A Gap in the Lower Main Sequence Revealed by Gaia Data Release 2

TL;DR

This paper reports the discovery of a narrow gap in the main sequence of the Hertzsprung-Russell Diagram near M3.0V, observed in Gaia DR2 data, likely related to the transition to full stellar convection.

Contribution

It identifies a previously unrecognized narrow gap in the main sequence, linking it to the onset of full convection in M dwarf stars, based on Gaia DR2 data analysis.

Findings

The gap appears in Gaia and near-IR photometry.

It is located near the M3.0V spectral type.

The gap is approximately 0.05 mag wide.

Abstract

We present the discovery of a gap near $M_{G}\approx$10 in the main sequence on the Hertzsprung-Russell Diagram (HRD) based on measurements presented in Gaia Data Release 2 (DR2). Using an observational form of the HRD with $M_{G}$ representing luminosity and $G_{BP}-G_{RP}$ representing temperature, the gap presents a diagonal feature that dips toward lower luminosities at redder colors. The gap is seen in samples extracted from DR2 with various distances, and is not unique to the {\it Gaia} photometry --- it also appears when using near-IR photometry ($J-K_{s}$ vs $M_{K_{s}}$). The gap is very narrow ($\sim$0.05 mag) and is near the luminosity-temperature regime where M dwarf stars transition from partially to fully convective, i.e., near spectral type M3.0V. This gap provides a new feature in the H-R Diagram that hints at an underlying astrophysical cause and we propose that it is linked to the onset of full convection in M dwarfs.