* Astronomy

Blobrana · L

Type II Cepheids

Blobrana · L

Title: Type II Cepheids as Extragalactic Distance Candles
Authors: Daniel J. Majaess, David G. Turner, David J. Lane

Potential Type II Cepheids are identified in surveys of the galaxies IC 1613, M33, M101, M106, M31, NGC 4603, and the SMC, complementing previous discoveries of Type II Cepheids in NGC 5128, NGC 3198, and the LMC. The most distant Type II Cepheid observed may be associated with NGC 4603 (d~29 Mpc). Preliminary results confirm that Type II Cepheids could play an important role as standard candles, in constraining the effects of metallicity on Cepheid distances to galaxies, and in mapping extinction.

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Blobrana · L

Title: The Chemical Compositions of the Type II Cepheids -- The BL Her and W Vir Variables
Authors: Thomas Maas (1), Sunetra Giridhar (2), David L. Lambert (1) ((1) The W.J. McDonald Observatory (2) Indian Institute of Astrophysics)

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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