A discontinuity in the luminosity-mass relation and fluctuations in the evolutionary tracks of low-mass and low-metallicity stars at the Gaia M-dwarf gap

TL;DR

This study uses detailed stellar models to investigate the Gaia M-dwarf gap, revealing a luminosity-mass discontinuity and convective instabilities in low-metallicity, low-mass stars that may explain the observed feature.

Contribution

It provides the first detailed stellar evolution models at very low metallicities with fine mass and time steps, uncovering a luminosity-mass discontinuity and convective instabilities related to the Gaia M-dwarf gap.

Findings

Discontinuity in the luminosity-mass relation at low metallicities.

Fluctuations in stellar properties due to convective merging events.

Longer and more intense convective kissing instability in low-metallicity stars.

Abstract

The Gaia M-dwarf gap is a recently discovered feature in the colour-magnitude diagram that shows a deficiency of low-mass and low-metallicity stars at the lower end of the main sequence. We aim at performing theoretical stellar modelling at low metallicities using a fine mass step and a fine time step, looking specifically for the transition of models from partially to fully convective, since the convective kissing instability that occurs at this transition is believed to be the cause of the gap. Stellar evolution models with metallicities of Z = 0.01, Z = 0.001 and Z = 0.0001 are performed using MESA, with a mass step of 0.00025 M$_{\odot}$ and a time step of 50,000 years. The small time step produced models that experience loops in their evolutionary tracks in the Hertzsprung-Russell (HR) diagram. The fluctuations in effective temperature and luminosity correspond to repeated events in which the bottom of the convective envelope merges with the top of the convective core, transporting $^3$He from the core to the surface. In addition to the episodes of switching from partially to fully convective, several near-merger events that produced low amplitude fluctuations were also found. Low-metallicity models undergo the convective kissing instability for longer portions of their lifetime and with higher fluctuation amplitudes than models with higher metallicities. The small mass step used in the models revealed a discontinuity in the luminosity-mass relation at all three metallicities.