Three-dimensional simulations of the upper radiation-convection transition layer in subgiant stars

Abstract

This paper describes three‐dimensional (3D) large eddy simulations of stellar surface convection using realistic model physics. The simulations include the present Sun, a subgiant of one solar mass and a lower‐gravity subgiant, also of one solar mass. We examine the thermal structure (superadiabaticity) after modification by 3D turbulence, the overshoot of convective motions into the radiative atmosphere and the range of convection cell sizes. Differences between models based on the mixing length theory (MLT) and the simulations are found to increase significantly in the more evolved stages as the surface gravity decreases. We find that the full width at half maximum (FWHM) of the turbulent vertical velocity correlation provides a good objective measure of the vertical size of the convective cells. Just below the convection surface, the FWHM is close to the mean vertical size of the granules and 2 × FWHM is close to the mean horizontal diameter of the granules. For the Sun, 2 × FWHM = 1200 km, a value close to the observed mean granule size. For all the simulations, the mean horizontal diameter is close to 10 times the pressure scaleheight at the photospheric surface, in agreement with previous work.

FROM THE COLOR-MAGNITUDE DIAGRAM OF Ω CENTAURI AND (SUPER-)ASYMPTOTIC GIANT BRANCH STELLAR MODELS TO a GALACTIC PLANE PASSAGE GAS PURGING CHEMICAL EVOLUTION SCENARIO

“…However, both simulations as well as semi-empirical evidence suggest that the deep convection zones of giant envelopes, comprising essentially fully convective configurations, are better described by an increased mixing-length parameter. A variable (i.e., non-constant) α MLT parameter was already suggested by the radiation-hydrodynamics simulations by Ludwig et al (1999), also see the later work by Robinson et al (2004). Specifically relevant to giant stars, Porter & Woodward (2000) presented 3D-simulations of deep envelope convection that were best reproduced within the mixing-length picture if α MLT = 2.68, assuming the formulation of the MLT given in Cox & Giuli (1968).…”

Numerical Simulations of a Rotating Red Giant Star. I. Three-Dimensional Models of Turbulent Convection and Associated Mean Flows

“…Other three-dimensional simulations of the convection in the upper envelope and the atmosphere of red giants have been performed in a more local approach, within the framework of realistic simulation of spectral lines formation in these stars (e.g., Chiavassa et al 2006;Collet et al 2007;Robinson et al 2004). In these works, the hydrodynamical simulation of convection itself is not discussed in details.…”

“…They find no striking difference in the convective pattern compared to their nonrotating case. Concerning rotation, they find a strong differential rotation, which is anti-solar in latitude, and very strong meridional circulation flows, comparable to typical convective velocities.Other three-dimensional simulations of the convection in the upper envelope and the atmosphere of red giants have been performed in a more local approach, within the framework of realistic simulation of spectral lines formation in these stars (e.g., Chiavassa et al 2006;Collet et al 2007;Robinson et al 2004). In these works, the hydrodynamical simulation of convection itself is not discussed in details.…”

Hydrodynamic Simulations of He Shell Flash Convection

“…Other simulations include work on deep envelope convection in giants (Porter & Woodward 1994, 2000Freytag et al 2002;Robinson et al 2004), shallow surface convection in, for example, A stars and white dwarfs (Freytag et al 1996), and the general properties of slab convection (Hurlburt et al 1986). Hydrodynamic simulations of stellar interiors include an investigation of semiconvection in massive stars (Merryfield 1995) and core convection in rotating A-type stars (Browning et al 2004).…”