Convective Dynamos and the Minimum X‐Ray Flux in Main‐Sequence Stars

Abstract

The objective of this paper is to investigate whether a convective dynamo can account quantitatively for the observed lower limit of X-ray surface flux in solar-type main-sequence stars. Our approach is to use three-dimensional numerical simulations of a turbulent dynamo driven by convection to characterize the dynamic behavior, magnetic field strengths, and filling factors in a nonrotating stratified medium and to predict these magnetic properties at the surface of cool stars. We use simple applications of stellar structure theory for the convective envelopes of main-sequence stars to scale our simulations to the outer layers of stars in the F0-M0 spectral range, which allows us to estimate the unsigned magnetic flux on the surface of nonrotating reference stars. We find agreement between our G0 star calculations and the observed unsigned magnetic flux density in the quiet Sun. With these magnetic flux estimates we use the recent results of Pevtsov et al. to predict the level of X-ray emission from such a turbulent dynamo and find that our results compare well with observed lower limits of surface X-ray flux. If we scale our predicted X-ray fluxes to Mg II fluxes, we also find good agreement with the observed lower limit of chromospheric emission in K dwarfs. This suggests that dynamo action from a convecting, nonrotating plasma is a viable alternative to acoustic heating models as an explanation for the basal emission level seen in chromospheric, transition-region, and coronal diagnostics from late-type stars.