[mesa-users] high Carbon in opacity tables

[Bill Paxton]

Hi Dave,

On Oct 24, 2013, at 2:31 PM, David Arnett wrote:

> During C burning, the opacity is close to the electron scattering limit, and radiation diffusion is weaker than neutrino cooling (not to  mention that for high densities, electron conduction also beats out radiation diffusion). Probably all that is needed for opacity is a value that makes a "graceful exit".

Good points -- and basically that saves us in mesa for typical situations.  

Here's my standard plot of how we carve up (T,Rho) for opacities:
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logR = logRho - 3*logT + 18, and the tables are for logR <= 1 (except for the wonderful ones from Freedman)
To the right of the dotted blue line electron conduction dominates.

So for "normal" C burning and beyond, we are off the tables and into the regions where we just use Compton & electron conduction.

It might not be a very graceful exit, but I think it works okay in practice.  As a computer scientist I'm always thinking about the odd-ball cases that might possibly happen to cause problems.  For opacities this includes scenarios involving taking ashes of C burning and bringing them up out of the hot dense regions into cooler, less dense near the surface.  The sort of thing that no self-respecting star would ever do, right? ;p

btw: this plot shows that there is a danger zone where logT is too high for Freedman and logR > 1 and < the blue dotted line so we can't just hand off to electron conduction.  we have no data in that zone!!  all we can do is hope that the star is convective in those areas so we don't really care about getting an accurate opacity.

-Bill