Stellar X-rays and magnetic activity in 3D MHD coronal models

Abstract

Observations suggest a power-law relation between the coronal emission in X-rays, $L_{\mathrm{X}}$, and the total (unsigned) magnetic flux at the stellar surface, $\Phi$. The physics basis for this relation is poorly understood. We use three-dimensional (3D) magnetohydrodynamics (MHD) numerical models of the coronae above active regions, that is, strong concentrations of magnetic field, to investigate the $L_{\mathrm{X}}$ versus $\Phi$ relation and illustrate this relation with an analytical model based on simple well-established scaling relations. In the 3D MHD model horizontal (convective) motions near the surface induce currents in the coronal magnetic field that are dissipated and heat the plasma. This self-consistently creates a corona with a temperature of 1 MK. We run a series of models that differ in terms of the (unsigned) magnetic flux at the surface by changing the (peak) magnetic field strength while keeping all other parameters fixed. In the 3D MHD models we find that the energy input into the corona, characterized by either the Poynting flux or the total volumetric heating, scales roughly quadratically with the unsigned surface flux $\Phi$. This is expected from heating through field-line braiding. Our central result is the nonlinear scaling of the X-ray emission as $L_{\mathrm{X}}\propto {\Phi }^{3.44}$. This scaling is slightly steeper than found in recent observations that give power-law indices of up to only 2 or 3. Assuming that on a real star, not only the peak magnetic field strength in the active regions changes but also their number (or surface filling factor), our results are consistent with observations. Our model provides indications of what causes the steep increase in X-ray luminosity by four orders of magnitude from solar-type activity to fast rotating active stars.

arXiv commentUpdated version after review: 15 pages, 12 figures, Accepted for publication in Astronomy and Astrophysics

References in corpus (28)

• The stellar activity-rotation relationship and the evolution of stellar dynamos
• The stellar atmosphere simulation code Bifrost
• Stellar magnetism: empirical trends with age and rotation
• Heating of the Solar Chromosphere and Corona by Alfven Wave Turbulence
• An Ab Initio Approach to the Solar Coronal Heating Problem
• CHIANTI - an atomic database for emission lines - Paper XV: Version 9, improvements for the X-ray satellite lines
• Extension of the MURaM radiative MHD code for coronal simulations
• Generalized investigation of the rotation-activity relation: Favouring rotation period instead of Rossby number
• Nonlinear Dynamics of the Parker Scenario for Coronal Heating
• The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained
• An Ab Initio approach to Solar Coronal Loops
• The influence of numerical resolution on coronal density in hydrodynamic models of impulsive heating
• Coronal heating through braiding of magnetic field lines
• Intermittent heating in the solar corona employing a 3D MHD model
• Relation of X-ray activity and rotation in M dwarfs and predicted time-evolution of the X-ray luminosity
• Hidden magnetic fields of young suns
• Numerical Simulations of Coronal Heating through Footpoint Braiding
• Mass Loss Rates from Coronal Mass Ejections: A Predictive Theoretical Model for Solar-Type Stars
• A model for the formation of the active region corona driven by magnetic flux emergence
• Observational Signatures of Coronal Loop Heating and Cooling Driven by Footpoint Shuffling
• Nanoflare statistics in an active region 3D MHD coronal model
• Detection of the strongest magnetic field in a sunspot light bridge
• Data-driven model of the solar corona above an active region
• Using coronal seismology to estimate the magnetic field strength in a realistic coronal model
• Full compressible 3D magnetohydrodynamic simulation of solar wind
• Scaling laws of coronal loops compared to a 3D MHD model of an Active Region
• Stellar coronal X-ray emission and surface magnetic flux
• Testing Alfvén wave propagation in a "realistic" set-up of the solar atmosphere

Cited by in corpus (2)

• Magnetic Activity-Rotation-Age-Mass Relations in Late Pre-main Sequence Stars
• X-ray Super-Flares From Pre-Main Sequence Stars: Flare Modeling