Asteroseismic Signature of a Large Active Region. (arXiv:1911.11812v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Papini_E/0/1/0/all/0/1">Emanuele Papini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Gizon_L/0/1/0/all/0/1">Laurent Gizon</a>
Axisymmetric magnetic activity on the Sun and Sun-like stars increases the
frequencies of the modes of acoustic oscillation. However, it is unclear how a
corotating patch of activity affects the oscillations, since such a
perturbation is unsteady in the frame of the observer. In this paper we
qualitatively describe the asteroseismic signature of a large active region in
the power spectrum of the dipole and quadrupole p modes. In the corotating
frame of the active region, the perturbations due to (differential) rotation
and the active region completely lift the $(2ell + 1)$-fold azimuthal
degeneracy of the frequency spectrum of modes with harmonic degree $ell$. In
the frame of the observer, the unsteady nature of the perturbation leads to the
appearance of $(2ell+1)^2$ peaks in the power spectrum of a multiplet. These
peaks blend into each other to form asymmetric line profiles. In the limit of a
small active region, we approximate the power spectrum of a multiplet in terms
of $2times(2ell+1)$ peaks, whose amplitudes and frequencies depend on the
latitude of the active region and the inclination angle of the star’s rotation
axis. In order to check the results and to explore the nonlinear regime, we
also perform numerical simulations using the 3D time-domain pseudo-spectral
linear pulsation code GLASS.
Axisymmetric magnetic activity on the Sun and Sun-like stars increases the
frequencies of the modes of acoustic oscillation. However, it is unclear how a
corotating patch of activity affects the oscillations, since such a
perturbation is unsteady in the frame of the observer. In this paper we
qualitatively describe the asteroseismic signature of a large active region in
the power spectrum of the dipole and quadrupole p modes. In the corotating
frame of the active region, the perturbations due to (differential) rotation
and the active region completely lift the $(2ell + 1)$-fold azimuthal
degeneracy of the frequency spectrum of modes with harmonic degree $ell$. In
the frame of the observer, the unsteady nature of the perturbation leads to the
appearance of $(2ell+1)^2$ peaks in the power spectrum of a multiplet. These
peaks blend into each other to form asymmetric line profiles. In the limit of a
small active region, we approximate the power spectrum of a multiplet in terms
of $2times(2ell+1)$ peaks, whose amplitudes and frequencies depend on the
latitude of the active region and the inclination angle of the star’s rotation
axis. In order to check the results and to explore the nonlinear regime, we
also perform numerical simulations using the 3D time-domain pseudo-spectral
linear pulsation code GLASS.
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