[mesa-users] type 2 opacities

[Bill Paxton]

Hi Radek,

On Oct 23, 2013, at 12:27 AM, Radek Smolec wrote:

> Hi Bill,
> 
> Thank you for very detailed explanations. I have two more comments.
> 
> But even after dealing with the type1 vs type2 question, I've still had problems with the next question: how do you do the interpolation in the type2 tables for a given composition?  Each type2 table is for a given X, Zbase, dXC, and dXO.  The values dXC and dXO are the (non-negative) enhancements in carbon and oxygen.  So the actual Z for the table is Zbase + dXC + dXO.   So the seemingly simple question is given the cell abundances, which type2 tables do you use to interpolate?  The code in mesa/kap is my current attempt do answer this, but there may be better solutions.   It is all there for anyone who would like to dig into the details.
> I browsed descriptions of input physics in other codes and it seems that in several of them type 2 opacities are simply not used (even though tracks are published up till thermal pulses) or their use is not described. Parsec (Bressan et al 2012) is an exception here, as they nicely describe what they did. Their approach is different however. Their type 2 tables are in fact H-free tables and so are used only then hydrogen is depleted. Then they interpolate in R, T, Y, Z and XC/(XC+XO).

Interesting!   That's a much cleaner approach.  We need a volunteer to create a similar set of tables for use in mesa.  I'd be willing to add the necessary routines to mesa/kap to interpolate them. 
> 
>       real(dp) :: kap_type2_logT_lower_bdy = 3.8d0
>       ! below this, turn off type2
> A comment on the default values. Of course it is the MESA-user job to check whether the default values will work for his/her problem, but logT=3.8 is very low. Taking into account its priority with respect to X-controls, the type 2 opacities once turned on are in fact used in all zones, maybe except the thin atmospheric layers of cool stars, during full evolution, also MS or even PMS, when no significant enhancement in C/O is expected. Indeed, the MS tracks computed with type 2 opacities differ from those computed with type I opacities (assuming default values for X/T transitions). The differences are likely caused by larger interpolation gaps in type 2 opacities (interpolation is less accurate), but also in my specific case, by different solar mixtures. With the default logT transition to Type 2 we actually use gn93 mix in all the star, independent of what was set for Type 1 mix (with kappa_file_prefix, gs98 in my case).

I'm confused here --- what do you think the default should be?   Or do you think this control shouldn't exist?   Or what?   Keep in mind the cases with C/O rich material near the surface.

Cheers,
Bil



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