On the period distribution of cluster RR Lyrae stars to constrain their helium content: the case of omega Centauri

Abstract

We present new sets of nonlinear, time-dependent convective hydrodynamical models of RR Lyrae stars assuming two metal (Z=0.0005, Z=0.001) and three helium abundances (Y=0.24, 0.30, 0.38). For each chemical composition we constructed a grid of fundamental (FU) and first overtone (FO) models covering a broad range of stellar masses and luminosities. To constrain the impact of the helium content on RR Lyrae properties, we adopted two observables --period distribution, luminosity amplitudes-- that are independent of distance and reddening. The current predictions confirm that the helium content has a marginal effect on the pulsation properties. The key parameter causing the difference between canonical and He-enhanced observables is the luminosity. We compared current predictions with the sample of 189 RR Lyrae stars in omega Cen and we found that the period range of He-enhanced models is systematically longer than observed. These findings apply to metal-poor and metal-intermediate He-enhanced models. To further constrain the impact of He-enhanced structures on the period distribution we also computed a series of synthetic HB models and we found that the predicted period distribution, based on a Gaussian sampling in mass, agrees quite well with observations. This applies not only to the minimum fundamentalized period of RR Lyrae stars (0.39 vs 0.34 day), but also to the fraction of Type II Cepheids (2% vs 3%). We also computed a series of synthetic HB models assuming a mixed HB population in which the 80% is made of canonical HB structures, while the 20% is made of He-enhanced (Y=0.30) HB structures. We found that the fraction of Type II Cepheids predicted by these models is almost a factor of two larger than observed (5% vs 3%). This indicates that the fraction of He-enhanced structures in omega Cen cannot be larger than 20%.

accepted for publication on ApJ

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• On a new theoretical framework for RR Lyrae stars II: Mid--Infrared Period--Luminosity--Metallicity Relations
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• On the Impact of Helium Content on the RR Lyrae Distance Scale
• The ancient stellar population of M32: RR Lyr Variable stars confirmed
• On the separation between RR Lyrae and Type II Cepheids and their importance for distance determination: the case of $\omega$ Cen
• An Updated Census of RR~Lyrae Stars in the Globular Cluster $\omega$~Centauri (NGC\,5139)
• Gauging the Helium Abundance of the Galactic Bulge RR Lyrae Stars
• The Carina Project. VI. The helium burning variable stars
• A theoretical framework of BL Her stars. I. Effect of metallicity and convection parameters on period-luminosity and period-radius relations
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• On the variation of light curve parameters of RR Lyrae variables at multiple wavelengths
• Near-IR period-luminosity relations for pulsating stars in $\omega$ Centauri (NGC 5139)
• The Horizontal Branch of NGC 1851: Constraints from its RR Lyrae Variables
• A stellar population synthesis approach to the Oosterhoff dichotomy
• Predicting light curves of RR Lyrae variables using artificial neural network based interpolation of a grid of pulsation models
• LAMOST medium-resolution spectroscopic survey of binarity and exotic star (LAMOST-MRS-B): Observation strategy and target selection
• A theoretical framework for BL Her stars IV. New period-luminosity relations in the Rubin-LSST filters