Evolutionary Tracks and Isochrones

  Stellar evolutionary tracks and isochrones have a wide range of applications in astrophysics. They are the basic tool for age-dating resolved stellar populations and for modelling the spectrophotometric evolution of distant galaxies. The new grid of evolutionary models and isochrones for an extended region of the Z-Y plane enables users to test different modes of helium enrichment for the interpretation of stellar populations in clusters and in galaxies. Moreover, the extension of stellar models and isochrones to the end of the thermal pulses along the AGB is particularly relevant for the analysis of the stellar population observed at the near infrared, where the contribution of AGB stars to the integrated photometric properties is significant.

 Stellar models for a wide range of Z-Y values.Stellar evolutionary tracks covering a wide range of initial masses, helium content and metallicity have been recently computed, adopting recent improvements in the input physics and including updated thermally pulsing asymptotic giant branch (TP-AGB) models. The range of metal content is between Z=0.0001 and 0.070, and of the helium content is between Y=0.23 and 0.46. The evolutionary phases extend from the ZAMS to the TP-AGB phase or C-ignition depending on the mass. The first release deals with the masses from 0.15 to 2.5 Mo, so as to have available isochrones from the Hubble time down to 1 Gyr for any chemical composition inside of the range of Z-Y values. The tracks for masses between 2.5 and 20 Mo have been computed for the same chemical compositions and the preparation of the relative isochrones is in progress. We are going to compute tracks and isochrones taking into account alpha-enhancement in order to extend the database to study of related astrophysical problems.

 Isochrones. The algorithm of equivalent evolutionary points used in  in Bertelli et al. (1994) and Girardi et al. (2000) has been updated and improved to construct isochrones from the new tracks. Previously for each of a few discrete values of the metallicity of the evolutionary tracks there was only one value of the He content (derived from a fixed enrichment law). Now we have about 40 sets of new stellar tracks in the plane Z-Y and it is possible to derive isochrones for whatever Z-Y combination inside the provided range (by means of a double interpolation in Y-Z) and/or simulated stellar populations with various Y(Z) enrichment laws. These isochrones are provided in various photometric systems. We have verified that the enhanced He content has just a modest impact on the bolometric corrections and colour-Teff relations.

Database of isochrones and related services. Extensive tables of stellar evolutionary tracks, isochrones, and bolometric corrections in many photometric systems are being provided in the traditional Padova database of stellar evolutionary tracks and isochrones. A new web server has been recently mounted in STEV, and is providing additional services.

People: P. Bertelli, L. Girardi, E. Nasi, A. Bressan

Collaboration: P. Marigo, C. Chiosi (Padova Univ.), F. Castelli (INAF OA Trieste), A. Weiss (MPA Garching)

Publications: Girardi et al. (2007), A&A 468, 657; Bertelli et al. (2008), A&A 484, 815; Marigo et al. (2008), A&A 482, 883

Link:  STEV

 Calibrating the TP-AGB phase

 The TP-AGB of intermediate- and low-mass stars is one of the most uncertain phases of stellar evolution, although it is crucial to understand the integrated properties of galaxies. The TP-AGB contributes with about half the metals produced in stars, and affects significantly the red-infrared integrated spectra of stellar populations with ages between 0.1 and 10 Gyr. We are carrying on an extensive modelling of TP-AGB evolution as a function of mass and metallicity, by means of synthetic models. The latest improvements include the proper consideration of variable molecular opacities in their external layers, updated prescriptions for the dredge-up efficiency, pulse cycle variations, mass loss rates, etc. The models are calibrated to reproduce both the carbon star luminosity functions in the Magellanic Clouds as well as their lifetimes in star clusters. Such lifetimes have now been measured and are significantly larger (a few Myr) than assumed in the past. Models have been applied to the study of variations in star formation history across the Magellanic Clouds.

 The work proceeds with the further extension of model parameters, building theoretical isochrones for many photometric systems that are relevant for the study of AGB stars, obtaining measurement of observational parameters such as pulsation periods, C/M ratios, luminosity functions, mid-infrared colours, etc., and modelling of AGB populations in nearby galaxies.

People: L. Girardi, A. Bressan, E. Held, M. Gullieuszik, G. Bertelli, E. Nasi

Collaboration: P. Marigo (Padova Univ.), M. Groenewegen, E. Vanhollebeke (Leuven Univ.), B. Aringer (Vienna Univ.), M.-R. Cioni (Hertfordshire Univ., UK), H. Habing (Leiden Obs.), A. Weiss (MPA Garching)

Publications: Marigo et al. (2008),  A&A 482,883; Marigo & Girardi (2007), A&A 469,239; Girardi & Marigo (2007), A&A 462,237