Fundamental properties of low-mass stars

TL;DR

This paper reviews the discrepancy between observed and theoretical properties of low-mass stars, highlighting stellar activity as a key factor affecting their radii and temperatures, and discusses recent observational evidence and implications.

Contribution

It provides new observational data and reviews the role of stellar activity, especially starspots, in explaining the mismatch between models and observations of low-mass stars.

Findings

Stellar activity may cause radius inflation and temperature suppression in low-mass stars.

Starspot coverage significantly impacts stellar property measurements.

Evidence supports activity as a primary factor in the observed discrepancies.

Abstract

Numerous investigations on the fundamental properties of low-mass stars using eclipsing binaries indicate a strong discrepancy between theory and observations that is still awaiting explanation. Current models seem to predict radii for stars less massive than the Sun that are some 10% smaller than observed, while their effective temperatures are some 5% larger. Here we discuss recent new observational data that are relevant to this issue and review the progress made in understanding the origin of the important differences with theoretical calculations. Notably, we provide evidence that stellar activity may be responsible for the mismatch between observations and theory through two different channels: inhibition of convection or effects of a significant starspot coverage. The activity hypothesis is put to a test with observational diagnostics and some of the consequences of the large starspot coverage are evaluated. We conclude that stellar activity likely plays a key role in defining the properties of active low-mass stars and that this should be properly taken into account when investigating young, active stars in clusters or star-forming regions.