Gaia Gaps and the Physics of Low-Mass Stars. I. The Fully Convective Boundary

TL;DR

This study confirms that the Gaia M-dwarf gap results from $^{3}$He fusion instabilities in low-mass stars, using stellar models to replicate observed features and explore the physics of fully convective stars.

Contribution

The paper demonstrates that the Gaia M-dwarf gap is caused by $^{3}$He fusion instabilities, providing a new way to test low-mass stellar physics and star formation history.

Findings

The synthetic gap matches observed features like slope and prominence.

The gap's position is sensitive to stellar interior conditions.

Models reproduce over-densities above the gap.

Abstract

The Gaia M-dwarf gap is a significant under-density of stars observed near $M_G = 10.2$ in a color-magnitude diagram for stars within 200 pc of the Sun. It has been proposed that the gap is the manifestation of structural instabilities within stellar interiors due to non-equilibrium $^{3}$He fusion prior to some stars becoming fully convective. To test this hypothesis, we use Dartmouth stellar evolution models, MARCS model atmospheres, and simple stellar population synthesis to create synthetic $M_G$-($G_{\rm BP} - G_{\rm RP})$ color-magnitude diagrams. We confirm that the proposed $^{3}$He instability is responsible for the appearance of the M-dwarf gap. Our synthetic gap shows qualitatively similar features to the observed gap including: its vertical extent in $M_G$, its slope in the color-magnitude diagram, and its relative prominence at bluer colors as compared to redder colors. Furthermore, corresponding over-densities of stars above the gap are reproduced by the models. While qualitatively similar, the synthetic gap is approximately 0.2 magnitudes bluer and, accounting for this color offset, 0.16 magnitudes brighter than the observed gap. Our results reveal that the Gaia M dwarf gap is sensitive to conditions within cores of M dwarf stars, making the gap a powerful tool for testing the physics of M dwarf stars and potentially using M dwarfs to understand the local star formation history.