The distance to NGC 6397 by M-subdwarf main-sequence fitting

Recent years have seen a substantial improvement both in photometry of low- luminosity stars in globular clusters and in modeling the stellar atmospher es of late-type dwarfs. We build on these observational and theoretical adv ances in undertaking the first determination of the distance to a globular cluster by main-sequence fitting using stars on the lower main sequence. Th e calibrating stars are extreme M subdwarfs, as classified by Gizis, with p arallaxes measured to a precision of better than 10%. Matching against King et al.'s deep [V (V-I)] photometry of NGC 6397, and adopting EB-V = 0.18 m ag, we derive a true distance modulus of 12.13 +/- 0.15 mag for the cluster . This compares with (m - M)(0) = 12.24 +/- 0.1 derived through conventiona l main-sequence fitting in the [V, (B - V)] plane. Allowing for intrinsic d ifferences due to chemical composition, we derive a relative distance modul us of delta(m - M)(0) = 2.58 mag between NGC 6397 and the fiducial metal-po or cluster M92. we extend this calibration to other metal-poor clusters and examine the resulting RR Lyrae (M-V, [Fe/H]) relation.