MID INFRARED COLORS OF EARLY TYPE GALAXIES ALESSANDRO BRESSAN, HERVÉ AUSSEL and GIAN LUIGI GRANATO Osservatorio Astronomico di Padova, Padova, IT GIULIA RODIGHIERO Dipartimento di Astronomia, Padova, IT PASQUALE PANUZZO and LAURA SILVA SISSA, Trieste, IT Abstract. We compare the mid infrared colors of early type galaxies with isochrones that suitably account for the effects of dust in the circumstellar envelopes of AGB stars. We show that these stars affect significantly the integrated colors in the ISO bands. The excess at 10μm observed in some early type galaxies combined with the optical spectrum, is compatible with a 100% old metal rich population. Limits on the additional young components are: age 0.2 Gyr, mass=0.5% and E(B- V)≥0.5 or age 3 Gyr, mass 10% and E(B-V)≥0.2. Keywords: AGB stars: dust – galaxies: stellar content – infrared: galaxies 1. Isochrones in the Mid Infrared Bressan, Granato and Silva (1998) have suggested that the presence of dust en- shrouded AGB stars should significantly affect the infrared colors of simple stellar populations (SSP). With the advent of ISO observations of the galactic bulge and nearby elliptical galaxies it has become possible to quantify these predictions. Fig- ure 1 shows the HR diagram of Bulge stars in the field of NGC 5266 (Glass et al., 1999) superimposed to isochrones by Bressan et al., 1998. The new isochrones rest on a quite elaborate picture by means of which it is possible to relate a dusty envelope of a given optical depth with the basic parameters of the star along the isochrone. Key ingredients are the empirical relation connecting the mass-loss rate and the pulsational period of the star, and the relation between the dust to gas ratio (δ ) and the luminosity and escape velocity of the star. The isochrones in Figure 1 nicely reproduce the observed slope of the AGB. Assuming for instance a constant δ (eg proportional to the metallicity of the system) would steepen the theoretical AGB in a unacceptable way. We compare the mid-infrared integrated colors of isochrones with observations of early type galaxies (Madden, Vigroux, 1999) in Figure 2. The age of the isochrones is decreasing from right to left (shown in Gyr), and the metal content is Z = 0.008 (dashed line) and Z = 0.02 (solid line). A silicate mixture (Bressan et al., 1998) has been adopted. By contrast the asterisk in the upper right, indicates the region occupied by isochrones of different age and metallicity computed without accounting for dusty envelopes around AGB stars. Astrophysics and Space Science 277 (Suppl.): 251–254, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.