1999MNRAS.308...97D

We discuss an alternative approach to constrain the absolute bolometric luminosity of zero-age horizontal branch (ZAHB) structures by using the observational pulsational properties of ab-type RR Lyrae stars, theoretical relations connecting these properties to the evolutionary ones, i.e. luminosity, mass and effective temperature, and also the location in the Hertzsprung-Russell diagram of the instability strip boundaries for fundamental pulsators. As the main goal of this work is to obtain an evaluation of the ZAHB bolometric luminosity as independent of stellar evolution theory as possible, we have minimized the use of evolutionary prescriptions, the only adopted evolutionary input being the allowed mass range for fundamental pulsators. Nevertheless, the effects on our final results related to these evolutionary prescriptions have been carefully checked.

In order to test the accuracy of the current framework, we have carefully investigated the effective temperature scale provided by De Santis and adopted in the present work. It has been found that the pulsational colour-effective temperature scale fixed by this temperature scale and the relation between intrinsic (B-V)₀ colour, blue amplitude and metallicity given by Caputo & De Santis appear fully consistent with both theoretical colour transformations, given by Castelli, Gratton & Kurucz and Buser & Kurucz, and a semi-empirical one, provided by Green. In contrast, there is a large discrepancy between this pulsational colour-temperature scale and the one based on the Kurucz model atmospheres. The effects of adopting the temperature scale of Catelan, Sweigart & Borissova have been investigated.

The reliability of the method is shown by applying it to a selected sample of globular clusters (GCs), the heavy element abundances of which cover almost all the complete GC metallicity range. In order to verify the accuracy of the results obtained by using ab-type RR Lyrae stars, a similar analysis has also been performed by using both observational and theoretical evidence for first-overtone variables. The results obtained for the ZAHB bolometric luminosities have been critically analysed and a comparison with recent evolutionary prescriptions has also been performed. The existence of evident mismatches between current results and some evolutionary models has been verified and discussed.

Finally, our investigation has been extended to field variables to check for a luminosity difference between cluster RR Lyrae stars and field ones as suggested by Gratton. The comparison between the pulsational properties of field and cluster variables does not show the existence of any significant difference in their intrinsic luminosity, thus providing further support to the results obtained by Catelan.