On the pulsation parallax of the variable star RR Lyr

Abstract

We show that a straightforward application of the predicted relation connecting the absolute K magnitude of fundamental RR Lyrae variables to their period and metal content, together with current evolutionary predictions on the mass and luminosity of Horizontal Branch stars, supply a distance estimate to the prototype star RR Lyr in close agreement with the recent parallax determination by the Hubble Space Telescope (HST; Benedict et al.), largely unaffected by the interstellar extinction correction. Adopting <A_V>=0.12+/-0.10mag as a safe estimate of the extinction correction to RR Lyr, we obtain a `pulsation' parallax π_puls=3.858+/-0.131mas which agrees quite well, and with a smaller formal error, with the HST measurement (π_abs=3.82+/-0.20mas) and with the weighted average of HST, Hipparcos, and ground-based determinations (<π_abs>=3.87+/-0.19mas). This result suggests that near-infrared magnitudes and pulsation models could constrain the distance to well-studied RR Lyrae stars, both in the field and in globular clusters, with an accuracy better than current direct trigonometric measurements.