AAS 201st Meeting, January, 2003
Session 17. Stellar Evolution and Metal-Poor Stars
Poster, Monday, January 6, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[17.05] The Halo Distance Scale: A Statistical Parallax Analysis

M. M. Brewer, B. W. Carney (UNC), D. W. Latham (CfA), J. B. Laird (Bowling Green State Univ.), L. Aguilar (UNAM)

We present a statistical parallax study of main sequence metal-poor field stars to calibrate the zero point of the RR Lyrae distance scale. Kinematic and photometric data of 620 dwarfs with [Fe/H] \leq\ -1.0 were taken from the samples of Carney & Latham (1987) and Ryan & Norris (1991). All stars have metallicites determined from high-resolution, low-S/N spectra. To determine relative MV values and hence relative distances given observed V0 magnitudes, we employed (B-V)0 colors with model isochrones and the globular cluster ridge lines of M5, M3, M92, and M30 to bring all stars of a given metallicity to a common relative distance. To interpolate as a function of relative metallicity, we adopted a relation between MV(RR) and [Fe/H] with a slope of 0.20 mag/dex for the calibrating clusters and isochrones. Following Hawley's (1986) method of statistical parallax using maximum likelihood estimation, a distance was found for the reference color on the main sequence of M5. The average de-reddened V magnitude of the RR Lyrae variables in M5 then gives a zero point for the RR Lyrae magnitude-metallicity relation. We find MV(RR) = 0.64 ±0.09 at [Fe/H]~=~-1.17. The stars from the Carney and Latham sample were also analyzed with statistical weights that correct for the bias introduced from a proper motion selected sample. The weighted sample gives MV(RR) = 0.68 ±0.09 at the metallicity of M5.

We thank the NSF for finanical support of this work at UNC.

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.