Evolution of binary stars and its implications for evolutionary population synthesis

TL;DR

This paper discusses how binary star interactions significantly influence stellar evolution and population synthesis, affecting observable features like the far-UV excess in elliptical galaxies and challenging previous age and metallicity assumptions.

Contribution

It demonstrates that including binary interactions in population synthesis models explains the far-UV excess and alters understanding of elliptical galaxy evolution.

Findings

Binary interactions produce hot stars like blue stragglers and hot subdwarfs.

Binary interactions make stellar populations appear younger and hotter.

Far-UV excess in ellipticals is likely due to binaries, not age or metallicity.

Abstract

Most stars are members of binaries, and the evolution of a star in a close binary system differs from that of an ioslated star due to the proximity of its companion star. The components in a binary system interact in many ways and binary evolution leads to the formation of many peculiar stars, including blue stragglers and hot subdwarfs. We will discuss binary evolution and the formation of blue stragglers and hot subdwarfs, and show that those hot objects are important in the study of evolutionary population synthesis (EPS), and conclude that binary interactions should be included in the study of EPS. Indeed, binary interactions make a stellar population younger (hotter), and the far-ultraviolet (UV) excess in elliptical galaxies is shown to be most likely resulted from binary interactions. This has major implications for understanding the evolution of the far-UV excess and elliptical galaxies in general. In particular, it implies that the far-UV excess is not a sign of age, as had been postulated prviously and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.