[Paper Review] The modelling of intermediate-age stellar populations: I- near-infrared properties

M. Mouhcine, A. Lançon|ArXiv.org|May 30, 2002

Stellar, planetary, and galactic studies50 references51 citations

TL;DR

This paper presents a new synthetic stellar population model that self-consistently includes the effects of mass loss, third dredge-up, and envelope burning on thermally pulsing AGB stars to accurately predict near-infrared (near-IR) spectral and photometric properties of intermediate-age stellar populations. The model successfully reproduces observed bolometric and K-band luminosities, carbon star contributions, and colour evolution in Magellanic Cloud clusters without ad hoc corrections, showing that envelope burning smooths the redward colour evolution and that carbon stars dominate at low metallicities.

ABSTRACT

In this paper, we discuss how the integrated properties of intermediate-age single burst population, especially in the near-infrared, behave as a function of age and metallicity. Our models take into account all stellar evolutionary phases that affect the evolution of the integrated optical and near-infrared spectrum of such a population. Particular care was dedicated to the Asymptotic Giant Brach stars, which can be dominant at near-infrared wavelengths. First we present new synthetic model that takes into account the relevant physical processes that control the evolution through the thermally pulsing AGB. In the isochrone presented in this paper the lifetime and the nature of the AGB stars are established as consequences of the interplay between the physical processes that control the AGB star evolution. The contribution of these stars to the integrated light of the population is thus obtained in a consistent way. We optimize our models by using a new stellar library that explicitly takes into account the spectral features thar characterize only AGB stars in comparison to other cool and luminous stars (abriged)

Motivation & Objective

• To improve the accuracy of evolutionary population synthesis models for intermediate-age stellar populations in the near-infrared.
• To resolve discrepancies between observed and predicted near-IR properties of stellar populations, particularly those dominated by AGB stars.
• To incorporate the physical processes governing AGB evolution—mass loss, third dredge-up, and envelope burning—into a self-consistent model.
• To evaluate the contribution of oxygen-rich and carbon-rich AGB stars to integrated light, especially in the K band and bolometric luminosity.
• To test the viability of using near-IR broad- and narrow-band colours to identify post-starburst populations and constrain metallicity.

Proposed method

• Developed a new grid of synthetic evolutionary tracks for AGB stars, explicitly modeling mass loss, third dredge-up, and envelope burning using analytical prescriptions.
• Calibrated the model using a purpose-built stellar spectral library that includes spectral features unique to AGB stars, distinguishing them from other cool, luminous stars.
• Computed integrated near-IR magnitudes and colours by summing contributions from all stellar evolutionary phases, with special attention to the TP-AGB phase.
• Used observational data from Magellanic Cloud star clusters (e.g., Frogel et al. 1990; Ferraro et al. 1995) as benchmarks for model validation.
• Re-analyzed narrow-band indices (Lançon et al. 1999) sensitive to AGB stars to assess their utility in detecting intermediate-age populations.
• Compared model predictions with observed VJHK colours and luminosity functions to test consistency across different ages and metallicities.

Experimental results

Research questions

• RQ1 How do envelope burning and third dredge-up affect the luminosity and spectral energy distribution of intermediate-age stellar populations in the near-IR?
• RQ2 What is the contribution of bright AGB stars and carbon stars to the integrated bolometric and K-band luminosity, and how does it vary with age and metallicity?
• RQ3 Why do observed near-IR colours of intermediate-age clusters deviate from classical models, and can these deviations be explained by AGB physics?
• RQ4 Can near-IR broad- and narrow-band colours reliably identify post-starburst populations, and what role do carbon-rich AGB stars play in this?
• RQ5 How sensitive are model predictions to uncertainties in the effective temperature scale of upper AGB stars?

Key findings

• The model reproduces the observed contribution of bright AGB stars to the bolometric luminosity and K-band light in Magellanic Cloud clusters without requiring ad hoc correction factors.
• The contribution of carbon stars to the bolometric luminosity increases significantly with decreasing metallicity, consistent with observations.
• Envelope burning causes a gradual, smoothed redward evolution of near-IR colours, eliminating the sharp colour jump predicted by earlier models.
• The inclusion of carbon stars makes the predicted colours significantly redder than models without them, with the difference increasing at higher metallicities.
• The model successfully predicts the age at which AGB contributions peak in bolometric and K-band luminosity, matching observational trends.
• The model's predictions for the initial mass–final mass relation and period–luminosity relation of AGB stars are consistent with observed single AGB star properties.