M dwarfs: effective temperatures, radii and metallicities

TL;DR

This study empirically determines effective temperatures, radii, and metallicities for nearby M dwarfs using a new flux ratio technique, revealing a radius increase at the K to M dwarf transition not captured by models.

Contribution

Introduces a novel flux ratio method to estimate temperatures and metallicities of M dwarfs, extending to very cool stars and comparing results with theoretical models.

Findings

Temperature scale extends below 3000 K.

Metallicities agree with independent methods within 0.2 dex.

Identifies a radius increase at the K to M dwarf transition.

Abstract

We empirically determine effective temperatures and bolometric luminosities for a large sample of nearby M dwarfs, for which high accuracy optical and infrared photometry is available. We introduce a new technique which exploits the flux ratio in different bands as a proxy of both effective temperature and metallicity. Our temperature scale for late type dwarfs extends well below 3000 K (almost to the brown dwarf limit) and is supported by interferometric angular diameter measurements above 3000 K. Our metallicities are in excellent agreement (usually within 0.2 dex) with recent determinations via independent techniques. A subsample of cool M dwarfs with metallicity estimates based on hotter Hipparcos common proper-motion companions indicates our metallicities are also reliable below 3000 K, a temperature range unexplored until now. The high quality of our data allow us to identify a striking feature in the bolometric luminosity versus temperature plane, around the transition from K to M dwarfs. We have compared our sample of stars with theoretical models and conclude that this transition is due to an increase in the radii of the M dwarfs, a feature which is not reproduced by theoretical models.