The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables

Details The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables

2007-06-14

arxiv.org/abs/0706.2029v1

Astrophys.J.666:378-392,2007

Astronomy and Astrophysics

Astrophysics

41 pages including 11 figures and 4 tables; Accepted for publication in ApJ

Scientific paper

10.1086/520081

Abundance analyses from high-resolution optical spectra are presented for 19 Type II Cepheids in the Galactic field. The sample includes both short-period (BL Her) and long-period (W Vir) stars. This is the first extensive abundance analysis of these variables. The C, N, and O abundances with similar spreads for the BL Her and W Vir show evidence for an atmosphere contaminated with $3\alpha$-process and CN-cycling products. A notable anomaly of the BL Her stars is an overabundance of Na by a factor of about five relative to their presumed initial abundances. This overabundance is not seen in the W Vir stars. The abundance anomalies running from mild to extreme in W Vir stars but not seen in the BL Her stars are attributed to dust-gas separation that provides an atmosphere deficient in elements of high condensation temperature, notably Al, Ca, Sc, Ti, and $s$-process elements. Such anomalies have previously been seen among RV Tau stars which represent a long-period extension of the variability enjoyed by the Type II Cepheids. Comments are offered on how the contrasting abundance anomalies of BL Her and W Vir stars may be explained in terms of the stars' evolution from the blue horizontal branch.

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