Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator δ Cephei

We present an absolute parallax and relative proper motion for the fundamental distance scale calibrator δ Cep. We obtain these with astrometric data from FGS 3, a white-light interferometer on the Hubble Space Telescope (HST). Utilizing spectrophotometric estimates of the absolute parallaxes of our astrometric reference stars and constraining δ Cep and reference star HD 213307 to belong to the same association (Cep OB6), we find πabs = 3.66 ± 0.15 mas. The larger than typical astrometric residuals for the nearby astrometric reference star HD 213307 are found to satisfy Keplerian motion with P = 1.07 ± 0.02 yr, a perturbation and period that could be due to an F0 V companion ∼7 mas distant from and ∼4 mag fainter than the primary. Spectral classifications and VRIJHKT2M and DDO51 photometry of the astrometric reference frame surrounding δ Cep indicate that field extinction is high and variable along this line of sight. However the extinction suffered by the reference star nearest (in angular separation and distance) to δ Cep, HD 213307, is lower and nearly the same as for δ Cep. Correcting for color differences, we find ⟨AV⟩ = 0.23 ± 0.03 for δ Cep and hence an absolute magnitude MV = -3.47 ± 0.10. Adopting an average V magnitude, ⟨V⟩ = 15.03 ± 0.03, for Cepheids with log P = 0.73 in the large Magellanic Cloud (LMC) from Udalski et al., we find a V-band distance modulus for the LMC, m - M = 18.50 ± 0.13, or 18.58 ± 0.15, where the latter value results from a highly uncertain metallicity correction. These agree with our previous RR Lyr HST parallax-based determination of the distance modulus of the LMC.