RR Lyrae and Type II Cepheid Variables Adhere to a Common Distance Relation

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Daniel J. Majaess

Saint Mary’s University, Department of Astronomy and Physics, Halifax, Nova Scotia, B3H 3C3, Canada
The Abbey Ridge Observatory, Stillwater Lake, Nova Scotia, Canada


Received October 30, 2009; revised December 21, 2009; accepted December 21, 2009

Preliminary evidence is presented reaffirming that SX Phe, RR Lyrae, and Type II Cepheid variables may be characterized by a common Wesenheit period-magnitude relation, to first order. Reliable distance estimates to RR Lyrae variables and Type II Cepheids are ascertained from a single VI-based reddening- free relation derived recently from OGLE photometry of LMC Type II Cepheids. Distances are computed to RR Lyrae (d ≅260 pc) and variables of its class in the galaxies IC 1613, M33, Fornax dSph, LMC, SMC, and the globular clusters M3, M15, M54, ω Cen, NGC 6441, and M92. The results are consistent with literature estimates, and in the particular cases of the SMC, M33, and IC 1613, the distances agree with those inferred from classical Cepheids to within the uncertainties: no corrections were applied to account for differences in metallicity. Moreover, no significant correlation was observed between the distances computed to RR Lyrae variables in ω Cen and their metallicity, despite a considerable spread in abundance across the sample. In sum, concerns regarding a sizeable metallicity effect are allayed when employing VI-based reddening-free Cepheid and RR Lyrae relations.

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