The Absolute Magnitude and Kinematics of RR Lyrae Stars Via Statistical Parallax
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Abstract
We present new statistical parallax solutions for the absolute magnitude and kinematics of RR Lyrae stars. We have combined new proper motions from the Lick Northern Proper Motion program with new radial velocity and abundance measures to produce a data set that is 50% larger, and of higher quality, than the data sets employed by previous analyses. Based on an a priori kinematic study, we separated the stars into halo and thick disk sub-populations. We performed statistical parallax solutions on these sub-samples, and found Mv(RR)=+0.71±0.12 at <[Fe/H]>=-1.61 for the halo (162 stars), and Mv(RR)=+0.79±0.30 at <[Fe/H]>=-0.76 for the thick disk (51 stars). The solutions yielded a solar motion <V>=-2l0±12 km s-1 and velocity ellipsoid (σU,σV,σW) = (168±13, l02±8, 97±7)km s-1 for the halo. The values were <V>=-48±9km s-1 and (σU,σV,σW) = (56±8,51±8,31±5) km s-1 for the thick disk. Both are in good agreement with estimates of the halo and thick disk kinematics derived from both RR Lyrae stars and other stellar tracers. Monte Carlo simulations indicated that the solutions are accurate, and that the errors may be smaller than the estimates above. The simulations revealed a small bias in the disk solutions, and appropriate corrections were derived. The large uncertainty in the disk Mv(RR) prevents ascertaining the slope of the Mv(RR)-[Fe/H] relation. Using a zero point defined by our halo solution and adopting a slope of 0.15 mag dex-1, we find that (1) the distance to the Galactic Center is 7.6±0.4 kpc; (2) the mean age of the 17 oldest Galactic globular clusters is 16.522.1/11.9 Gyr; and (3) the distance modulus of the LMC is 18.28t 0.13 mag. Estimates of H0 which are based on an LMC distance modulus of 18.50 (e.g., Cepheid studies) increase by 10% if they are recalibrated to match, our LMC distance modulus.
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