# A discontinuity in the $T_{\rm eff}$-radius relation of M-dwarfs

## TL;DR

This study uses high-precision interferometry and Gaia data to refine M-dwarf stellar parameters, revealing a temperature-radius discontinuity likely due to the transition from partially to fully convective stars around 3200-3340 K.

## Contribution

It provides the first observational evidence of a discontinuity in the $T_{\rm eff}$-radius relation of M-dwarfs, pinpointing the transition to full convection.

## Key findings

- Identified a $T_{\rm eff}$-radius discontinuity between 3200 K and 3340 K.
- Refined the $M_{G}$-$T_{\rm eff}$ relation for M-dwarfs.
- Found metallicity influences stellar radii, with metal-rich stars being slightly inflated.

## Abstract

We report on 13 new high-precision measurements of stellar diameters for low-mass dwarfs obtained by means of near-infrared long-baseline interferometry with PIONIER at the Very Large Telescope Interferometer. Together with accurate parallaxes from Gaia DR2, these measurements provide precise estimates for their linear radii, effective temperatures, masses, and luminosities. This allows us to refine the effective temperature scale, in particular towards the coolest M-dwarfs. We measure for late-type stars with enhanced metallicity slightly inflated radii, whereas for stars with decreased metallicity we measure smaller radii. We further show that Gaia DR2 effective temperatures for M-dwarfs are underestimated by $\sim$ 8.2 % and give an empirical $M_{G}$-$T_{\rm eff}$ relation which is better suited for M-dwarfs with $T_{\rm eff}$ between 2600 and 4000 K. Most importantly, we are able to observationally identify a discontinuity in the $T_{\rm eff}$-radius plane, which is likely due to the transition from partially convective M-dwarfs to the fully convective regime. We found this transition to happen between 3200 K and 3340 K, or equivalently for stars with masses $\approx 0.23 M_{\odot}$. We find that in this transition region the stellar radii are in the range from 0.18 to 0.42$R_{\odot}$ for similar stellar effective temperatures.