The Mount Wilson Observatory Metallicity Index, C RV: Comparison with Other Photometric Systems

Abstract

A new spectrophotometric index, C<SUB>RV</SUB>, is assessed as a metallicity indicator for late-type stars. The index is the ratio of the measured photospheric fluxes in 20 Å wide passbands centered at 400t and 3901 Å. C<SUB>RV</SUB> correlates directly with the metallicity index, m<SUB>1</SUB>, of the Strömgren uvby system and with the metallicity index hk, of Anthony-Twarog et al. (1991). <P />Using observations of 236 dwarfs and 140 giants combined with stellar atmosphere models (Kurucz 1991), we compared the sensitivity of the C<SUB>RV</SUB>, m<SUB>1</SUB>, and hk indices to metal abundance. We also studied the sensitivity of the C<SUB>RV</SUB>, C<SUB>1</SUB>, and hk indices to surface gravity. The effect of interstellar extinction on all the indices was also studied from published mean extinction laws. <P />We find that the C<SUB>RV</SUB> index is sensitive to the variation of metal abundance, [M], over the range examined (-5.0 ≲ [M] ≲ 0.5). C<SUB>RV</SUB> is also more sensitive than the m<SUB>1</SUB> index at metal-poor conditions ([M] ≲ -2.0). The C<SUB>RV</SUB> index has the following advantages: (1) the passbands of C<SUB>RV</SUB> are dominated by Fe lines, which reduce the uncertainty that may be introduced by the presence of lines of α-process elements with enhanced abundances at metal-poor conditions; (2) the effect of interstellar reddening is limited because the two passbands are separated in wavelength by only 100 Å. We also find that the atmospheric models produce results that agree qualitatively with the trends of observed indices on stellar parameters such as effective temperature, metallicity, and surface gravity.