2007A&A...476..779B

This work uses nonlinear convective models of RR Lyrae stars and evolutionary predictions of low-mass helium burning stellar structures to constrain the properties of cluster and field RR Lyrae variables. In particular, we address two problems: is the Period-Amplitude (PA_V) plane of fundamental (RR_ab) variables a good diagnostic for the metal abundance? Is the M_V(RR)-[Fe/H] relation of field and cluster variables linear over the whole metal abundance range of [Fe/H]~-2.5 to ∼0? We perform a detailed comparison between theory and observations for fundamental RR Lyrae variables in the solar neighborhood and in both Oosterhoff type I (OoI) and type II (OoII) Galactic globular clusters. We show that the distribution of cluster RR_ab variables in the PA_V plane depends not only on the metal abundance, but also on the cluster Horizontal Branch (HB) morphology. We find that on average the observed pulsation parameter k_puls connecting the period to the visual amplitude increases when moving from metal-poor to metal-rich GGCs. However, this parameter shows marginal changes among OoI clusters with intermediate to red HB types and iron abundances -1.8≤[Fe/H]≤-1.1, whereas its value decreases in OoII clusters with the bluer HB morphology, although these clusters are also the less metal-poor ones of the group. Moreover, at [Fe/H]=-1.7±0.1 the OoI clusters present redder HB types and larger <k_puls> values than the OoII clusters. The RR_ab variables in ω Cen and in the solar neighborhood further support the evidence that the spread in [Fe/H], at fixed k_puls, is of the order of ±0.5dex. Using the results of synthetic HB simulations, we show that the PA_V plane can provide accurate cluster distance estimates. We find that the RR_ab variables in OoI and in OoII clusters with very blue HB types obey a well-defined M_V(RR)-k_puls relation, while those in OoII clusters with moderately blue HB types present a zero-point that is ∼0.05mag brighter. Regarding field variables, we show that with [Fe/H]≥-1.0 a unique M_V(RR)-k_puls relation can be adopted, independently of the color distribution of the parent HB star population. Current findings suggest that the PA_V distribution is not a robust diagnostic for the metal abundance of RR_ab variables. However, the same observables can be used to estimate the absolute magnitude of globular cluster and field RR_ab variables. We show that over the metallicity range -2.4≤[Fe/H]≤0.0 the M_V(RR)-[Fe/H] relation is not linear but has a parabolic behavior.