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Simulating the signature of starspots in stellar oscillations
von Emanuele PapiniIt has been known for a few decades that acoustic oscillations are affected by stellar activity. In the Sun, global acoustic modes show a variation with the 11-year cycle and a similar phenomenon has been observed in other stars. In this thesis, I investigate the effects of a large starspot on the global low-degree modes of stellar oscillations.
I first consider the problem of convective stabilization, common to every linear oscillation code in the time domain. A general method to build a convectively stable background starting from a given stellar model is presented. I also propose a perturbative approach to approximately recover the acoustic wavefield of the original unstable stellar model.
In the second part of the thesis I study the effects of a localized sound speed perturbation placed at the north pole (i. e. a polar spot) on radial, dipole, and quadrupole modes of oscillation. For large starspots, numerical simulations show that the changes in the axisymmetric modes cannot be modeled by linear theory.
Finally, I consider the asteroseismic signature of a large starspot rotating with the star. Such a perturbation is unsteady in the observer’s frame, thus each multiplet appears as blended peaks in the power spectrum, whose amplitudes depend on the star’s inclination and on the latitude of the starspot. I simulate example power spectra using both perturbation theory and numerical simulations. This work ought to be useful in interpreting oscillation power spectra of spotted pulsators.
I first consider the problem of convective stabilization, common to every linear oscillation code in the time domain. A general method to build a convectively stable background starting from a given stellar model is presented. I also propose a perturbative approach to approximately recover the acoustic wavefield of the original unstable stellar model.
In the second part of the thesis I study the effects of a localized sound speed perturbation placed at the north pole (i. e. a polar spot) on radial, dipole, and quadrupole modes of oscillation. For large starspots, numerical simulations show that the changes in the axisymmetric modes cannot be modeled by linear theory.
Finally, I consider the asteroseismic signature of a large starspot rotating with the star. Such a perturbation is unsteady in the observer’s frame, thus each multiplet appears as blended peaks in the power spectrum, whose amplitudes depend on the star’s inclination and on the latitude of the starspot. I simulate example power spectra using both perturbation theory and numerical simulations. This work ought to be useful in interpreting oscillation power spectra of spotted pulsators.