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 delta 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 Cep and reference star HD 213307 to belong to the same association (Cep OB6), we find pi(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 similar to7 mas distant from and similar to4 mag fainter than the primary. Spectral classifications and VRIJHKT(2)M and DDO51 photometry of the astrometric reference frame surrounding delta 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 Cep, HD 213307, is lower and nearly the same as for delta Cep. Correcting for color differences, we find [A(V)] = 0.23 +/- 0.03 for delta 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 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.