[Mesa-users] Turning off hydrogen recombination energy

Wed Aug 9 10:16:09 UTC 2023

Hi Evan,

That’s super helpful thank you and has cleared up much of my confusion!

Many thanks,
Natalie

From: Evan Bauer <evan.bauer.astro at gmail.com>
Date: Monday, 7 August 2023 at 19:22
To: Rees, Natalie R (PG/R - Maths & Physics) <n.rees at surrey.ac.uk>
Cc: Ebraheem Farag <ekfarag at asu.edu>, mesa-users <mesa-users at lists.mesastar.org>
Subject: Re: [Mesa-users] Turning off hydrogen recombination energy
Hi Natalie,

Instead of having an explicit source term for ionization energy, it can be included as part of the definition of "specific internal energy" (e) reported by the EOS, depending on which EOS is being employed. That means that if your EOS tracks the various ionization states and accounts for the energy associated with them, then that energy term can end up in the energy equation as something like de/dt or eps_grav, depending on which form of the energy equation you have selected for MESA to use.

There's a carefully worked out example of this in section 8.4 and figures 53-55 of the 4th MESA instrument paper:
https://iopscience.iop.org/article/10.3847/1538-4365/aaa5a8#apjsaaa5a8s8-4

More recent versions of MESA now default to using FreeEOS rather than OPAL in the regime relevant to that worked example, so it might be worth reworking that exercise in your version of MESA to see for sure whether both the H ionization energy and molecular H2 energy are both accounted for with the more recent EOS.

Cheers,
Evan

On Aug 7, 2023, at 10:12 AM, Natalie Rees via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>> wrote:

Hi Ebraheem,

Sorry for the delay in getting back to you. Yes I am evolving models with a range of masses through the TP-AGB and it occurs for M>2Msun when a significant amount of the envelope mass has been lost. I am currently using version 15140 however I am not opposed to updating if that would be helpful.

I guess what I’m trying to get to the bottom of is where the energy released by recombination is accounted for in the energy equation. I can see from the code these are the contributing terms however it is not clear to me where ionization/ recombination comes into this

From hydro_energy.f90:

         esum_18 = - dL_dm_18 + sources_18 + others_18 - dEturb_dt_18 - dwork_dm_18 + eps_grav_18

         s% energy_sources(k) = sources_18%val

            ! nuclear heating, non_nuc_neu_cooling, irradiation heating, extra_heat, eps_mdot

         s% energy_others(k) = others_18%val

            ! eps_WD_sedimentation, eps_diffusion, eps_pre_mix, eps_drag, Eq

I am also wondering whether it would be better to use other_energy or other_energy_implicit. Any thoughts on these things would be greatly appreciated!

Many thanks,
Natalie

From: Ebraheem Farag <ekfarag at asu.edu<mailto:ekfarag at asu.edu>>
Date: Thursday, 27 July 2023 at 17:20
To: Rees, Natalie R (PG/R - Maths & Physics) <n.rees at surrey.ac.uk<mailto:n.rees at surrey.ac.uk>>
Cc: mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org> <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>>
Subject: Re: [Mesa-users] Turning off hydrogen recombination energy
Hello Natalie,

Have you evolved a stellar evolution model through the TP-AGB and encountered this instability? Would you also mind sharing which version of MESA you are using?

-EbF

On Thu, Jul 27, 2023 at 2:46 AM Natalie Rees via Mesa-users <mesa-users at lists.mesastar.org<mailto:mesa-users at lists.mesastar.org>> wrote:
Dear mailing list,

I am trying to prevent the hydrogen recombination instability in TP-AGB Stars (Wagenhuber & Weiss 1994, https://ui.adsabs.harvard.edu/abs/1994A%26A...290..807W/abstract<https://urldefense.com/v3/__https://ui.adsabs.harvard.edu/abs/1994A*26A...290..807W/abstract__;JQ!!IKRxdwAv5BmarQ!YxGy-aFmrhOGNZFQT85QQgwqwyfSS1mavuTbVhkxcQFrOHyHw7wz1UoEXEkRMHZnQZin14bXIOZhPDtBzW6nMl4U$>). To do so I want to try removing the energy produced by hydrogen recombination in the outer envelope temporarily. Does anyone have any guidance to as to how I could achieve this? I’ve tried playing around with the other_energy hook a bit but even if I inject a small amount of negative heat (small compared to the current energy of the shell) it causes convergence difficulties. Any advice or suggestions would be greatly appreciated.

Many thanks,
Natalie Rees

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