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

TL;DR

This study examines how alpha-element enhancement and TP-AGB stars influence surface brightness fluctuation magnitudes and colors, highlighting their sensitivity and implications for stellar evolution models.

Contribution

It provides new insights into the effects of alpha-element enhancement and TP-AGB stars on SBF magnitudes and colors using updated isochrones, and compares different stellar models.

Findings

Alpha-element enhancement makes I-band SBF magnitudes brighter by ~0.35 mag.

SBF magnitudes are highly sensitive to the presence of TP-AGB stars.

Empirical SBF trends can constrain uncertain stages of stellar evolution.

Abstract

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.