Simulating the outer layers of Procyon A: a comparison with the Sun

Abstract

Compared to the Sun, the atmospheric structure and convective flow in Procyon A exhibit the following characteristics: (1) the highly superadiabatic transition layer (SAL) is located at much shallower optical depth; it is in a dynamically active region, and its outer region is located part of the time in the optically thin atmosphere; (2) the outer region of the SAL moves from an optically thin region to thick region and back again over a time of 20-30 minutes. This motion, which is driven by the granulation, takes place in a time approximately half the turnover time of the largest granules; The main reason for the radically different radiative-convective behaviour in Procyon A compared to the Sun is the role played by turbulent eddies in determining the overall flow/thermal structure. The turbulent pressure and turbulent kinetic energy can exceed 50 % of the local gas pressure (compared to about 10-20 % in the Sun). The Procyon A simulation thus reveals two distinct timescales - the autocorrelation time of the vertical velocity and the characteristic timescale of the SAL which is tied to granulation. Just below the surface the autocorrelation decay time is about 5 minutes in Procyon A, and the SAL motion timescale is 20-30 mins. When the SAL penetrates the optically thin region there are efficient radiative losses and the peak of the SAL is low. We speculate that these losses damp out the relative amplitudes in luminosity (temperature fluctuations) compared to velocity (Doppler). Although this will not affect the frequencies of the peaks in the power spectrum, it will probably lower the average amplitude of the peaks relative to the noise background.

18 pages 10 figures

References in corpus (6)

• Spectroscopic Signatures of Convection in the Spectrum of Procyon. Fundamental Parameters and Iron Abundance
• 3D convection simulations of the outer layers of the Sun using realistic physics
• Inclusion of turbulence in solar modeling
• Solar-like oscillations in Procyon A
• p-mode frequencies in solar-like stars : I. Procyon A
• Atomic diffusion in star models of type earlier than G

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• Improved calibration of the radii of cool stars based on 3D simulations of convection: implications for the solar model
• Entropy in adiabatic regions of convection simulations
• Helium-Abundance and Other Composition Effects on the Properties of Stellar Surface Convection in Solar-like Main-sequence Stars
• Searching for p-modes in MOST Procyon data: Another view
• Recent MOST space photometry
• Simulating the outer layers of rapidly rotating stars