Effects of Alpha-Element Enhancement and the Thermally Pulsing-Asymptotic Giant Branch on Surface Brightness Fluctuation Magnitudes and Broadband Colors

We investigate the effects of alpha-element enhancement and the thermally pulsing-asymptotic giant branch (TP-AGB) stars on the surface brightness fluctuation (SBF) magnitudes and broadband colors of simple stellar populations and compare to the empirical calibrations. We consider a broad range of ages and metallicities using the recently updated Teramo BaSTI isochrones. We find that the alpha-element enhanced I-band SBF magnitudes are about 0.35 mag brighter and their integrated V - I colors are about 0.02 mag redder, mostly because of oxygen enhancement effects on the upper red giant branch and asymptotic giant branch. We also demonstrate, using both the Teramo BaSTI and Padova isochrones, the acute sensitivity of SBF magnitudes to the presence of TP-AGB stars, particularly in the near-IR, but in the I-band as well. Empirical SBF trends therefore hold great promise for constraining this important but still highly uncertain stage of stellar evolution. In a similar vein, non-negligible disparities are found among several different models available in the literature due to intrinsic model uncertainties.

💡 Research Summary

Surface brightness fluctuations (SBF) have become a cornerstone technique for measuring extragalactic distances and probing the unresolved stellar content of galaxies. The amplitude of SBF is dominated by the brightest stars in a population—principally red‑giant branch (RGB) stars and asymptotic giant branch (AGB) stars—so any physical process that alters the luminosities, temperatures, or lifetimes of these phases will be reflected in the SBF magnitude and its colour dependence. In this paper the authors examine two such processes: (1) enhancement of α‑capture elements (O, Mg, Si, Ca, etc.) and (2) the treatment of the thermally pulsing AGB (TP‑AGB) phase. Using the latest Teramo BaSTI isochrones, supplemented by Padova models for cross‑checking, they compute SBF magnitudes and integrated V‑I colours for simple stellar populations (SSPs) spanning ages from 1 to 14 Gyr and metallicities from