The interplay between magnetic fields and Rossby modes in intermediate-mass stars

All stars exhibit brightness variability in time. The source of brightness variation can be due to e.g., magnetic activity, stellar pulsations, stellar or planetary companions. The detection of variability is very much dependent on the instrument, and the advancement of space photometry has allowed us to characterize stellar variability with high precision. The Kepler mission not only discovered thousands of exoplanets but, through its long and continuous observations, allowed the study of stellar variability, which helped in better understanding the stellar physics of both the interior and the surface.

The main purpose of this thesis is the study of a sample of stars observed by Kepler, which exhibits a particular periodic feature named in the scientific literature hump and spike. This feature was detected in the Fourier spectra of the light curves of more than 200 intermediate-mass stars. The hump is a broad collection of unresolved peaks, followed by a sharp narrow peak - the spike. It has been theorized that the hump feature is due to Rossby modes, which are a type of stellar oscillations. Rossby modes occur in rotating fluids and consist of retrograde motions restored by the Coriolis force.

The spike part of the feature is interpreted as rotational modulation and has been theorized to be caused by either stellar spots co-rotating with the stellar surface or by convective modes originating from the convective core of the star. Stellar spots are a product of magnetic fields generated in the convective regions close to the stellar surface. If stellar spots cause the spike, its frequency corresponds to the rotation frequency at the surface. Convective modes become overstable in time, gain an oscillatory character, and resonantly excite gravity modes (waves that have buoyancy as the restoring force). If the spike is caused by modes that originate from the convective core, then its frequency would
correspond to the rotation frequency of the core. In the stars studied in this work, the Rossby modes that constitute the hump are believed to be mechanically excited by either: deviated flows which stellar spots cause, or due to gravity modes that are resonantly excited by the convective modes (the spike).

In this work, the spike feature is characterized in terms of, e.g., frequency, amplitude, and width. Together with stellar parameters from the Gaia mission and stellar evolutionary models, the parameters extracted from observations are used for estimating the magnetic field strength, which is then compared to theoretical predictions. We find a good agreement between the magnetic field estimates and the predictions by current theoretical models. We also investigate whether the spike can be evidence of convective-core rotation. We take into consideration the harmonic signature and the strength of the resonance between convective modes and gravity modes. We assess that neither of these scenarios can be excluded without further observations and analysis, such as measuring the magnetic field strength and specific modelling tailored for these stars.

Furthermore, our analysis revealed that the power confined in the hump in the Fourier spectrum strongly correlates with the amplitude of the spike, confirming that the two phenomena are connected. Spectroscopic observations aided the photometric data in revealing that, as theory predicts, Rossby modes tend to be confined to the mid-latitudinal bands of the star.

This study is the first ensemble study of hump and spike stars where observations are put into the context of theoretical predictions of magnetic fields, convective core rotation, and Rossby modes.