paper

Relation of X-ray activity and rotation in M dwarfs and predicted time-evolution of the X-ray luminosity

arXiv:2004.02904 · doi:10.1051/0004-6361/201937408

Abstract

We present a sample of 14 M dwarf stars observed with XMM-Newton and Chandra, for which we also computed rotational periods from Kepler Two-Wheel (K2) Mission light curves. We compiled X-ray and rotation data from the literature and homogenized all data sets to provide the largest uniform sample of M dwarfs (302 stars) for X-ray activity and rotation studies to date. We then fit the relation between using three different mass bins to separate partially and fully convective stars. We found a steeper slope in the unsaturated regime for fully convective stars and a nonconstant level in the saturated regime for all masses. In the space we discovered a remarkable double gap that might be related to a discontinuous period evolution. Then we combined the evolution of predicted by angular momentum evolution models with our new results on the empirical relation to provide an estimate for the age decay of X-ray luminosity. We compare predictions of this relationship with the actual X-ray luminosities of M stars with known ages from 100 Myr to a few billion years. We find remarkably good agreement between the predicted and the observed values for partially convective stars. However, for fully convective stars at ages of a few billion years, the constructed age relation overpredicts the X-ray luminosity because the angular momentum evolution model underpredicts the rotation period of these stars. Finally, we examine the effect of different parameterizations for the Rossby number () on the shape of the activity-rotation relation in space, and we find that the slope in the unsaturated regime and the location of the break point of the dual power-law depend sensitively on the choice of .

Accepted for publication by A&A

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