The Chemical Compositions of the Type II Cepheids—The BL Herculis and W Virginis Variables

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 process and CN-cycling products. A notable anomaly of the BL Her stars is an overabundance of Na by a factor of about 5 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.

Type II Cepheids in the Milky Way disc

“…The depletion is more severe for DD Vel as the underabundances with respect to an average thick disc star reach ≈−0.30 dex for Ti, ≈−0.45 dex for Nd and Ca, ≈−0.80 dex for Sc and ≈−0.90 dex for Y. To further support the dust-gas separation, we note that most of the individual abundances fall below those of the thick-disc reference star for the elements with 50% T c > 1400 K. Hints or even clear evidence of dust-gas separation in Type II Cepheids have already been reported for ST Pup by and for CO Pup, V1711 Sgr, MZ Cyg, and SZ Mon by Maas et al (2007). On the other hand, these authors find the signature of dust-gas separation less convincing in RX Lib and W Vir because it relies mostly on the depletion in Sc.…”

“…As we discuss in more detail in the next paragraph, severe dust-gas separation has also been reported in most of the RV Tau stars (see Giridhar et al 2005, and references therein). Maas et al (2007) found a Type II Cepheid (CC Lyr) with an extreme dust-gas separation, which is larger than in any RV Tau star. It is important to note that all the Type II Cepheids with a signature of dust-gas separation are W Vir stars and not BL Her stars.…”

“…Surprisingly, Type II Cepheids did not receive much attention (see references in Maas et al 2007) after the early spectroscopic analyses of Rodgers & Bell (1963, 1968, Barker et al (1971), and Anderson & Kraft (1971). Indeed, modern high resolution spectroscopic studies have been limited to only a few stars: TX Del (Andrievsky et al 2002), ST Pup , V553 Cen , and RT Tra (Wallerstein et al 2000) before Maas et al (2007) analysed 19 BL Her and W Vir stars.…”

“…Indeed, modern high resolution spectroscopic studies have been limited to only a few stars: TX Del (Andrievsky et al 2002), ST Pup , V553 Cen , and RT Tra (Wallerstein et al 2000) before Maas et al (2007) analysed 19 BL Her and W Vir stars. The study of Maas et al (2007) is, to date, the unique extensive abundance analysis of Type II Cepheids. Amongst other results, they made three significant points regarding the classification of Type II Cepheids:…”

“…Since the Maas et al (2007) paper, only two new Type II Cepheids have been studied in detail: QQ Per by Wallerstein et al (2008) and W Vir by Kovtyukh et al (2011). After a brief description of the data in Sect.…”