Synthetic properties of bright metal-poor variables

Abstract

We present new grids of evolutionary models for the so-called "Anomalous" Cepheids (ACs), adopting Z = 0.0001 and various assumptions on the progenitor mass and mass-loss efficiency. These computations are combined with the results of our previous set of pulsation models and used to build synthetic populations of the predicted pulsators as well as to provide a Mass-Luminosity relation in the absence of mass-loss. We investigate the effect of mass-loss on the predicted boundaries of the instability strip and we find that the only significant dependence occurs in the Period-Magnitude plane, where the synthetic distribution of the pulsators is, on average, brighter by about 0.1 mag than the one in absence of mass-loss. Tight Period-Magnitude relations are derived in the K band for both fundamental and first overtone pulsators, providing a useful tool for distance evaluations with an intrinsic uncertainty of about 0.15 mag, which decreases to ∼0.04 mag if the mass term is taken into account. The constraints provided by the evolutionary models are used to derive evolutionary (i.e., mass-independent) Period-Magnitude-Color relations which provide distance determinations with a formal uncertainty of the order of ∼0.1 mag, once the intrinsic colors are well known. We also use model computations from the literature to investigate the effect of metal content both on the instability strip and on the evolutionary Period-Magnitude-Color relations. Finally, we compare our theoretical predictions with observed variables and we confirm that a secure identification of actual ACs requires simultaneous information on period, magnitude and color, that also provide constraints on the pulsation mode.

The ancient stellar population of M 32: RR Lyrae variable stars confirmed

“…It is not possible to make a clear classification between these two class of variables. We have plotted the cold (red) and the warm (blue) edges of the pulsation instability strip as derived by theoretical models (Fiorentino et al 2006). Top: wesenheit plane, where the separation between the anomalous Cepheids and Population II Cepheids is clearer.…”

Life in the Outer Limits: Insights on Hierarchical Assembly from Stellar Halos in the Local Universe

“…The scatter in Figure 6.4b is comparable to the scatter in the stellar mass of individual Anomalous Cepheids and observational errors for the mean optical magnitude (Fiorentino et al 2006). Pulsation mode cannot always be determined from the light curve alone (Bono et al 1997;Fiorentino et al 2006), but the two modes separate rather cleanly in the PLR diagram.…”

“…The scatter in Figure 6.4b is comparable to the scatter in the stellar mass of individual Anomalous Cepheids and observational errors for the mean optical magnitude (Fiorentino et al 2006). Pulsation mode cannot always be determined from the light curve alone (Bono et al 1997;Fiorentino et al 2006), but the two modes separate rather cleanly in the PLR diagram. Once identified, masses of individual stars can be derived from the theoretical periodmagnitude-amplitude (PMA) and period-magnitude-color relationships (PMC) for the given pulsation mode Marconi et al 2004;Fiorentino et al 2006).…”

“…Light curves, from which pulsation modes can be determined, will be fit to derive periods and amplitudes using methods developed by Layden et al (2000). The physical parameters of the variable stars can be derived using the NIR magnitudes, which better approximate the static luminosity, using theoretical Period-Magnitude-Amplitude and Period-Magnitude-Color relationships Marconi et al 2004;Fiorentino et al 2006). …”

“…Pulsation mode cannot always be determined from the light curve alone (Bono et al 1997;Fiorentino et al 2006), but the two modes separate rather cleanly in the PLR diagram. Once identified, masses of individual stars can be derived from the theoretical periodmagnitude-amplitude (PMA) and period-magnitude-color relationships (PMC) for the given pulsation mode Marconi et al 2004;Fiorentino et al 2006). These relationships exist for both optical and NIR bands and with our combined optical+NIR data we will be able to average estimates from multiple PMA and PMC relationships to reduce observational scatter.…”