Abstract
Millimetre continuum observations of the Sun have been used in tandem with 3D radiation magnetohydrodynamic (rMHD) solar atmosphere simulations as a tool for studying the thermal structure of the solar chromosphere with promising results. Extending such observations and simulations to include other main-sequence stars has the potential to provide great insight into the structure of stellar chromospheres in general. In this study we produce a parameter grid of chromospheric stellar atmosphere simulations using the CO5BOLD code. The grid covers effective temperatures from 3200K to 6500K and initial magnetic field strengths from 0G to 500G. Synthetic millimetre continuum observations are produced using the radiative transfer codes Linfor3D and ART and compared with solar and stellar observations to test the accuracy of the model atmospheres. In particular, we compare synthetic spectral energy distributions (SED) with a set of stellar observations compiled by Mohan et al. (2021). We find that our numerical models produce basic features of the chromosphere, like the characteristic shock wave pattern, which are consistent with earlier simulations and with observations of the solar chromosphere. We however find consistent underestimations of the synthetic spectral energy distributions at millimetre wavelengths compared with data from our stellar sample. This is due to too low chromospheric temperatures in our model atmospheres. We suggest that improvements to the CO5BOLD are needed to improve the realism of the simulations, particularly producing an average chromospheric temperature rise that can reproduce the observed SEDs. Potential improvements include adding the treatment of non-equilibrium ionisation to the simulations, as well as ion-neutral effects (ambipolar diffusion). We also argue that more stellar millimetre observations are needed in order to facilitate comparative studies by the use of 3D atmosphere simulations.
Supervisor: Professor Sven Wedemeyer, Institute of Theoretical Astrophysics, UiO
Intern. assessor: Professor David F. Mota, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Dr. Jaime de la Cruz Rodriguez, Stockholm University