Carbon star populations in systems with different metallicities: statistics in Local Group galaxies

Abstract

We present evolutionary population synthesis models for the study of the cool and luminous intermediate age stellar populations in resolved galaxies with particular emphasis on carbon star populations. We study the effects of the star formation history, the age and the metallicity on the populations of intermediate mass stars. In the case of instantaneous bursts, we confirm that lower metallicity results in higher contributions of carbon stars to the total star number, and in higher number ratios of carbon stars to late-type M stars. Chemically consistent models are used to study the effect of the star formation history on the relations between carbon star population properties and global parameters of the parent galaxy (age, metallicity). Our models are able to account, for the first time, for those correlations, as observed in the galaxies of the Local Group. For stellar populations older than about 1 Gyr, the properties of carbon star populations are linked to the current metallicity in a way that is quite independent of the star formation scenario. The number ratio of carbon stars to late-type M stars forms a metallicity sequence along which stellar populations with very different star formation histories are found. For the same populations, we find that both the mean bolometric luminosity of carbon stars and their normalized number to the luminosity of the parent galaxy are quite independent of metallicity over a large range in metallicity. This is in good agreement with the observational constraints. The observed statistics of carbon star populations can be interpreted by two principal effects: (i) the carbon star formation efficiency is higher in metal-poor systems, (ii) the typical star formation time-scale along the Hubble sequence of galaxies is much longer than the typical time-scale for the production of carbon stars at any metallicity.