A new generation of PARSEC-COLIBRI stellar isochrones including the TP-AGB phase

Abstract: We introduce a new generation of PARSEC-COLIBRI stellar isochrones that include a detailed treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase, and covering a wide range of initial metallicities (0.0001<Zi<0.06). Compared to previous releases, the main novelties and improvements are: use of new TP-AGB tracks and related atmosphere models and spectra for M and C-type stars; inclusion of the surface H+He+CNO abundances in the isochrone tables, accounting for the effects of diffusion, dredge-up episodes and hot-bottom burning; inclusion of complete thermal pulse cycles, with a complete description of the in-cycle changes in the stellar parameters; new pulsation models to describe the long-period variability in the fundamental and first overtone modes; new dust models that follow the growth of the grains during the AGB evolution, in combination with radiative transfer calculations for the reprocessing of the photospheric emission. Overall, these improvements are expected to lead to a more consistent and detailed description of properties of TP-AGB stars expected in resolved stellar populations, especially in regard to their mean photometric properties from optical to mid-infrared wavelengths. We illustrate the expected numbers of TP-AGB stars of different types in stellar populations covering a wide range of ages and initial metallicities, providing further details on the C-star island that appears at intermediate values of age and metallicity, and about the AGB-boosting effect that occurs at ages close to 1.6 Gyr for populations of all metallicities. The isochrones are available through a new dedicated web server.

• The paper introduces updated TP-AGB tracks that accurately capture long-period variability in M and C-type stars over a wide metallicity range.
• It employs advanced chemical and dust modeling to track surface abundance changes and mass-loss rates essential for stellar evolution analyses.
• The work enhances isochrone precision by incorporating full thermal pulse cycles, improving age and metallicity estimates in star clusters and galaxies.

An Overview of the New Generation of PARSEC-COLIBRI Stellar Isochrones

The reported research by Marigo et al. introduces a sophisticated generation of PARSEC-COLIBRI stellar isochrones, which incorporate a detailed treatment of the thermally-pulsing asymptotic giant branch (TP-AGB) phase. This advancement is situated within the broader framework of understanding stellar evolution, and the inclusion of TP-AGB stars in models serves a crucial role due to their significant contribution to the chemical enrichment of galaxies. Here, we provide an analysis of the key aspects and implications of the new isochrones.

Key Innovations and Methodological Enhancements

The newly introduced release of PARSEC-COLIBRI isochrones builds upon prior model iterations by offering several notable improvements:

1. Updated TP-AGB Tracks: The inclusion of new TP-AGB tracks represents a significant step forward. These tracks allow for a more comprehensive portrayal of the long-period variabilities typical of M and C-type stars. This coverage extends through a wide range of initial stellar metallicities from 0.0001 to 0.06, thereby better accommodating observed stellar metallicity distributions.
2. Chemical Composition Dynamics: The isochrones account for surface abundances of H, He, and CNO, thereby capturing alterations due to diffusion, third dredge-up episodes, and hot-bottom burning. These dynamics are essential for interpreting intrinsic stellar properties across evolutionary phases.
3. Dust Modeling: The integration of detailed dust growth models in stellar envelopes underpins the reprocessing of photospheric emissions, enhancing the modeling of mass loss and radiative properties of TP-AGB stars.
4. Thermal Pulse Cycles: A complete depiction of thermal pulse cycles (TPCs) and their effect on stellar parameters has been included. Such detailed modeling facilitates predictions regarding the variability and evolutionary stage of TP-AGB stars.
5. Availability and Accessibility: The isochrones are accessible through a dedicated web-based interface, allowing for broad applicability in astrophysical research by providing real-time isochrone calculations.

Implications and Applications

The PARSEC-COLIBRI isochrones have significant implications for both theoretical and observational astrophysics. Theoretically, the improvements assist in refining stellar population synthesis models. The detailed incorporation of TP-AGB star evolution into these models enables better predictions of the integrated light from stellar populations, particularly in galaxies where TP-AGB stars contribute heavily to the infrared emission.

Observationally, the new isochrones can significantly improve the fitting of color-magnitude diagrams, enhancing our ability to infer ages and metallicities of observed star clusters. With more precise isochrones, comparisons between predicted and observed TP-AGB stars provide insights into the star formation history and chemical evolution of galaxies. Moreover, the ability to model detailed variations along thermal pulse cycles enables better interpretation of photometric and spectroscopic variability in TP-AGB variables.

Future Speculations

As computational techniques and astrophysical models continue to evolve, future iterations of the PARSEC-COLIBRI isochrones may incorporate additional phases or refine existing processes, such as more comprehensive treatment of binary interactions and a wider range of pollution by other elements. Additionally, as observations from new telescopes increase in resolution and coverage, the demand for highly detailed isochrones will likely grow, further advancing our understanding of complex stellar lifecycle stages, including the enigmatic TP-AGB phase.

In summary, the advancements presented by Marigo et al. in the context of PARSEC-COLIBRI isochrones represent a substantial enrichment of stellar evolution models. By providing detailed TP-AGB treatments, these models not only improve our understanding of stellar lifecycles but also bolster our interpretation of galactic chemical and photometric evolution.