Toward more accurate RR Lyrae metallicities

For the 197 stars with multiple spectra we find overall [Fe/H] standard deviations of 0.167 (as published), 0.145 (shifted by data source zero points) and 0.121 (both zero point shifted and corrected to the static gravity).

These improvements are significant at the ~2 sigma level at each correction step, leading to a clearly significant improvement after both corrections applied. The higher quality of the gravity-corrected metallicities is strongly supported also by the tighter correlation with the metallicities predicted from the period and Fourier phase phi_31_. This work underlines the need for using some external estimates of the temporal gravity in the chemical abundance analysis rather than relying on a full-fetched spectrum fit that leads to large correlated errors in the estimated parameters.