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Effects of inclusion of small-scale dynamo in near-surface structure of F-stars

Published onAug 18, 2020
Effects of inclusion of small-scale dynamo in near-surface structure of F-stars

The presence of (unresolved) small-scale mixed polarity regions in the quiet Sun photosphere plays an important role in determining the basal magnetic flux. Observationally, the magnitude of the vertical component of this field is estimated to be ~50-100 G on the Sun. This field is important for determining the energy balance in the chromosphere and may also subtly affect the radiative properties of the photosphere. These fields are believed to be the result of a small-scale dynamo (SSD) operating near the surface. While significant progress has been made in investigating the role of the SSD in the Sun, it is unclear what effects SSDs have on other stars. In particular, for F-stars, the photosheric kinetic and internal energies seem to be of the same order of magnitude. Since there is a rough equipartition in energies for a saturated SSD, deviations from a pure hydrodynamic (HD) stratification are expected. We aim to characterize these deviations. Box simulations of the upper convection zone and the photosphere are carried out using the radiative MHD code MURaM. To obtain SSD simulations, we use initial HD simulations and seed a magnetic field of negligible strength and zero net flux, which we then run till the magnetic field reaches saturation. We consider two different lower boundary conditions (BCs) for the magnetic field to characterize BC-effects: a) only vertical magnetic field is allowed, b) both vertical and horizontal magnetic field is allowed. Both boundary conditions exhibit SSD action. We observe slight increase (fraction of a percent) in the horizontally-averaged temperature profile for both the cases. Other thermodynamic quantities exhibit deviations (~ a percent) depending on the boundary condition considered. In addition, the spatial power spectra of the bolometric intensity shows deviations from the corresponding HD (without magnetic field) run, implying larger power at smaller spatial scales for SSD case. The presence of a SSD results in a significant amount of “quiet”-star magnetic flux with associated changes in the stratification of the atmosphere and spatial distribution of the bolometric intensity.


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