Fully compressible simulations of waves and core convection in main-sequence stars

Abstract

Context. Recent, nonlinear simulations of wave generation and propagation in full-star models have been carried out in the anelastic approximation using spectral methods. Although it makes long time steps possible, this approach excludes the physics of sound waves completely and requires rather high artificial viscosity and thermal diffusivity for numerical stability. A direct comparison with observations is thus limited. Aims. We explore the capabilities of our compressible multidimensional Seven-League Hydro (SLH) code to simulate stellar oscillations. Methods. We compare some fundamental properties of internal gravity and pressure waves in 2D SLH simulations to linear wave theory using two test cases: (1) an interval gravity wave packet in the Boussinesq limit and (2) a realistic 3 M⊙ stellar model with a convective core and a radiative envelope. Oscillation properties of the stellar model are also discussed in the context of observations. Results. Our tests show that specialized low-Mach techniques are necessary when simulating oscillations in stellar interiors. Basic properties of internal gravity and pressure waves in our simulations are in good agreement with linear wave theory. As compared to anelastic simulations of the same stellar model, we can follow internal gravity waves of much lower frequencies. The temporal frequency spectra of velocity and temperature are flat and compatible with the observed spectra of massive stars. Conclusion. The low-Mach compressible approach to hydrodynamical simulations of stellar oscillations is promising. Our simulations are less dissipative and require less luminosity boosting than comparable spectral simulations. The fully-compressible approach allows for the coupling of gravity and pressure waves in the outer convective envelopes of evolved stars to be studied in the future.

Probing the interior physics of stars through asteroseismology

“…Both formalisms are suitable to treat gapped nonequidistant time-series data while requiring only a short computation time (Kurtz, 1985). Horne and Baliunas (1986) provided guidance for estimation of the number of independent frequencies as well as a method for detecting the presence of alias frequencies caused by the interaction between the window function and the observed data values. In the limit of N → ∞, one has…”

“…A second frequency is then searched for by computing the LS amplitude spectrum for the residual light curve [t i ; x R ðt i Þ] and optimizing its values ν 2 , A 2 , ψ 2 , etc. This procedure is repeated until the periodogram no longer leads to frequencies that are significant for a specified criterion, such as the ones discussed by Horne and Baliunas (1986), Breger et al (1993), and Degroote et al (2009). For a Kepler light curve, this procedure is tedious and time consuming, as it leads to hundreds of significant frequencies.…”

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

“…A recent discovery made using the CoRoT, K2, and TESS missions was that the vast majority of early-type stars have significant low-frequency variability in photometry (Bowman et al, 2019a,b). Such stochastic variability is not predicted from the κ-mechanism, but is expected from convectively-driven internal gravity waves (IGWs) excited by core convection (Rogers et al, 2013;Rogers, 2015;Edelmann et al, 2019;Horst et al, 2020). 2D and 3D hydrodynamical simulations predict that IGWs reach the surface with significant amplitudes and provide detectable perturbations in temperature and velocity (Edelmann et al, 2019), and that IGWs are also extremely efficient at transporting angular momentum and chemical elements (Rogers, 2015;Rogers and McElwaine, 2017).…”

Characterization of the variability in the O+B eclipsing binary HD 165246

“…Bowman et al 2020). Additionally, the photometric signal associated with IGWs has been independently reproduced via both 2D (Ratnasingam et al 2020;Horst et al 2020) and 3D hydrodynamical simulations (Edelmann et al 2019).…”