Super metal-rich clusters g158 and g177 in m31 : clues on the inner bulge/disc transition

Abstract

The star clusters G 158 and G 177, located at projected distances smaller than 1.5 kpc from the semi-stellar nucleus of M 31, show spectra extreme in absorption-line strengths, comparable to those from the nuclei of giant ellipticals or from the M 31 nucleus itself. A synthesis has been performed of the integrated spectrum of the Galactic globular cluster 47 Tucanae ([Fe/H]= –0.7) with a solar neighbourhood stellar library: as expected, lines of elements like Mg and Fe are stronger in the solar metallicity model. In turn, 47 Tuc presents a blue-violet blanketing excess which, from a detailed study of the spectral residuais (47 Tuc-model) using laboratory molecular patterns, was found to arise from molecules containing C, N and O. In this Research Note we discuss, at a much higher metallicity level, a very similar blanketing excess observed in G 177 with respect to G 158. We suggest that G 177 is an inner bulge cluster, whereas G 158 is an inner disc one, in analogy with the difference found between the halo cluster 47 Tuc and the model which simulates an old cluster with disc stars. In this scenario the [O/Fe] overabundance, known to exist in metal poor Galactic halo stars as compared to the disc, would persist through the bulge and inner bulge. The inner disc stars might have in common with solar neighbourhood ones the solar CNO/Fe ratio.