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Session 108 - Cosmology and Distance Indicators.
Oral session, Thursday, January 16
Frontenac Ballroom,

[108.01] Metallicity Effects in the Cepheid P-L Relation

A. M. Fry, B. W. Carney (U. North Carolina)

I am working on a recalibration of the cepheid distance scale. In order to optimize the main sequence fitting that is done to calibrate the zero point of the P-L relation, it is necessary to know the difference in metallicity between the cluster being fitted and the Pleiades; a 0.1 dex difference in metallicity can result in an error of 0.1 mag in derived distance. I obtained 101 (each co-add of 3) high resolution, high S/N echelle spectra of 24 Galactic cepheids, including 15 of the cepheids in open clusters which have been used as primary calibrators of the P-L relation. Constancy of [Fe/H] at different pulsational phases, and hence temperatures, is an indication that the spectroscopically derived temperature scale is consistent. Agreement among the abundances derived for 2 dwarfs in M 25 and U Sgr, a cepheid in the same cluster is a check the applicability of static, LTE, plane-parallel stellar atmosphere models to pulsating supergiants such as cepheids. I find a spread of 0.4 dex in [Fe/H] and a spread of 0.2 dex in [\alpha/Fe] among the cepheids. I have obtained new photometry of 9 of the clusters containing cepheids to derive new main sequence fitting distances. I have UBVJK photometry of NGC 6087 (S Nor), NGC 129 (DL Cas), NGC 7790 (CE Cas A and B and CF Cas), and M 25 (U Sgr), UBV photometry of NGC 5662 (V Cen), and JK photometry of Lynga 6 (TW Nor), C1814-191 (WZ Sgr) and Trumpler 35 (RU Sct). When the photometry is completed, I will be able to derive distances for all of these clusters relative to the Pleiades with higher precision than previously achievable. Initial results will be discussed.

Program listing for Thursday