The Absolute Magnitude Of RRc Variables From Statistical Parallax

Abstract

We present the first definitive measurement of the absolute magnitude of RR Lyrae c-type variable stars (RRc) determined purely from statistical parallax. We use a sample of 242 RRc variables selected from the All Sky Automated Survey for which high-quality light curves, photometry, and proper motions are available. We obtain high-resolution echelle spectra for these objects to determine radial velocities and abundances as part of the Carnegie RR Lyrae Survey. We find that M-V,M-RRc = 0.59 +/- 0.10 at a mean metallicity of [Fe/H] = -1.59. This is to be compared with previous estimates for RRab stars (M-V,M-RRab = 0.76 +/- 0.12) and the only direct measurement of an RRc absolutemagnitude (RZ Cephei, M-V,M-RRc = 0.27 +/- 0.17). We find the bulk velocity of the halo relative to the Sun to be (W-pi, W-theta, W-z) = (12.0,-209.9, 3.0) km s(-1) in the radial, rotational, and vertical directions with dispersions (sigma(W pi), sigma(W theta), sigma(Wz)) = (150.4, 106.1, 96.0) km s(-1). For the disk, we find (W-pi, W-theta, W-z) = (13.0,-42.0,-27.3) km s(-1) relative to the Sun with dispersions (sigma(W pi), sigma(W theta), sigma(Wz)) = (67.7, 59.2, 54.9) km s(-1). Finally, as a byproduct of our statistical framework, we are able to demonstrate that UCAC2 proper-motion errors are significantly overestimated as verified by UCAC4.