[mesa-users] max Z for Type2 opacities -- Z vs. Z_base

[Bill Paxton]

On Oct 24, 2013, at 12:00 PM, Radek Smolec wrote:

> Indeed, as helium burning is going, not only we move quickly from Z=0.03 to 0.1 but also to much larger Z. And once helium is depleted we actually have Z=1 in the core. So it seems reasonable to have tables for much larger Z, up till Z=1 and not to extrapolate(?) from Z=0.1 table. I don't think the temperatures are larger than 8.7 at this point and we are in the Compton region.  But maybe my understanding is wrong here? After all OPAL published C/O enhanced tables for gn93 only up to Z=0.1. The highest possible value of Z is 0.2 according to their www.
> Radek 

this is where the distinction of Z vs Z_base comes in.    if you start with Z = 0.02 before doing He burning, then you increase Z to 1.0 by converting He to C and O, you should still be okay --- just use the Type2 table with Z_base = 0.02 and set the dXC and dXO enhancements to reflect the added C and O.   Mesa will interpolate your dXC and dXO using the various tables for different C/O enhancements to the base amounts of C and O.  It's tricky, but I think it works.
There are problems later when you start burning C but until then, as long as your starting Z is < the max Z_base you should be okay -- modulo the problems described in the following.

NOTE: Things get complicated when your starting Z doesn't match any of the table Z_base values. If your starting Z = 0.015 and the nearest tables are for Z_base = 0.01 and 0.02, what do you do?  If you have only a small enhancement in C/O, then you might not be able to use the Z_base = 0.02 tables since you'd need a negative enhancement relative to that base to get your actual C & O abundances and the tables don't support negative enhancements.  If you skip using the Z_base = 0.02 table to avoid that problem, then you are stuck with Z_base = 0.01 and that will fail to reflect the abundances of non-C/O metals in your Z = 0.015 material.  What's the right answer?  I don't know.  Perhaps there isn't any good solution.

As you point out, the mesa tables currently use OPAL Type2 tables with max Z_base of 0.1 
I think that was the limit back in the old days, but you correctly note that the webpage now gives 0.2 as a max for custom built tables.

The higher value might be of use during and after C burning -- I don't know.
But if someone makes a case for adding them, it is certainly possible.

Cheers,
Bill



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