RR Lyrae stars in Galactic globular clusters

Free access article

Issue		A&A Volume 476, Number 2, December III 2007


Page(s)		779 - 790
Section		Galactic structure, stellar clusters, and populations
DOI		http://dx.doi.org/10.1051/0004-6361:20078206

A&A 476, 779-790 (2007)
DOI: 10.1051/0004-6361:20078206

VI. The period-amplitude relation

G. Bono^{1, 2}, F. Caputo¹, and M. Di Criscienzo^{1, 3}

¹ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
e-mail: [bono;caputo;dicriscienzo]@mporzio.astro.it
² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei Munchen, Germany
³ INAF-Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy

(Received 2 July 2007 / Accepted 18 September 2007 )

Abstract
Aims.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 $]\sim -$ 2.5 to ~0?
Methods.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.
Results.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 $k_{\rm puls}$ 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 $-1.8\le[$ Fe/H $]\le-1.1$ , 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 $]=-1.7\pm0.1$ the OoI clusters present redder HB types and larger $\langle k_{\rm puls}\rangle$ values than the OoII clusters. The RR_ab variables in $\omega$ Cen and in the solar neighborhood further support the evidence that the spread in [Fe/H], at fixed $k_{\rm puls}$ , is of the order of $\pm$ 0.5 dex. 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)- $k_{\rm puls}$ relation, while those in OoII clusters with moderately blue HB types present a zero-point that is ~0.05 mag brighter. Regarding field variables, we show that with [ Fe/H $]\ge -$ 1.0 a unique M_V(RR)- $k_{\rm puls}$ relation can be adopted, independently of the color distribution of the parent HB star population.
Conclusions. 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 $-2.4\le[{\rm Fe/H}]\le 0.0$ the M_V(RR)-[Fe/H] relation is not linear but has a parabolic behavior.

Key words: Galaxy: globular clusters: general -- stars: evolution -- stars: horizontal-branch -- stars: oscillations -- stars: variables: RR Lyr

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