Tracers of Stellar Mass-loss - II. Mid-IR Colors and Surface Brightness Fluctuations

TL;DR

This paper models mid-infrared colors and surface brightness fluctuations of stellar populations, emphasizing the impact of dusty envelopes around TP-AGB stars and their relation to mass-loss rates, metallicity, and age.

Contribution

It introduces stellar population synthesis models incorporating dusty TP-AGB envelopes and analyzes their mid-IR properties against observational data, highlighting the importance of mass-loss rates.

Findings

Higher mass-loss rates are needed to fit extreme AGB star colors.

Surface brightness fluctuations are sensitive to metallicity at 4.5 μm and longer wavelengths.

SB fluctuations serve as diagnostics for TP-AGB mass-loss in intermediate-age populations.

Abstract

I present integrated colors and surface brightness fluctuation magnitudes in the mid-IR, derived from stellar population synthesis models that include the effects of the dusty envelopes around thermally pulsing asymptotic giant branch (TP-AGB) stars. The models are based on the Bruzual & Charlot CB* isochrones; they are single-burst, range in age from a few Myr to 14 Gyr, and comprise metallicities between $Z$= 0.0001 and $Z$ = 0.04. I compare these models to mid-IR data of AGB stars and star clusters in the Magellanic Clouds, and study the effects of varying self-consistently the mass-loss rate, the stellar parameters, and the output spectra of the stars plus their dusty envelopes. I find that models with a higher than fiducial mass-loss rate are needed to fit the mid-IR colors of "extreme" single AGB stars in the Large Magellanic Cloud. Surface brightness fluctuation magnitudes are quite sensitive to metallicity for 4.5 um and longer wavelengths at all stellar population ages, and powerful diagnostics of mass-loss rate in the TP-AGB for intermediate-age populations, between 100 Myr and 2-3 Gyr.