Abstract
The chemical composition of 28 Population II Cepheids and one RR Lyrae variable has been studied using high-resolution spectra. The chemical composition of W Vir variable stars (with periods longer than 8 d) is typical for the halo and thick disc stars. However, the chemical composition of BL Her variables (with periods of 0.8-4 d) is drastically different, although it does not differ essentially from that of the stars belonging to globular clusters. In particular, the sodium overabundance ([Na/Fe] ≈ 0.4) is reported for most of these stars, and the Na-O anticorrelation is also possible. The evolutionary tracks for BL Her variables (with a progenitor mass value of 0.8 solar masses) indicate that mostly helium-overabundant stars (Y = 0.30-0.35) can fall into the instability strip region. We suppose that it is the helium overabundance that accounts not only for the existence of BL Her variable stars but also for the observed abnormalities in the chemical composition of this small group of pulsating variables.
Highlights
Type II Cepheids are the stars that fall into the instability strip between RR Lyrae and RV Tau stars (Wallerstein 2002; Feast et al 2008)
T2C are divided into sub-classes: the BL Her stars with pulsation period of 1–4 d and W Vir stars with pulsation period of 4–20 d (Soszynski et al 2008)
Nitrogen, oxygen, and sodium were treated under NLTE approximation
Summary
Type II Cepheids (here after T2C) are the stars that fall into the instability strip between RR Lyrae and RV Tau stars (Wallerstein 2002; Feast et al 2008). To extend the analysis of metallicities and relative atomic abundances, Maas et al (2007) analysed spectra of 19 halo and thick disc Cepheids, with periods ranging from 1.1 to 28.6 d and a range of metallicity from solar to [Fe/H] = −2 with one exceptional object near −4.0, which they noted to be similar to post-AGB stars (van Winckel 2003). Of particular interest is the presence of many BL Her stars with near solar metallicity despite the absence of such stars in globulars like 47 Tuc. Exceptions may be the peculiar clusters NGC 6388 and 6441, which have metallicities similar to that of 47 Tuc and unusually long period RR Lyrae stars. The description for individual stars and remarks on their classification as T2C are provided
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