Abstract:
The use of microturbulence as a free parameter is often done in stellar and solar astrophysics. This is a parameter that gets added when comparing the equivalent width of observations to the models when looking at stellar abundances (Mucciarelli 2011) or for the purpose of inversions (Socas-Navarro 2015). However, neither the value of this microturbulence nor the roots are universally agreed upon. For the photosphere 3D modelling has been shown to reduce or completely remove the need for microturbulence in the model (Asplund 2005), however in the chromosphere microturbulence is still added to both 1D and 3D models (da Silva Santos et al. 2020; Socas-Navarro 2015). The value of microturbulence and its roots are important as microturbulence is used to find values for physical parameters in the star or the abundance in the star. Therefore, finding the right microturbulence value or disregarding the microturbulence completely is of importance for the information we find. We made one-dimensional optical depth averages (h3Diτ ) by averaging a Bifrost 3D model, and added four different microturbulence recipes to this average. Then we compared Ca II 854.2 nm synthetic line profiles for these h3Di models to the line profile from the 3D model. From this we found that although the equivalent width of the h3Di model is closer to that of the 3D model when a microturbulence of 1 3 q v 2 x + v 2 y + v 2 z is added, the shapes of the lines are not a good fit. Further, we have split the 3D atmospheres into smaller boxes to look more closely at the fit in different areas. The atmosphere is split into boxes of around 230km×230km and 1150km×1150km horizontally. We found that when looking at the smallest boxes microturbulence gives the h3Di lines a worse fit, while in the larger boxes the the addition of microturbulence does not change the overall goodness of fit. When looking into the roots of the microturbulence, we found that there seems to be a weak correlation between the high temperature and velocity areas and the areas with a large difference in the line profile, but neither of these parameters is enough to explain the microturbulence on its own. Overall, the main difference between between the spectra from the 3D model and the h3Di model comes from the averaging itself, and the addition of microturbulence reduces the difference on a larger scale, but not on a smaller scale. Therefore, we concluded that the microturbulence works as a smoothing parameter to compensate for 3D motions in the h3Di model, but is not a physical parameter.
This is a hybrid examination. Participants can join in person (Peisestua 304) or by zoom link.
Supervisior: Assistant Professor Tiago M. D. Pereira, Institute of Theoretical Astrophysics, UiO (main-supervisor)
Intern. assessor: Professor Boris V. Gudiksen, Institute of Theoretical Astrophysics, UiO
Extern. assessor: Assistant Professor Karin Lind, department of Astronomy, SU