Stellar Structure - Stellar structure and evolution

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. Padova provides some of the most used sets of stellar evolutionary tracks and isochrones, through the web interfaces http://stev.oapd.inaf.it/cmd and http://stev.oapd.inaf.it/YZVAR/cgi-bin/form.

The activity in the last couple of years has been concentrated in two fronts:

1) A deep revision has been performed in our stellar evolution code, now called the Padova-Trieste Stellar Evolution Code (PARSEC; Bressan et al. 2012). PARSEC is the result of a thorough revision of the major input physics, together with the inclusion of the pre-main sequence phase, not present in our previous releases of stellar models. Another innovative aspect is the possibility of promptly generating accurate opacity tables fully consistent with any selected initial chemical composition, by coupling the Opacity Project At Livermore (OPAL) data at high temperatures to the molecular opacities computed with our AESOPUS code. The present Solar Model complies very well with the stringent contraints provided by helioseismology and by the radiometric age of the Solar System. A thoroughful revision of parameters related with convection is being performed.  

One of the many sets of PARSEC evolutionary tracks.

2) A new code, COLIBRI (Marigo et al. 2013), has been build for computing the evolution of stars along the thermally-pulsing asymptotic giant branch (TP-AGB) phase. Compared to purely synthetic TP-AGB codes, COLIBRI relaxes a significant part of their analytic formalism in favour of a detailed physics applied to a complete envelope model, in which the stellar structure equations are integrated from the atmosphere down to the bottom of the hydrogen-burning shell. This allows to predict self-consistently: (i) the effective temperature, and more generally the convective envelope and atmosphere structures, correctly coupled to the changes in the surface chemical abundances and gas opacities; (ii) sphericity effects in the atmospheres; (iii) the core mass-luminosity relation and its break-down due to hot bottom burning (HBB) in the most massive AGB stars, (iv) the HBB nucleosynthesis via the solution of a complete nuclear network (pp chains, and the CNO, NeNa, MgAl cycles), including also the production of 7Li via the Cameron-Fowler beryllium transport mechanism; (v) the chemical composition of the pulse-driven convective zone; (vi) the onset and quenching of the third dredge-up, with a suitable temperature criterion. At the same time COLIBRI pioneers new techniques in the treatment of the physics of stellar interiors. It is the first evolutionary code ever to use accurate on-the-fly computation of the equation of state for roughly 800 atoms, ions, molecules, and of the Rosseland mean opacities throughout the deep envelope. Another distinguishing aspect of COLIBRI is its high computational speed. This feature is necessary for calibrating the uncertain parameters and processes that characterize the TP-AGB phase, a step of paramount importance for producing reliable stellar population synthesis models of galaxies up to high redshift.

 WORK IN PROGRESS:

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 Isoch 

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

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 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.

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 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

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