Modeling the corona and XUV emission of the Sun and Sun-like stars

Munehito Shoda, Shinsuke Takasao

Published 2021-06-16Version 1

The X-ray and extreme-ultra-violet (EUV) emissions from the low-mass stars significantly affect the evolution of the planetary atmosphere. It is, however, observationally difficult to constrain the stellar high-energy emission because of the interstellar extinction. In this work, we simulate the XUV (X-ray+EUV) emission from the Sun-like stars by extending the solar coronal heating model that self-consistently solves the surface-to-corona energy transport, turbulent energy dissipation, and coronal thermal response by conduction and radiation with sufficiently high resolution. The simulations are performed for a range of loop lengths and magnetic filling factors at the stellar surface. When applied to the solar corona, our model is found to reproduce the observed solar XUV spectrum below the Lyman edge, which validates the capability of our model in predicting the XUV spectra of other Sun-like stars. The nearly-linear relation between the unsigned magnetic flux and X-ray luminosity is also reproduced self-consistently. From the simulation runs with various loop lengths and filling factors, the following scaling relations are found. $\log L_{\rm EUV} = 9.93 + 0.67 \log L_{\rm X}$, $\log \Phi^{\rm EUV}_{\rm photon} = 20.40 + 0.66 \log L_{\rm X}$, where $L_{\rm EUV}$ and $L_{\rm X}$ are the cgs-unit luminosity in the EUV and X-ray range, respectively, and $\Phi_{\rm photon}^{\rm EUV}$ is the total number of EUV photons emitted per second. This study demonstrates a refined picture of solar and stellar coronal heating and provides the above observable relations that will be useful for estimating the luminosity of the hidden stellar EUV from X-ray observations.