[Mesa-users] Hydrogen burning with no hydrogen?

[Richard Townsend]

Hi Sunny —

This is a good analysis, but I think because Nicholas has dxdt_nuc_factor = 0, no H will be produced.

However, the energy generation from 3He + 3He (and 3He + 4He) will still generate energy, and this energy will be counted as “hydrogen burning” since it is part of the pp chains.

Either way, your fix of setting initial_he3 should work fine.

cheers,

Rich

> On Jul 25, 2025, at 6:42 PM, Sunny Wong via Mesa-users <mesa-users at lists.mesastar.org> wrote:
> 
> Hi Nicholas, 
> 
> Here's my guess for what happened (and others please 
> correct me):
> 
> If you didn't change the initial_he3 flag in controls, MESA will 
> assume that he4/he3 has the same value as the Sun. Then, 
> once the temperature reaches ~10^7 K, part of the pp chain 
> starts: He3 + He3 -> He4 + 2 H1 . 
> 
> Once some He burning has happened, some C12 is produced. 
> Since there is some H1 lying around, you can then start the 
> CNO cycle. 
> (Fun MESA problem: even if you start with a zero metallicity 
> H-rich star, it will eventually do CNO cycle on the main sequence 
> via this way, because there's a tiny amount of He burning that 
> produces C12. )
> 
> In short, you can set
> initial_he3 = 0d0 
> on top of what you already have (initial_y = 1d0)
> and I believe this would solve the H burning issue. 
> 
> As an aside, I notice that you set initial_z = 0d0. 
> You might want to set Zbase = 0d0 in your &kap section as well. 
> 
> Cheers,
> Sunny
> 
> 
> 
> 
> 
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--
Rich Townsend • Professor of Astronomy
Astronomy Department • University of Wisconsin-Madison
Phone: 608-262-1752 • E-mail: rhtownsend at wisc.edu