The Rr Lyrae Distance Scale

We review seven methods of measuring the absolute magnitude M V of RR Lyrae stars in light of the Hipparcos mission and other recent developments. We focus on identifying possible systematic errors and rank the methods by relative immunity to such errors. For the three most robust methods, statistical parallax, trigonometric parallax, and cluster kinemat-ics, we find M V (at [Fe/H]= −1.6) of 0.77 ± 0.13, 0.71 ± 0.15, 0.67 ± 0.10. These methods cluster consistently around 0.71±0.07. We find that Baade-Wesselink and theoretical models both yield a broad range of possible values (0.45–0.70 and 0.45–0.65) due to systematic uncertainties in the temperature scale and input physics. Main-sequence fitting gives a much brighter M V = 0.45 ± 0.04 but this may be due to a difference in the metallic-ity scales of the cluster giants and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67 ± 0.13 and is potentially very robust, but at present is too new to be fully tested for systematics. If the three most robust methods are combined with Walker's mean measurement for 6 LMC clusters, V 0,LMC = 18.98 ± 0.03 at [Fe/H]=−1.9, then µ LMC = 18.33 ± 0.08.