A pulsational approach to near-infrared and visual magnitudes of RR Lyr stars

In this paper, we present an improved theoretical scenario concerning near-infrared and visual magnitudes of RR Lyr variables, as based on up-to-date pulsating models. New relations connecting V and K absolute magnitudes with periods, mass, luminosity and metal content are discussed separately for fundamental and first-overtone pulsators. We also show that the V-K colours are predicted to supply tight constraints on the pulsator intrinsic luminosity. On this basis, we revisit the case of the prototype variable RR Lyr, showing that the parallax inferred by this new pulsational approach appears in close agreement with Hubble Space Telescope absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyr variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity (M_K-[Fe/H]-logP) as well as a new calibration of the M_V-[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyr stars in the field and in galactic globular clusters (GGCs) should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of the current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade-Wesselink (BW) method suggest relations with a zero-point which is fainter than is predicted by pulsation models, together with a milder metallicity dependence. However, preliminary results based on a new calibration of the BW method provided by Cacciari et al. (2000) for RR Cet and SW And appear in a much better agreement with the pulsational predictions.