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| Paper: |
Period Change Rate and Chemical Composition of the Anomalous Cepheid V19 in NGC 5466 |
| Volume: |
135, A Half Century of Stellar Pulsation Interpretations: a Tribute to Arthur N. Cox |
| Page: |
57 |
| Authors: |
Nemec, J.; McCarthy, J. K. |
| Abstract: |
A detailed model atmosphere and chemical composition analysis has been made of the brightest known Anomalous Cepheid (AC), V19 in the globular cluster NGC 5466. The study is based on new CCD photometry and echelle spectrograms acquired at minimum light with the HIRES spectrograph on the 10-m Keck-I telescope. Atmospheric parameters and a new mass estimate are presented. EW measurements of 82 iron lines give [Fe/H] = - -1.92 1 0.03 dex, and other iron-peak elements (Cr, Mn, Fe, and Ni) give a mean metallicity of [M/H] = -1.90 1 0.15 dex. Abundances of other elements (V, Co, Cu Zn, Ba, La, Ce, Nd, Mg, Si, S, Ti) will also be discussed. In general the abundance ratios to iron closely match the normal, well-established pattern for metal-poor stars. V 19Us pulsation period is found to be P = 0.8213010 +/- 0.0000003 day (in 1996) and decreasing at the rate dP/dt = -0.28 +/0 0.19 d/Myr, implying evolution towards a hotter temperature. The residuals in the O-C diagram suggest a possible long-term (Porb ~ 10,000 day) binary motion. The line widths in the spectrum of V19 are quite narrow, with v sin(i) < 18 km/s. Given that V19 is the first AC to be observed at such high spectroscopic resolution, the rotational velocity is significant in view of the hypothesis that ACs are coalesced binaries. The upper limit on v sin(i) for V19 appears to contradict this hypothesis, unless sin(i) is very small. Finally, radial velocities derived from each HIRES spectrogram are smaller than would be expected at minimum light based on previous, though much less precise, radial velocity measurements. The lack of a detectable change in radial velocity over 311 days, if reinforced by continued monitoring, calls into question the evolutionary scenerio explaining V19 as the result of mass transfer in a close binary system presently composed of an AC plus a white dwarf. If time permits I will also briefly discuss CCD photometry of the double-mode RR Lyrae stars in the globular cluster IC 4499, and new variable blue stragglers in the outer regions of the globular cluster M3. |
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