Beryllium in Ultra-Lithium-Deficient Halo Stars: The Blue Straggler Connection

• Published in: The Astrophysical journal Vol. 667; no. 2; pp. 1196 - 1205
• Main Author: Boesgaard, Ann Merchant
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 01-10-2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Galaxy halo; Late type stars; Blue stragglers; Population II; stars: evolution; Stellar abundance; Stellar evolution; Subdwarf; stars: abundances; Galaxy: halo; stars: Population II; stars: late-type; subdwarfs
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/521075

There exists a small group of metal-deficient stars that have Li abundances well below the Li plateau that is defined by over 100 unevolved stars with temperatures above 5800 K and values of [Fe/H] < -1.0. Abundances of Be have been determined for most of these ultra-Li-deficient stars in order to investigate the cause of the Li deficiencies. These Li-deficient stars have implications on the value of primordial Li. High-resolution and high signal-to-noise ratio spectra have been obtained in the Be II spectral region near 3130 AA for six ultra-Li-deflcient stars with the Keck I telescope and its new UV-sensitive CCD on the upgraded HIRES. The spectrum synthesis technique has been used to determine Be abundances. All six stars are found to have Be deficiencies also. Two have measurable but reduced Be, and four have only upper limits on Be. These results are consistent with the idea that these Li- and Be-deficient stars are analogous to blue stragglers. The stars have undergone mass transfer events (or mergers) that destroy or dilute both Li and Be. The findings cannot be matched by the models that predict that the deficiencies are due to extramixing in a subset of halo stars that were initially rapid rotators, with the possible exception of one star, G139-8. Because the ultra-Li-deficient stars are also Be-deficient, they appear to be genuine outliers in the population of halo stars used to determine the Li plateau in that they no longer have the Li in their atmospheres that was produced in the big bang.