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(σ_x̄) ± 5(σ) 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 H₀.