2018ApJ...861L..11J

We present the discovery of a gap near M_G ≃ 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 Gaia photometry-it also appears when using near-IR (NIR) photometry (J - K_s versus M_Ks_). The gap is very narrow (∼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 HRD that hints at an underlying astrophysical cause, and we propose that it is linked to the onset of full convection in M dwarfs.