A comparative study of Na I and Ca II infrared lines in stars, star clusters and galaxy nuclei : an alternative to the dwarf-enriched population

Abstract

We make a comparative study of the near-infrared Na I doublet and the Ca II triplet, among sets ofstars, star clusters and galaxy nuclei. In particular, we study the semistellar nucleus of M31. We find evidence that the enhancement of the absorption feature around 8200 Å in galaxy nuclei is not due to a strengthening of the Na I lines, but rather to another absorber, possibly a molecular band at λ ~ 8205 Å which varies strongly with θеff and metallicity. Therefore, an alternative explanation to the classical discussion about dwarf/giant star content in M31, is that of an increased metallicity in the semi-stellar nucleus of M31, with respect to its bulge or to the nucleus of M32. A similar result is found from molecular bands in the visible part o f the spectrum, like CN and C₂ which show highly non-linear dependences on the metallicity.