Models for extremely metal-poor halo stars

¹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85748 Garching, Federal Republic of Germany
² Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK
³ INAF – Osservatorio Astronomico Collurania, via Mentore Maggini, 64100 Teramo, Italy

Corresponding author: A. Weiss, weiss@mpa-garching.mpg.de

Received: 11 February 2004
Accepted: 2 April 2004

Abstract

Two alternative scenarios concerning the origin and evolution of extremely metal-poor halo stars are investigated. The first one assumes that the stars have been completely metal-free initially and produced observed carbon and nitrogen overabundances during the peculiar core helium flash typical of low-mass Population III stars. The second scenario assumes that the initial composition resulted from a mixture of primordial material with ejecta from a single primordial supernova. Both scenarios are shown to have problems in reproducing C, N, and O abundances simultaneously, and both disagree with observed -ratios, although in different directions. We concentrate on the most iron-poor, carbon-rich object of this class, HE 0107–5240, and conclude that the second scenario presently offers the more promising approach to understand these objects, in particular because evolutionary tracks match observations very well.