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

TL;DR

This study provides the first extensive high-resolution spectral analysis of 19 Type II Cepheids, revealing distinct chemical abundance patterns linked to their evolutionary stages and dust-gas separation effects.

Contribution

It offers new detailed abundance data for both BL Her and W Vir stars, highlighting differences in element overabundances and dust-gas separation phenomena not previously documented.

Findings

BL Her stars show a fivefold Na overabundance.

W Vir stars exhibit abundance anomalies due to dust-gas separation.

Both groups show evidence of atmosphere contamination from nuclear processing.

Abstract

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.