2006A&A...453..895W

The iron abundance of halo RR Lyrae stars can provide important information about the formation history of the Galactic halo. We determine the [Fe/H] of the sample of halo RRab stars by using the P-φ₃₁-[Fe/H] relation developed by Jurcsik & Kovacs (1996A&A...312..111J) based on their light curves. We need to extend the relation from the V band to our unfiltered CCD band. To do this, we use the low-dispersion spectroscopic [Fe/H] of literatures and the photometric data released by the first-generation Robotic Optical Transient Search Experiment (ROTSE-I) project. We do regression analyses for the calibrating sample using a linear function and test its validity by comparing of the predicted [Fe/H] with the spectroscopic [Fe/H]. In general, the fit accuracy for the two different [Fe/H] is better than 0.19dex. We derive an empirical P-φ₃₁-[Fe/H] linear relation for the unfiltered CCD band (ROTSE-I), i.e. [Fe/H]=-3.766-5.350P+1.044φ₃₁. In our test, the P-φ₃₁-[Fe/H] relation is also fit for our unfiltered CCD band. In addition, another linear relation, φ₃₁V_{=0.882+0.792φ}31_W, is also derived for the transformation between the V and W bands. We present the predicted [Fe/H] of the sample (the 31 halo RRab stars) in a catalog. The mean [Fe/H] of the sample is -1.63 with dispersion of 0.45 dex in distribution, which is consistent with the results derived from the blue horizontal branch star candidates by Kinnman et al. (2000, A&A, 364, 102). The mean [Fe/H] values of the RRab stars in the range of 1 kpc, 2 kpc, and 3 kpc from the star 91 (a double-mode RR Lyrae star), are all lower than that of the background halo stars. These values are consistent with that of star 91 suggested by Wu et al. (2005AJ....130.1640W), which indicates they might have a common origin.