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 = 0.61. This provides a distance modulus for the Large Magellanic Cloud (LMC) of m - M = 18.38–18.53, 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.