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Evolution of lithium beyond the solar age: A Li survey ofthe old open cluster NGC 188^*

¹ INAF/Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Dipartimento di Astronomia, Università di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy
³ INAF/Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Corresponding author: S. Randich, randich@arcetri.astro.it

Received: 1 August 2002
Accepted: 27 November 2002

Abstract

We have determined Li abundances for 11 G–type stars inthe 6–8 Gyr old open clusterNGC 188. These data significantly enlarge the number of cluster stars withLi measurements,allowing us to extend the investigation of Li depletion inopen clusters to ages well beyond the age of the Sun. We have also inferred the cluster metallicity which turns out to be solar.We find thatsolar–type stars inNGC 188 are only slightly more Li depletedthan the much youngerHyades and no more Li depleted than stars of similartemperature in the 2–4 Gyr younger clusterM 67.At variance withM 67,NGC 188 members show virtually no scatter intheir Li abundances. Surprisingly, no solar–type star inNGC 188 appears asLi depleted as the Sun or as the most Li depleted stars inM 67. We discussthe implications of these results for mechanisms of internalmixing and Li depletion in main sequence stars.