Exploring the solar paradigm to explain stellar variability
by Nina-Elisabeth Nemec
Date of Examination:2021-01-29
Date of issue:2021-07-13
Advisor:Dr. Alexander I. Shapiro
Referee:Prof. Dr. Stefan Dreizler
Referee:Dr. Alexander I. Shapiro
Persistent Address:
http://dx.doi.org/10.53846/goediss-8683
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Abstract
English
The unprecedented precision of broadband stellar photometry achieved with the planet hunting missions CoRoT and Kepler initiated a new era in examining the magnetically driven brightness variations of hundreds of thousands of stars. Such brightness variations are well studied and understood for the Sun. The plethora of data allows to accurately compare solar and stellar brightness variations. An intriguing question is whether the observed trends in the stellar photometric variability (e.g. the dependence of the variability on the stellar rotation period) can be explained by utilising the solar paradigm, in particular the physical concepts of brightness variations learnt from the Sun. The goal of this work is to find out, through comparison of observational and simulated data, if any physical concepts of solar brightness variability have to be altered to reproduce the distribution of Sun-like stars variabilities. Comparisons between solar and stellar variability su er from several observational biases. Stellar brightness variations are routinely measured in various spectral passbands and direct measurements of solar variability in these passbands do not exist. Therefore, measurements of stellar variability are often compared to measurements of the Total Solar Irradiance variability (i.e. the spectrally integrated solar radiative flux at 1 AU from the Sun), introducing potential biases. Additionally, observations of solar variability are made from the equatorial plane, corresponding to a right angle between the Sun’s rotation axis and the line-of-sight (the 7.25 tilt between the solar rotation axis and the ecliptic plane can be neglected). In this thesis I build a model based on a surface flux transport model (SFTM) and the Spectral And Total Irradiance REconstruction (SATIRE) approach to calculate the e ffect of the inclination and di fferent passbands on the solar variability on both the activity cycle (11 years) and the rotational (27 days) timescales. This model is presented in Sect. 2. We quantify the rotational variability of the Sun as it would be observed by diff erent space missions and the eff ect of the inclination in Sect. 3. In the next step we extend our model to stars that are more active than the Sun. This extension is based on the observation that the solar disk coverage by spots increases faster with the activity than that by faculae. Until now such a behaviour has not been explained. I demonstrate in Sect. 4 that the cancellation of small magnetic flux concentrations, which are associated with faculae, is able to explain this behaviour. In Sect. 5 I present calculations of brightness variations for fast-rotating stars. I conclude that in order to model the observed dependence of the stellar variability on the rotation period, the degree of nesting (i.e. the tendency of active regions to emerge in the vicinity of previous emergences) of active regions should increase with decreasing rotation periods.
Keywords: Sun; Stars; Variability; Kepler; Inclination; Rotation