Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR Lyrae

We present an absolute parallax and relative proper motion for the fundamental distance scale calibrator, RR Lyrae. We obtain these with astrometric data from FGS 3, a white-light interferometer on the Hubble Space Telescope. We find π_abs=3.82+/-0.2 mas. Spectral classifications and VRIJHKT₂M and DDO 51 photometry of the astrometric reference frame surrounding RR Lyr indicate that field extinction is low along this line of sight. We estimate <A_V>=0.07+/-0.03 for these reference stars. The extinction suffered by RR Lyr becomes one of the dominant contributors to the uncertainty in its absolute magnitude. Adopting theaverage field absorption <A_V>=0.07+/-0.03, we obtain M^RR_V=0.61^-0.11_+0.10. This provides a distance modulus for the Large Magellanic Cloud (LMC) of m-M=18.38-18.53^-0.11_+0.10, with the average extinction-corrected magnitude of RR Lyrae variables in the LMC, <V(RR)>, remaining a significant uncertainty. We compare this result with more than 80 other determinations of the distance modulus of the LMC. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.