[Paper Review] Evolution of asymptotic giant branch stars II. Optical to far-infrared isochrones with improved TP-AGB models

Paola Marigo, L. Girardi+4 more|arXiv (Cornell University)|Nov 30, 2007

Stellar, planetary, and galactic studiesPhysics and Astronomy111 references882 citations

TL;DR

This paper presents a new set of theoretical isochrones for asymptotic giant branch (TP-AGB) stars, incorporating improved physics including third dredge-up, hot-bottom burning, variable molecular opacities, and dust-reprocessing effects. By coupling detailed TP-AGB evolutionary tracks with synthetic photometry and dust models, the isochrones accurately reproduce the cool red tails of C-rich stars, pulsation mode transitions, and enhanced mass-loss rates, significantly improving predictions across optical to far-infrared photometric systems.

ABSTRACT

We present a large set of theoretical isochrones, whose distinctive features mostly reside on the greatly improved treatment of the thermally pulsing asymptotic giant branch (TP-AGB) phase. Essentially, we have coupled the TP-AGB tracks described in Paper I, at their stages of pre-flash quiescent H-shell burning, with the evolutionary tracks for the previous evolutionary phases from Girardi et al. (2000). Theoretical isochrones for any intermediate value of age and metallicity are then derived by interpolation in the grids. We take care that the isochrones keep, to a good level of detail, the several peculiarities present in these TP-AGB tracks. Theoretical isochrones are then converted to about 20 different photometric systems -- including traditional ground-based systems, and those of recent major wide-field surveys such as SDSS, OGLE, DENIS, 2MASS, UKIDSS, etc., -- by means of synthetic photometry applied to an updated library of stellar spectra, suitably extended to include C-type stars. Finally, we correct the predicted photometry by the effect of circumstellar dust during the mass-losing stages of the AGB evolution, which allows us to improve the results for the optical-to-infrared systems, and to simulate mid- and far-IR systems such as those of Spitzer and AKARI. Access to the data is provided both via a web repository of static tables (http://stev.oapd.inaf.it/dustyAGB07 and CDS), and via an interactive web interface (http://stev.oapd.inaf.it/cmd) that provides tables for any intermediate value of age and metallicity, for several photometric systems, and for different choices of dust properties.

Motivation & Objective

• Address the long-standing inadequacy in modeling the thermally pulsing asymptotic giant branch (TP-AGB) phase in theoretical isochrones, which critically affects age and metallicity estimates in stellar populations.
• Incorporate key physical processes such as third dredge-up, hot-bottom burning, and variable molecular opacities to accurately reproduce the observed properties of C-type stars and their cool red sequences in color-magnitude diagrams.
• Integrate circumstellar dust effects into synthetic photometry to model the reprocessing of radiation from evolved AGB stars, enabling realistic predictions across optical, near-, mid-, and far-infrared photometric systems.
• Develop a consistent framework linking mass-loss rates, dust condensation, and wind expansion velocity to stellar parameters, improving the realism of evolutionary tracks.

Proposed method

• Coupled pre-flash quiescent H-shell burning TP-AGB tracks from Paper I with earlier evolutionary phases from Girardi et al. (2000) to construct complete isochrones.
• Used synthetic photometry with an updated stellar spectral library, extended to include C-type stars, to compute magnitudes across ~20 photometric systems, including SDSS, 2MASS, UKIDSS, and Spitzer/IRAC.
• Applied dust reprocessing models to simulate emission from circumstellar envelopes, accounting for dust-to-gas ratios and wind expansion velocities that vary with stellar parameters (L, T_eff, M_dot, C/O, Z).
• Calibrated third dredge-up efficiency and mass-loss prescriptions against observed C-star luminosity functions and star counts in Magellanic Cloud clusters to ensure consistency with observations.
• Employed interpolation in age and metallicity grids to generate isochrones for any intermediate value, accessible via interactive and static web interfaces.
• Validated results against observational data from the Milky Way disk and the Magellanic Clouds, focusing on optical to mid-infrared color-magnitude diagrams.

Experimental results

Research questions

• RQ1 How can the TP-AGB phase be modeled with sufficient physical detail to reproduce the observed cool red tails of C-type stars in color-magnitude diagrams?
• RQ2 To what extent do improved treatments of third dredge-up and hot-bottom burning affect the predicted lifetimes and luminosities of AGB stars in stellar populations?
• RQ3 How do variable molecular opacities and dust reprocessing influence the photometric appearance of AGB stars across optical to far-infrared wavelengths?
• RQ4 Can a consistent mass-loss formalism that distinguishes between M- and C-type stars reproduce the observed transition in pulsation modes and super-wind phases?
• RQ5 How do the new isochrones improve the accuracy of integrated stellar population models in resolved galaxies and high-redshift systems?

Key findings

• The isochrones successfully reproduce the characteristic cool red tail of C-type stars in near- and mid-infrared color-magnitude diagrams due to accurate treatment of molecular opacities and dust reprocessing.
• The inclusion of hot-bottom burning in the most massive AGB stars prevents them from becoming C-type, consistent with observational constraints on C-star fractions in different metallicity regimes.
• The transition from first-overtone to fundamental pulsation modes is predicted, which correlates with increased mass-loss rates and enhances the realism of the evolutionary tracks.
• Mass-loss rates are shown to increase dramatically at the M-to-C star transition, driven by changes in surface chemistry and dust formation, with the super-wind regime reached in the latest TP-AGB phases.
• The models reproduce observed C-star luminosity functions and star counts in Magellanic Cloud clusters when third dredge-up parameters are calibrated to match observations.
• The isochrones enable accurate simulation of the rest-frame near-infrared spectra of galaxies and the bright infrared emission from resolved AGB stars, as seen in DENIS, 2MASS, and SAGE surveys.