Asteroseismology with 3D magneto-hydrodynamical simulations of stellar convection

Abstract

The last decade has seen a rapid development in asteroseismology thanks to the CoRoT and Kepler missions. With more detailed asteroseismic observations available, it is becoming possible to infer exactly how oscillations are driven and dissipated in solar-type stars. To study this problem from a theoretical perspective, I carried out three-dimensional (3D) stellar atmosphere simulations together with one-dimensional (1D) stellar structural models of the Sun as well as key benchmark turn-off and subgiant stars. Mode excitation and damping rates are extracted from 3D and 1D stellar models based on analytical expressions. Mode velocity amplitudes are determined by the balance between stochastic excitation and linear damping, which then allows the estimation of the frequency of maximum oscillation power, $\nu_{\max}$, for the first time based on ab initio and parameter-free modelling. I have made detailed comparisons between my numerical results and observational data and achieved very encouraging agreement for all of the target stars. This opens the exciting prospect of using such realistic 3D hydrodynamical stellar models to predict solar-like oscillations across the Hertzsprung-Russell diagram, thereby enabling accurate estimates of stellar properties such as mass, radius, and age.

Solar-like oscillations can be observed in photometry and spectroscopy. The photometry method, represented by space-borne missions such as CoRoT, Kepler and TESS, detects stellar oscillations by measuring the variation of stellar luminosity. The spectroscopy method, represented by ground-based telescopes such as BiSON and SONG, use the wavelength shift of spectral lines as a probe to stellar oscillation. The relationship between the two types of measurement is of great importance, as it can not only guide asteroseismic observations but also serves as an additional constraint to stellar atmosphere models. I have carried out ab initio, 3D hydrodynamical numerical simulations of stellar atmosphere as well as realistic spectral line formation calculations to quantify the ratio between luminosity and radial velocity amplitude for the Sun and the red giant $\epsilon$ Tau. Luminosity amplitudes are computed based on the bolometric flux predicted by 3D simulations. Radial velocity amplitudes are determined from the wavelength shift of synthesized spectral lines with methods closely resemble BiSON and SONG observations. The resulting amplitude ratios are directly comparable with corresponding observations, and encouraging agreements between predicted and observed values are achieved for both the Sun and $\epsilon$ Tau. The numerical method presented here provides a novel way of simulating asteroseismic observations from detailed modelling and meanwhile opens an exciting prospect of bridging luminosity and radial velocity amplitude of solar-like oscillations with 3D stellar model atmospheres. Show more