together with the Hyades and the Pleiades, we compare quantitatively Li evolution from the ZAMS to 600 Myr with theoretical predictions of standard models.
Key words: stars: abundances / stars: evolution / open clusters and associations: individual: NGC 6475
© ESO, 2003
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