Concerning the classical Cepheid VI[subscript C] Wesenheit function's strong metallicity dependence

Abstract

Evidence is presented which supports findings that the classical Cepheid VI_C period Wesenheit function is relatively insensitive to metallicity. The viability of a recently advocated strong metallicity dependence was evaluated by applying the proposed correction (γ = −0.8 mag dex^−1) to distances established for the Magellanic Clouds via a Galactic VI_C Wesenheit calibration, which is anchored to 10 nearby classical Cepheids with measured Hubble Space Telescope (HST) parallaxes. The resulting γ-corrected distances for the Magellanic Clouds (e.g., Small Magellanic Cloud, μ0,γ ∼ 18.3) are in significant disagreement with that established from a mean of >300 published estimates (NED-D), and a universal Wesenheit template featuring 11 δ Scuti, SX Phe, RR Lyrae, and Type II Cepheid variables with HST/Hipparcos parallaxes. Conversely, adopting a null correction (i.e., γ = 0 mag dex^−1) 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 H₀. A new approach is described which aims to provide additional Galactic Cepheid calibrators to facilitate subsequent assessments of the VI_C Wesenheit function's relative (in) sensitivity to abundance changes. VVV/UKIDSS/Two Micron All Sky Survey JHK_s 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.