The role of dust in models of population synthesis

Abstract

We have employed state-of-the-art evolutionarymodels of low-and intermediate-mass asymptotic giant branch (AGB) stars and included the effect of circumstellar dust shells on the spectral energy distribution (SED) of AGB stars in order to revise the Padua library of isochrones, which covers an extended range of ages and initial chemical compositions. The major revision involves the thermally pulsingAGB phase, which is now taken from fully evolutionary calculations by Weiss & Ferguson. Two libraries of about 600 AGB dust-enshrouded SEDs each have also been calculated, one for oxygen-richMstars and one for carbon-rich C stars. Each library accounts for different values of input parameters like the optical depth τ , dust composition and temperature of the inner boundary of the dust shell. These libraries of dusty AGB spectra have been implemented into a large composite library of theoretical stellar spectra, to cover all regions of the Hertzsprung-Russell diagram (HRD) crossed by the isochrones.With the aid of the above isochrones and libraries of stellar SEDs, we have calculated the spectrophotometric properties (SEDs, magnitudes and colours) of single-generation stellar populations (SSPs) for six metallicities, more than 50 ages (from ~3Myr to 15 Gyr) and nine choices of the initial mass function. The new isochrones and SSPs have been compared with the colour-magnitude diagrams (CMDs) of field populations in the Large and SmallMagellanic Clouds, with particular emphasis on AGB stars, and the integrated colours of star clusters in the same galaxies, using data from the catalogue 'Surveying the Agents of Galaxy Evolution' (SAGE). We have also examined the integrated colours of a small sample of star clusters located on the outskirts of M31. The agreement between theory and observations is generally good. In particular, the new SSPs reproduce the red tails of the AGB star distribution in the CMDs of field stars in the Magellanic Clouds. Some discrepancies still exist and need to be investigated further. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.