New Evidence Supporting Cluster Membership for the Keystone Calibrator Delta Cephei

Abstract

New and existing UBVJHK_s , spectroscopic, NOMAD, Hubble Space Telescope, and revised Hipparcos observations are employed to determine properties for δ Cep and its host star cluster. The multi-faceted approach ensured that uncertainties were mitigated (σ/d ~ 2%). The following fundamental parameters were inferred for δ Cep: E(B - V) = 0.073 ± 0.018(σ), log τ = 7.9 ± 0.1, and d=272\,+/-\, 3(\sigma _{\bar{x}})\, +/-\, 5 (\sigma) pc. The cluster exhibits a turnoff near B6 (M _*/M _⊙ ~ 5), and the brightest host cluster members are the supergiants ζ Cep (K1.5Ib) and δ Cep. To within the uncertainties, the two stars share common astrometric parameters (π, μ_α, μ_δ, RV ~ -17 km s^-1) and are tied to bluer members via the evolutionary track implied by the cluster's UBVJHK_s color-color and color-magnitude diagrams. The cluster's existence is bolstered by the absence of an early-type sequence in color-magnitude diagrams for comparison fields. NOMAD data provided a means to identify potential cluster members (n ~ 30) and double the existing sample. That number could increase with forthcoming precise proper motions (DASCH) for fainter main-sequence stars associated with classical Cepheids (e.g., δ Cep), which may invariably foster efforts to strengthen the Galactic Cepheid calibration and reduce uncertainties tied to H ₀.