Correlations between Ca ii H and K Emission and the Gaia M Dwarf Gap

Abstract

The Gaia M dwarf gap, also known as the Jao Gap, is a novel feature discovered in the Gaia Data Release 2 G versus BP-RP color–magnitude diagram. This gap represents a 17% decrease in stellar density in a thin magnitude band around the convective transition mass (∼0.35 M ⊙) on the main sequence. Previous work has demonstrated a paucity of Hα emission coincident with the G magnitude of the Jao Gap in the solar neighborhood. The exact mechanism that results in this paucity is as of yet unknown; however, the authors of the originating paper suggest that it may be the result of complex variations to a star’s magnetic topology driven by the Jao Gap’s characteristic formation and breakdown of stars’ radiative transition zones. We present a follow-up investigating another widely used magnetic activity metric, Calcium ii H and K emission. Ca ii H and K activity appears to share a similar anomalous behavior as Hα does near the Jao Gap magnitude. We observe an increase in star-to-star variation of magnetic activity near the Jao Gap. We present a toy model of a star’s magnetic field evolution, which demonstrates that this increase may be due to stochastic disruptions to the magnetic field originating from the periodic-mixing events characteristic of the convective kissing instabilities that drive the formation of the Jao Gap.