The interstellar Na I strength versus reddening relationship : its incidence on stellar population synthesis

Abstract

We study the equivalent width W of the Na I D lines in 41 globular clusters with reddening O ≤ (B-V) ≤ 1.11 and a wide spatial distribution in the Galaxy. Contributions to the line absorption from the stellar and interstellar components are separated via comparisons with the Ca II K and the Mg I b lines. The value W(Na I) is found to be enhanced for reddened clusters. The relationship between interstellar Na I and E(B-V) presents a steeper slope for globular clusters than that derived for stars in the solar neighborhood. In view of stellar population synthesis in galaxies, we study the age and metallicity effects in the W(Na I) vs W(Mg I) plane by means of 15 Magellanic Cloud and 3 Galactic open clusters. For solar and lower than solar metallicities a large excess E(Na I) can be produced only by interstellar line absorption. We also analyze 154 normal and 11 active galactic nuclei. We find that the Na I line is enhanced with galaxy inclination in spirals. Very metallic early type galaxies deviate from linear relationship in the W(Na I) vs W(Mg I) plane. This excess of the Na I absorption can be equally accounted for by, (i) an interstellar contribution from the central regions and/or an extended halo compatible with the existing H I detections in early type galaxies, and (ii) an enhancement of W(Na I) with respect to W(Mg I) in the atmos­ pheres of very metal rich late type stars as suggested by synthetic spectra computations.