Concerning the Classical Cepheid VIc Wesenheit Function's Strong Metallicity Dependence

Abstract

Evidence is presented which supports findings that the classical Cepheid VIc period-Wesenheit function is relatively insensitive to metallicity. The viability of a recently advocated strong metallicity dependence was evaluated by applying the proposed correction (gamma=-0.8 mag/dex) to distances established for the Magellanic Clouds via a Galactic VIc Wesenheit calibration, which is anchored to ten nearby classical Cepheids with measured HST parallaxes. The resulting gamma-corrected distances for the Magellanic Clouds (e.g., SMC, mu(0,gamma)~18.3) are in significant disagreement with that established from a mean of >300 published estimates (NED-D), and a universal Wesenheit template featuring eleven delta Scuti, SX Phe, RR Lyrae, and Type II Cepheid variables with HST/Hipparcos parallaxes. Conversely, adopting a null correction (i.e., gamma=0 mag/dex) consolidates the estimates. In tandem with existing evidence, the results imply that variations in chemical composition among Cepheids are a comparatively negligible source of uncertainty for W(VIc)-based extragalactic distances and determinations of H_0. A new approach is described which aims to provide additional Galactic Cepheid calibrators to facilitate subsequent assessments of the VIc Wesenheit function's relative (in)sensitivity to abundance changes. VVV/UKIDSS/2MASS JHKs photometry for clusters in spiral arms shall be employed to establish a precise galactic longitude-distance relation, which can be applied in certain cases to determine the absolute Wesenheit magnitudes for younger Cepheids.