New Evidence Supporting Cluster Membership for the Keystone Calibrator Delta Cephei

TL;DR

This study combines multi-source observations to confirm delta Cephei's membership in its host star cluster, refining key parameters and supporting its role as a calibrator for cosmic distance measurements.

Contribution

It provides new evidence for delta Cephei's cluster membership using diverse observational data, improving parameter estimates and aiding in calibrating the cosmic distance scale.

Findings

Delta Cephei's distance is 272+-3(sem)+-5(sd) pc.

The cluster's turnoff is near B6, with key members zeta Cep and delta Cep sharing properties.

The study doubles the known cluster members, enhancing calibration efforts.

Abstract

New and existing UBVJHKs, spectroscopic, NOMAD, HST, and revised HIP observations are employed to determine properties for delta Cep and its host star cluster. The multi-faceted approach ensured that uncertainties were mitigated (~2%). The following fundamental parameters were inferred for delta Cep: E(B-V)=0.073+-0.018, log(t)=7.9+-0.1, and d=272+-3(se)+-5(sd) pc. The cluster exhibits a turnoff near B6 (M*/Ms~5), and the brightest host cluster members are the supergiants zeta Cep (K1.5Ib) and delta Cep. To within the uncertainties, the two stars share common astrometric parameters (pi, mu_ra, mu_dec, RV\sim-17 km/s) and are tied to bluer members via the evolutionary track implied by the cluster's UBVJHKs 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., delta Cep), which may invariably foster efforts to strengthen the Galactic Cepheid calibration and reduce uncertainties tied to H_0.