Shells of dust around AGB stars: effects on the integrated spectrum of Single Stellar Populations

In this paper we present models for SSPs of intermediate and old ages where dust enshrouded AGB stars are introduced. To this aim, particular care is devoted to follow the evolution of the AGB stars throughout the quiet and thermally pulsing regimes, to evaluate the effect of self contamination in the outermost layers by the third dredge-up mechanism, to follow the transition from oxygen-rich to carbon-rich objects, and finally to estimate the efficiency of mass-loss by stellar winds. In addition to this, accurate physical models of the dusty shells are presented in which the re-processing of radiation from the central stars is calculated. The resulting spectral energy distribution (SED) is examined to show how important features evolve with time. The SEDs are then convolved with the IRAS filters to obtain the flux in various pass-bands for individual AGB stars of different, mass, chemical composition, and age. The comparison is made by means of SSPs along which AGB stars of the same age but different initial masses are located. The theoretical results are compared to the observational data for selected groups of stars. The same is made for the J,H,K,L pass-bands of the Johnson system. Finally, from the integrated SEDs of the SSPs, we derive the integrated Johnson J,H,K,L magnitudes and colors to be compared to infrared data for star clusters of the Magellanic Clouds.Comment: 23 pages, 22 figures, accepted by A&

Measuring age, metallicity and abundance ratios from absorption line indices

In this study we present detailed calculations of absorption line indices on the Lick System based on the stellar models by Salasnich et al. (2000) calculated with enhanced mix of alpha-elements. Using the so-called Response Functions (RFs) of Tripicco & Bell (1995, TB95), we calculate the indices for SSPs of different age, metallicity and enhancement. We made use of the triplet Hb, Mgb and , and Minimum-Distance Method proposed by Trager et al. (2000, TFWG00) to estimate the age, metallicity and enhancement degree for the galaxies of the Gonzalez (1993) sample, and compare the results with those TFWG00 and Thomas et al (2003). Since very large differences are found, in particular as far as the age is concerned, we analyze in a great detail all possible sources of disagreement, going from the stellar models and SSPs to many technical details of the procedure to calculate the indices, and finally the pattern of chemical elements (especially when alpha-enhanced mixtures are adopted). The key issue of the analysis is that at given metallicity Z and enhancement factor, the specific abundance ratios [Xel/Fe] adopted for some elements (e.g. O, Mg, Ti, and likely others) dominate the scene because with the TB95 RFs they may strongly affect indices like Hb and the age in turn. Finally we have drawn some remarks on the interpretation of the distribution of early-type galaxies in popular two-indices planes, like Hb vs. [MgFe]. We argue that part of the scatter along the Hb axis observed in this plane could be attributed instead of the age, the current explanation, to a spread both in the degree of enhancement and some abundance ratios. The main conclusion of this study is that deriving ages, metallicities and degree of enhancement from line indices is a cumbersome affair whose results are still uncertain.Comment: 26 pages, 17 figures, accepted for publication in MNRAS. This is a revised version of our previous submission to astro-ph/030524

Synthetic Stellar libraries and SSP simulations in the Gaia Era

AbstractThe Gaia mission will obtain accurate positions, parallaxes and proper motions for 109object all over the sky. In addition, it will collect low resolution spectroscopy in the optical range for ~109objects, stars, galaxies, and QSOs. Parameters of those objects are expected to be part of the final Catalog. Complete and up-to-date libraries of synthetic stellar spectra are needed to train the algorithms to classify this huge amount of data. Here we focus on the use of the synthetic libraries of spectra calculated by the Gaia community to derive grids of Single Stellar Populations as building blocks of population synthesis models

Modelling galaxy spectra in presence of interstellar dust. II. From the UV to the far infrared

In this paper, we present spectrophotometric models for galaxies of different morphological type whose spectral energy distributions (SEDs) take into account the effect of dust in absorbing UV-optical light and re-emitting it in the infrared (IR). The models contain three main components: (i) the diffuse interstellar medium (ISM) composed of gas and dust, (ii) the large complexes of molecular clouds (MCs) in which new stars are formed and (iii) the stars of any age and chemical composition. The galaxy models stand on a robust model of chemical evolution taylored in such a way to match the gross properties of galaxies of different morphological type. A simple geometrical model is adopted for each type of galaxy. The total gas and star mass provided by the chemical model are distributed over the whole volume by means of suitable density profiles, one for each component and depending on the galaxy type (spheroidal, disk and disk plus bulge). The galaxy is then split in suitable volume elements to each of which the appropriate amounts of stars, MCs and ISM are assigned. We derive the SEDs of galaxies of different morphological type. First the the basic relations driving the interaction between the physical components of the galaxy are presented. Second, the main parameters are examined and their effects on the SED of three prototype galaxies (a disk, an elliptical and a starburster) are highlighted. The final part of the paper is devoted to assess the ability of our galaxy models in reproducing the SEDs of a few real galaxies of the Local Universe.Comment: 28 pages, 23 figures and 1 table. Accepted by MNRA