Iron abundances derived from RR Lyrae light curves and low-dispersion spectroscopy

Abstract

With the aid of the All Sky Automated Survey (ASAS) database on the Galactic field, we compare the iron abundances of fundamental mode RR Lyrae stars derived from the Fourier parameters with those obtained from low-dispersion spectroscopy. We show from a set of 79 stars, distinct from the original calibrating sample of the Fourier method and selected without quality control, that almost all discrepant estimates are the results of some defects or peculiarities either in the photometry or in the spectroscopy. Omitting objects deviating by more than 0.4dex, the remaining subsample of 64 stars yields Fourier abundances that fit the spectroscopic ones with a standard deviation of 0.20dex. Other, more stringent selection criteria and different Fourier decompositions lead to smaller subsamples and concomitant better agreement, down to 0.16dex standard deviation. Except perhaps for two variables among the 163 stars, comprised of the ASAS variables and those of the original calibrating set of the Fourier method, all discrepant values can be accounted for by observational noise and insufficient data coverage. We suggest that the agreement can be further improved when new, more accurate spectroscopic data become available for a test with the best photometric data. As a by-product of this analysis, we also compute revised periods and select Blazhko variables.