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Centro de Astrofísica da Universidade do Porto

Membership and lithium in the old, metal-poor open cluster Berkeley 32

S. Randich, G. Pace, L. Pastori, A. Bragaglia

Abstract
Context. Measurements of lithium (Li) abundances in open clusters provide a unique tool for following the evolution of this element with age, metallicity, and stellar mass. In spite of the plethora of Li data already available, the behavior of Li in solar-type stars has so far been poorly understood.
Aims. Using FLAMES/Giraffe on the VLT, we obtained spectra of 157 candidate members of the old, metal-poor cluster Berkeley 32, to determine membership and to study the Li behavior of confirmed members.
Methods. Radial velocities were measured, allowing us to derive both the cluster velocity and membership information for the sample stars. The Li abundances were obtained from the equivalent width of the Li I 670.8 nm feature, using curves of growth.
Results. We obtained an average radial velocity of 105.2 ± 0.86 km s−1, and 53% of the stars have a radial velocity consistent with membership. The Li – Teff distribution of unevolved members matches the upper envelope of M 67, as well as that of the slightly older and more metal-rich NGC 188. No major dispersion in Li is detected. When considering the Li distribution as a function of mass, however, Be 32 members with solar-like temperature are less massive and less Li-depleted than their counterparts in the other clusters. The mean Li of stars in the temperature interval 5750 ≤ Teff ≤ 6050 K is log n(Li) = 2.47 ± 0.16, less than a factor of two below the average Li of the 600 Myr old Hyades, and slightly above the average of intermediate age (1–2 Gyr) clusters, the upper envelope of M67, and NGC 188. This value is comparable to or slightly higher than the plateau of Pop. II stars. The similarity of the average Li abundance of clusters of different age and metallicity, along with its closeness to the halo dwarf plateau, is very intriguing and suggests that, whatever the initial Li abundance and the Li depletion histories, old stars converge to almost the same final Li abundance.

Keywords
stars: abundances, stars: evolution, stars: interiors, open clusters and associations: individual: Berkeley 32

Astronomy and Astrophysics
Volume 496, Page 441
March 2009

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