Small-scale dynamo in cool main-sequence stars

by Tanayveer Singh Bhatia
Doctoral thesis
Date of Examination:2022-10-17
Date of issue:2023-04-13
Advisor:Prof. Dr. Hardi Peter
Referee:Prof. Dr. Hardi Peter
Referee:Prof. Dr. Laurent Gizon
Persistent Address: http://dx.doi.org/10.53846/goediss-9768

 

 

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Abstract

English

Cool stars have an outer convection zone which can produce magnetic fields. These fields have a large scale component that is dependent on the solar cycle, as well as small-scale component largely independent of the cycle. The latter is thought to account for a substantial amount of unsigned quiet-sun magnetism, making it important for heating in the upper solar atmosphere. Simulations seem to suggest that a small-scale dynamo (SSD) is, at least partially, responsible for the quiet sun magnetic fields. In an SSD, magnetic fields are amplified by the small-scale turbulent motions of the plasma in the convection zone. SSD driven magnetism has been investigated thoroughly for the solar case, but its effect on models atmospheres of other cool main sequence stars (important for interpreting stellar observations and extracting fundamental stellar parameters like mass, age, surface gravity, metallicity and effective temperature) remains yet to be explored. In this thesis, I present the first radiative 3D MHD models of an SSD operating in the near surface layers of cool main-sequence stars. I show that SSD fields affect convective velocities and, in the case of hotter stars, even influence the thermodynamic structure near the surface. In addition, for all the stars considered (with a range in effective temperature of 3800 - 6800 K), the fields have a remarkably similar distribution in field strength, even for kilogauss concentrations (which are well-correlated with bright points). A crude estimate also shows a significant reduction of convective blueshift (a signature of granulation) for the hotter stars, necessitating computation of synthetic spectra to study affects of SSD magnetism on limb darkening, spectral line shifts and stellar variability as next steps.
Keywords: Convection; Stars: atmospheres; Dynamo; Stars: magnetic fields; Stars: late-type
 
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