Revisiting binary stars in population synthesis models

TL;DR

This paper presents an enhanced population synthesis model that incorporates the 2HeWD merger channel for EHB star formation, aligning well with observations and aiding the study of UV phenomena in early-type galaxies.

Contribution

It introduces a population synthesis model including the 2HeWD merger channel for EHB stars, with realistic parameters from BaSTI, improving understanding of UV features in galaxies.

Findings

Model matches observed CMD and SED of NGC 6791.

Successfully reproduces UV upturn in ETGs.

Aligns with traditional models except in binary-dominated spectra.

Abstract

We report results of a population synthesis model that follows the evolution of single and binary stars. In this model we include the 2HeWD merger channel, suggested by Han et al. (2002), for the formation of EHB stars. The physical parameters of the resulting EHB stars are derived from the BaSTI database, and are thus realistic and observationally supported. The predictions of this model are in good agreement with traditional population synthesis models, except when the spectrum of the stellar population is dominated by binary stars or their products, e.g., EHB stars in the UV of ETGs. We reproduce successfully the observed CMD and SED of the metal rich open cluster NGC 6791. The stellar population in this cluster may be archetypal of the stellar population in ETGs that show the UVX phenomenon. Our models should be appropriate to study the UV upturn in ETGs.