Beryllium abundances and star formation in the halo and in the thick disk ^*,**

¹ Universidade de São Paulo, IAG, Dpt. de Astronomia, Rua do Matão 1226, São Paulo-SP 05508-090, Brazil e-mail: rodolfo@astro.iag.usp.br
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
³ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France
⁴ INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34143 Trieste, Italy
⁵ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁶ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy

Received: 14 July 2008
Accepted: 26 January 2009

Abstract

Context. Beryllium is a pure product of cosmic ray spallation. This implies a relatively simple evolution in time of the beryllium abundance and suggests its use as a time-like observable.

Aims. Our goal is to derive abundances of Be in a sample of 90 stars, the largest sample of halo and thick disk stars analyzed to date.
We study the evolution of Be in the early Galaxy and its dependence on kinematic and orbital parameters, and investigate its use as a cosmochronometer. Abundances of Be, Fe, and α-elements of 73 stars are employed to study the formation of the halo and the thick disk of the Galaxy.

Methods. Beryllium abundances are determined from high-resolution, high signal-to-noise UVES spectra with spectrum synthesis. Atmospheric parameters and abundances of α-elements are adopted from the literature. Lithium abundances are used to eliminate mixed stars from the sample. The properties of halo and thick disk stars are investigated in diagrams of (Be/H) vs. [ α/H] , (Be/H) vs. [Fe/H], and [ α/Fe] vs. (Be/H) and with orbital and kinematic parameters.

Results. We present our observational results in various diagrams. (i) In a (Be/H) vs. [Fe/H] diagram we find a marginal statistical detection of a real scatter, above what is expected from measurement errors, with a larger scatter among halo stars. The detection of the scatter is further supported by the existence of pairs of stars with identical atmospheric parameters and different Be abundances; (ii) in a (Be/H) vs. [ α/Fe] diagram, the halo stars separate into two components; one is consistent with predictions of evolutionary models, while the other has too high α and Be abundances and is chemically indistinguishable from thick disk stars. This suggests that the halo is not a single uniform population where a clear age-metallicity relation can be defined; (iii) In diagrams of R_min vs. [ α/Fe] and (Be/H), the thick disk stars show a possible decrease in [ α/Fe] with R_min, whereas no dependence of Be with R_min is seen. This anticorrelation suggests that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation. The lack of correlation for Be indicates that it is insensitive to the local conditions of star formation.