Saturday, August 26, 2006

D. Arnett: Progress in 3D simulations of Stellar Convection (Review, Wed, Aug. 23)

This was one of the review talks of session D in the Convection Symposium: "Stellar evolution, nucleosynthesis, and convective mixing".

3D simulations with modest resources were presented, where oxygen burning allows thermal relaxation for quasi-adiabatic convection. The computational domain is a subsection of the star. Multi-fluid, realistic physics allows astronomical connections to be made. A careful treatment of boundary conditions and initial models allows long times to be simulated. Finally, extensive graphical and theoretical analysis was used to extract physics. Much of the work was done by Casey A. Meakin (PhD student).

A comparison to 2D simulations shows that 2D simulations overestimate velocities (angular momentum constraint) and incorrectly give inefficient turbulent mixing. Movies of the core showed that in 2D one gets much higher velocities and a less homogeneous core.

After a discussion of 2D burning of C, Ne, O (flames develop because of entrainment of nuclear fuel), the buoyancy frequency, and density fluctuations (occur mostly at interfaces), a comparison of oxygen in 2D and 3D was presented. In 3D, the oxygen abundance is much smoother, better behaved, and there are less fluctuations.

Next, it was pointed out that the solar convection zone can be seen in a plot of superadiabaticity vs. radius as a tiny spike at the surface, and that "Stein and Nordlund country" includes only 3% of the Sun.

Waves are generated at convective interfaces. This can be seen in simulations of turbulent, compressible convection: Velocities do not go to zero at the boundaries (as when using MLT). Those are not convection, but waves (g and p modes).

The convective core grows by entrainment. In a graphic of abundance gradients (e.g. oxygen) as a function of radius and time one could see the convective core growing with time. Patrick Young is another collaborator and has incorporated a first version of the entrainment in the TYCHO code. Previously successful results for wide, double-line eclipsing variables (which used a simpler model) are reproduced.

Implications for the standard solar model:
  • It has a problem - it does helioseismology too well.
  • It is static - adding even small dynamic effects may spoil it.
  • Dynamic effects on opacity diminish the helioseismologic discrepancies for the new abundances and increase them for the old composition.
  • The increased diffusion and the He surface abundance are at odds - rotational stirring may be required.
  • John Bahcall would have loved this new challenge.

Convection in Astrophysics, Session C, Oral Contributions (Wed, Aug. 23)

H.-G. Ludwig: Prospects of Using Simulations to Study the Photospheres of Brown Dwarfs

This talk presented ideas and problems when it comes to simulation of brown dwarf photospheres. A key aspect is the formation and transportation of dust grains. Up to now, all 3D simulations were extrapolations from simulations of hotter stars (M dwarfs), which were dust free. An extension to hot Jupiters (EGPs) is also forseen, which are quite similar to brown dwarfs, hydrodynamically speaking.

Simulating convection is relevant for the energy transport, the thermal structure of the convective layers and mixing of material. Maybe it will also be important for acoustic activity (mechanical heating) and local dynamo action (magnetic activity), we don't know yet.

In terms of radiation, RHD models will not add substantial information. Modelling radiation is of course important for energy transport and the thermal structure of the radiative layers, while radiation pressure is not important.

Another important ingredient of the simulations will be the formation of liquid and solid condensates ("dust"). This is an opacity source, for which knowing the albedo, the atmospheric chemical composition and the spatial distribution will be necessary.

Further modelling aspects include rotation (advective transport of dust) and external external irradiation by the host star for EGPs ("day/night"), causing a global circulation pattern.

Up to now, toy models without dust have been calculated to derive time scales. The results were presented in a graph showing time scales vs. geometrical height. Convection time scales are 10^2 to 10^3 s, the condensation time scale is about 10^2 s for 100 micron grains. This can be used to derive a grain size limit. The dynamic ranges of the simulation are 10^4 to 10^5 in time and 10^2 to 10^3 per dimension in space. The ratio of radius to convection cell size is about 10^4 (10^3 for EGPs). This shows that the simulations will be unable to encompass all spatial ranges (from global to convection) and one will have to separate global and local models.

References for global EGP models are Cho et al. and Burkert et al. (2005).

As an example for local models, a Teff = 1800 K RHD test model without dust by Ludwig, Allard and Hauschildt was shown, where the convective overshooting turns out to be about 0.35 pressure scale heights. The large range in velocities puts high demands on numerics. Rotation and thermal forcing cause an interplay between global and local models.

In conclusion, we are in the position to obtain realistic models of brown dwarfs including radiation and dust formation. But there will be no unified models in the near future.

The challenges for numerics are the substantial scale ranges (dust grain vs. convective velocities), developing an efficient procedure to treat scattering in the time-dependent multi-D case, the interaction between local and global transport processes, the dust cloud distribution and the local transport of momentum (turbulent velocity).

G. Wuchterl: Convection during the formation of gaseous giants and stars

Classical pre-main-sequence evolution starts somewhere in the middle of nowhere in the HR diagram (e.g. at the so-called "birthline"). The only other way is to calculate the full protostellar collapse, which involves challenging physics. To demonstrate this, eight equations for self-gravitating, convective, radiating fluids were shown (see Wuchterl and Tscharnuter 2003, A&A 398, 1081).

Wuchterl and Feuchtinger (1998) is on the supercritical protostellar accretion shock with convection. This requires very high resolution (order of 10^8) and one has to go to very high Courant numbers (10^12).

Improving and testing convection involves a physically plausible time dependent theory. A modified Kuhfuss theory is employed.

The calculations start with an equilibrium Bonnor-Ebert sphere, but the stars do not arrive on the Hayashi line (they are hotter and more luminous after one Myr).

In the HRD, star formation calculations are best displayed with isopleths (lines of constant mass), since the mass is not constant during evolution (accretion!).

Results of a solar mass collapse with convection show that after 1 million years, the cloud memory is lost. The differences to the static picture are that deuterium is burnt (so the birthline seems not to be a useful concept) and the core is radiative.

Two cases of extreme planet formation were presented:
  1. GQ Lupi b (ESO VLT NACO observations from June 2004). This is a nearby young star with a faint companion, with a projected separation of 0.7 arcsec (100 AU).
  2. HD 149026 b - a transiting Saturn mass planet with a 70 Earth masses core.
Models give predictions which are in accordance with the observations.