模拟斑点星中的声波
Simulating acoustic waves in spotted stars
摘要：
声波振荡模式受到恒星活动的影响，但目前还不清楚恒星黑子如何促成这些变化。在这里，我们研究了高活动恒星中星斑对具有角度$\ell \leq 2$的全局模式的影响，并在合成光变曲线上表征了点地震特征。我们进行3D时域模拟的线性声波，研究它们与模型星点的相互作用。我们将该点建模为相对于对流稳定恒星背景的声速分层的3D变化，该背景是从太阳模型S构建的。我们通过考虑不同的深度和扰动幅度进行参数研究。精确的数值模拟允许超越一阶微扰理论的波场光斑相互作用的调查。轴对称模与星斑的相互作用是强非线性的。随着模式频率的增加，径向模式的频移超过线性理论预测的值，而$\ell=2, m=0$模式的频移小于线性理论预测的值，在$m=0$和$m=1$模式频率之间形成避免交叉的模式。非线性行为随着光斑振幅的增加和/或深度的减小而增加。线性理论仍然再现了非轴对称模式的正确位移。在非线性区域，模式本征函数不是纯球谐函数，而是不同球谐函数的混合。这种模式混合，连同频率的变化，可能会导致在所观察到的声功率谱中的模式的错误识别。
英文摘要：
Acoustic modes of oscillation are affected by stellar activity, however it is unclear how starspots contribute to these changes. Here we investigate the nonmagnetic effects of starspots on global modes with angular degree $\ell \leq 2$ in highly active stars, and characterize the spot seismic signature on synthetic light curves. We perform 3D time-domain simulations of linear acoustic waves to study their interaction with a model starspot. We model the spot as a 3D change in the sound speed stratification with respect to a convectively stable stellar background, built from solar Model S. We perform a parametric study by considering different depths and perturbation amplitudes. Exact numerical simulations allow the investigation of the wavefield-spot interaction beyond first order perturbation theory. The interaction of the axisymmetric modes with the starspot is strongly nonlinear. As mode frequency increases, the frequency shifts for radial modes exceed the value predicted by linear theory, while the shifts for the $\ell=2, m=0$ modes are smaller than predicted by linear theory, with avoided-crossing-like patterns forming between the $m=0$ and $m=1$ mode frequencies. The nonlinear behavior increases with increasing spot amplitude and/or decreasing depth. Linear theory still reproduces the correct shifts for nonaxisymmetric modes. In the nonlinear regime the mode eigenfunctions are not pure spherical harmonics, but rather a mixture of different spherical harmonics. This mode mixing, together with the frequency changes, may lead to misidentification of the modes in the observed acoustic power spectra.
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