Beryllium abundances and star formation in the halo and in the thick disk

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 log(Be/H) vs. (ii) in a log(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 log(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.

New Beryllium results in halo stars from Keck/HIRES spectra

“…There seems to be little evidence in our data sample of a different distribution of Be with [O/Fe] for "accretive" vs. "dissipative" stars as determined by the criteria of Gratton et al (2003). Although the errors were large for [O/Fe] in the stars studied by Pasquini et al (2005) and in [α/Fe] in the stars for Smiljanic et al (2009), both groups found intriguing differences in the two sets of stars. They concluded that Be can be used as a chronometer in a subset of stars.…”

“…2. Data from Smiljanic et al (2009) A change in slope would be expected. In the oldest stars, Be would be formed mostly in the vicinity of SN II by the acceleration of CNO nuclei into protons, etc.…”

A primordial star in the heart of the Lion

“…One concern on the observability of these primordial stars is the possibility that their atmospheres may have been polluted by metals during encounters with molecular clouds during their long lifetimes. While early estimations of this process predicted significant pollution to take place (Yoshii et al 1995), subsequent observations failed to detect the predicted "accretion plateau" of light elements (Molaro et al 1997;Boesgaard et al 1999;Smiljanic et al 2009;Duncan et al 1997;Garcia Lopez et al 1998). More recent estimations of the accretion process efficiency (Frebel et al 2009) seem to exclude any significant pollution, especially if the primordial stars have a weak solarlike wind (Johnson & Khochfar 2011).…”

The Be-test in the Li-rich star #1657 of NGC 6397: evidence for Li-flash in RGB stars?

“…A strong conclusion is that star #1657 does not show any evidence of Be enhancement with respect to stars of the same cluster or with respect to field stars with similar metallicity. When assuming the linear relation derived in Smiljanic et al (2009) (Spite & Spite 1982;Lind et al 2009, González Hernández et al 2009, and that the polluting material had normal Be/Li composition (equivalent to A(Li) = 2.20 and log(Be/H) ∼ −12.8), the star should have accreted an amount of Be that would increase its chemical composition to log(Be/H) ∼ −11.5. Assuming the Be abundance from the field star regression (log(Be/H) = −12.8) as reference value is a conservative assumption, because this value is lower than what is observed in the TO stars of the cluster.…”