MESA meets MURaM

Abstract

Context. Space-based observations of solar-like oscillators have identified large numbers of stars in which many individual mode frequencies can be precisely measured. However, current stellar models predict oscillation frequencies that are systematically affected by simplified modelling of the near-surface layers. Aims. We use three-dimensional radiation hydrodynamics simulations to better model the near-surface equilibrium structure of dwarfs with spectral types F3, G2, K0 and K5, and examine the differences between oscillation mode frequencies computed in stellar models with and without the improved near-surface equilibrium structure. Methods. We precisely match stellar models to the simulations' gravities and effective temperatures at the surface, and to the temporally-and horizontally-averaged densities and pressures at their deepest points. We then replace the near-surface structure with that of the averaged simulation and compute the change in the oscillation mode frequencies. We also fit the differences using several parametric models currently available in the literature. Results. The surface effect in the stars of solar-type and later is qualitatively similar and changes steadily with decreasing effective temperature. In particular, the point of greatest frequency difference decreases slightly as a fraction of the acoustic cut-off frequency and the overall scale of the surface effect decreases. The surface effect in the hot, F3-type star follows the same trend in scale (i.e. it is larger in magnitude) but shows a different overall variation with mode frequency. We find that a two-term fit using the cube and inverse of the frequency divided by the mode inertia is best able to reproduce the surface terms across all four spectral types, although the scaled solar term and a modified Lorentzian function also match the three cooler simulations reasonably well. Conclusions. Three-dimensional radiation hydrodynamics simulations of near-surface convection can be averaged and combined with stellar structure models to better predict oscillation mode frequencies in solar-like oscillators. Our simplified results suggest that the surface effect is generally larger in hotter stars (and correspondingly smaller in cooler stars) and of similar shape in stars of solar type and cooler. However, we cannot presently predict whether this will remain so when other components of the surface effect are included.

Standing on the Shoulders of Dwarfs: the Kepler Asteroseismic LEGACY Sample. I. Oscillation Mode Parameters

“…These ratios are useful for model fitting, where they can be used instead of individual frequencies (Lebreton & Goupil 2014;Silva Aguirre et al 2015) because they are largely insensitive to the stellar surface layers (Kjeldsen et al 2008a;Ball & Gizon 2014;Roxburgh 2015;Ball et al 2016). For further details on the use of these ratios we refer to Roxburgh & Vorontsov (2003, Roxburgh (2005), Otí Floranes et al (2005), and Silva Aguirre et al (2011,2013).…”

Modelling Kepler red giants in eclipsing binaries: calibrating the mixing-length parameter with asteroseismology

“…The comparison between hydrodynamical simulations and stellar models (Ball et al 2016) suggested an increasing surface term for dwarfs from the spectral type of K5 to F3. Trampedach et al (2017) estimated the frequency shifts in various types of stars with a grid of convection simulations and also gave a clear correlation between the surface term and atmospheric parameters.…”

Determining stellar parameters of asteroseismic targets: going beyond the use of scaling relations

“…see Sonoi et al 2015;Ball et al 2016), and by trying to mitigate surface effects when comparing models and observations. In this respect a way forward would be to determine the star's mean density by using the full set of observed acoustic modes, not just their average frequency spacing.…”