2002AJ....124.1695B

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 π_abs=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 <A_V>=0.23±0.03 for δ Cep and hence an absolute magnitude M_V=-3.47±0.10. Adopting an average V magnitude, <V>=15.03±0.03, for Cepheids with logP=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.