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Title: The Chemical Compositions of the Type II Cepheids-The BL Herculis and W Virginis Variables
Authors: Maas, T
Giridhar, S
Lambert, D. L
Keywords: stars: abundances—stars: variables
Issue Date: Sep-2007
Publisher: University of Chicago Press
Citation: The Astrophysical Journal, Vol. 666, pp. 378 – 392
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 3α 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.
Appears in Collections:IIAP Publications

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