The RR Lyrae star period - K-band luminosity relation of the globular cluster M 3

That the RR Lyrae star period - K-band luminosity relation is a promising tool as a distance indicator in the Milky Way and Local Group of galaxies is apparent on observational and theoretical grounds in the literature. Less clear is the sensitivity of the relation, and consequently the physics of horizontal branch stars, to differences in stellar environment. In this paper, the first measurement of the (fundamental) period - K-band luminosity relation for the central region of a globular cluster is presented. It is based on a sample of seven RR Lyrae stars imaged with adaptive optics. In addition, the relation for the outer region has been reanalyzed, and is found to be in good agreement with both the previous estimate by Longmore et al. (1990), and with the inner region relation, especially when irregular and double-mode RR Lyrae stars are excluded. Importantly, there is no difference between the slope of the inner and outer region relation within measurement uncertainties, suggesting no difference in evolutionary state (luminosity). Taking the M 3 distance modulus as 15.0 +- 0.07 mag, the period-absolute K-band magnitude relation derived by linear least squares fitting is: … for the inner region. Excluding irregular variable stars, the outer region relation is: … . This good agreement provides further strong support for the near-IR period-luminosity relation as a distance indicator.