Fundamental properties of lower main-sequence stars

TL;DR

This paper reviews the fundamental properties of lower main-sequence stars, especially M dwarfs, highlighting discrepancies between observed and predicted stellar sizes and temperatures, and discusses observational and theoretical challenges.

Contribution

It provides a comprehensive overview of the current understanding and issues related to the physical properties of low-mass stars, emphasizing the need for improved measurements and models.

Findings

Stars are larger and cooler than stellar evolution models predict.

Systematic errors may contribute to scatter in mass-radius diagrams.

High activity levels in stars are linked to observed anomalies.

Abstract

The field of exoplanet research has revitalized interest in M dwarfs, which have become favorite targets of Doppler and transit surveys. Accurate measurements of their basic properties such as masses, radii, and effective temperatures have revealed significant disagreements with predictions from stellar evolution theory in the sense that stars are larger and cooler than expected. These anomalies are believed to be due to high levels of activity in these stars. The evidence for the radius discrepancies has grown over the years as more and more determinations have become available; however, fewer of these studies include accurate determinations of the temperatures. The ubiquitous mass-radius diagrams featured in many new discovery papers are becoming more confusing due to increased scatter, which may be due in part to larger than realized systematic errors affecting many of the published measurements. A discussion of these and other issues is given here from an observer's perspective, along with a summary of theoretical efforts to explain the radius and temperature anomalies.