Central Carbon burning almost finished before He depletion in massive stars

[Debraheem]

Hello,

I've been testing some models in r22.11.1 and I've noticed an "issue", which I can't identify as a runtime issue or bug?

The problem:

When running massive stars of any mass in r22.11.1, I've noticed that central carbon burning (c12ag) begins before He is depleted and depletes the core of Carbon before He burning is complete. This is a departure in burning behavior from what I've encountered in previous mesa versions as well as much of what I've read in the literature. Typically I would expect Helium burning to finish with some c12ag burning adjusting the C/O ratio before Carbon ignition (c12+c12) occurs later at higher rho and T. Instead I'm encountering models where the central core region is almost entirely depleted of Carbon before the convective core vanishes.

I've noticed this behavior across a variety models regardless of the inlist controls I choose, in stripped and non-stripped stars, when comparing back and forth from r22.11.1 to r15140.

To illustrate this I've taken the "20M_z2m2_high_rotation" test_suite case in r15140 and r22.11.1 and modified them both to be relatively identical. They share the following:
Machine: Mac, OS: Ventura 13.2, sdk version mesasdk-x86_64-macos-22.10.1.

• 20 Msun
• No mass loss
• Henyey convection (though turning on TDC doesn't change the result)
• no semi convection or thermohaline
• no rotation
• similar mesh and temporal resolution
• identical overshooting only in core.
• same abundance, kap, and eos
• identical rate preferences ( jina prefs =2 in r15140, which is essentially the same as r22.11.1 new default)
  ( I've tested changing any one of these, and the result in r22.11.1 is identical, so I'm pretty sure none of these inlist controls will effect the result anyway. )

I used the rates module to manually compare the r12+12 and r-3alpha rates to ensure they are identical, and I'm using the same c12ag rate in both models, which I also compared directly and from the rates cache.

For simplicity, I ran both models to center_he4 = 1d-15, but you could in principle restart from a photo and run them further.

In r15140, At the stopping condition center_He = 1d-15 a significant fraction of c12 remains in the core: center_c12 ~ 0.286
In r22.11.1, At the stopping condition center_He = 1d-15 little c12 remains in the core: center_c12 ~ 3e-6

Here are the models, if anyone would like to try running them, they run in 10-15 minutes on a 10 core machine. I've included pgstar movie in each model directory.
20Msun_C_burn_before_he_deplete_compare.tar.gz

Please, if anyone can help me figure out what is going on? Is this normal, Is this a bug? I'm quite curious!

Thanks!
-EbF

[rjfarmer]

Thanks Eb, I'm having a look. Just to note you do have a rate_tables/ folder which will override the default c12(a,g)o16 rate.

[rjfarmer]

Even switching back to the mesa defaults still shows this issue.

I used the rates module to manually compare the r12+12 and r-3alpha rates to ensure they are identical,

I've double-checked as well and agree they are the same.

Looking at the Kippenhahn I'm almost wondering if we got a breathing pulse? In 15140 the He core recedes smoothly. But in 22.11.1 as the core recedes, the recession stops and the core expands a little before receding again. If you look at the abundance profile in 22.11.1 you'll see a small pocket of c12 just below the He shell and the core (and the O16 abundances are not flat in the core).

Not sure why we are getting the difference but it may be a mixing issue rather than a rates issue.

[rjfarmer]

Though if you look at the composition plot as well, there is a lot of Ne and Mg being made as well. Almost like we are doing c12+c12 burning early?

[Debraheem]

Hey Rob, thanks for taking a look.

Looking at the Kippenhahn I'm almost wondering if we got a breathing pulse? In 15140 the He core recedes smoothly. But in 22.11.1 as the core recedes, the recession stops and the core expands a little before receding again. If you look at the abundance profile in 22.11.1 you'll see a small pocket of c12 just below the He shell and the core (and the O16 abundances are not flat in the core).

Not sure why we are getting the difference but it may be a mixing issue rather than a rates issue.

I agree it is likely not a rate issue, definitely not c12+c12. Also, I further compared o16_ag and ne20_ag, and mix and matched runs with the nacre/jina version in r15140, and no difference was observed.

I think your first guess was right on the money, it looks like a breathing pulse to me. The breathing pulse is clear in the Kip diagram by seeing the convective core grow just at the end, and the sharp increase in L_He as well as the power plot which is dominated by the alpha-capture rates c-alpha, o-alpha, and ne-alpha.

I tried turning overshooting off and using conv_premix with 'conv_premix_avoid_increase = .true.', but this did not prevent the second growth of the he core.

I also tried conv_pre_mix a second time with the experimental 'conv_premix_time_factor = 1d0' (instead of 0d0, default), and still I see a breathing pulse just at the end that consumes all the Carbon in the core.

I've attached two more movies of these two runs, if you'd like to take a look.

https://user-images.githubusercontent.com/63124736/235220647-86fc1319-c878-4ae9-a7f8-e89ffc55d644.mp4
https://user-images.githubusercontent.com/63124736/235220682-2bdeab88-3f36-4531-9c85-3e9989a80cd9.mp4

---

Presently, I can't make a single massive star model of any mass or input physics that doesn't display this same behavior. I agree with you that this could be a mixing related issue. The question now is, what changed between r15140 and r22.11.1 that would cause this?

(I tested in r22.05.1, and I get the same result as in r22.11.1, so the change must have actually somewhere between r15140 and r22.05.1)

[evbauer]

Could you check r21.12.1 as well? My guess is that the change is most likely something between r15140 and r21.12.1, but it would be good to be sure.

[Debraheem]

Could you check r21.12.1 as well? My guess is that the change is most likely something between r15140 and r21.12.1, but it would be good to be sure.

Yeah, give me a couple hours, downloading it now on this machine.

[Debraheem]

I tested in r21.12.1 and I get the same result as in r22.05.1 and r22.11.1, so the change is likely occurred between r15140 and r21.12.1, as Evan suggested.

[evbauer]

Hmm, I just quickly ran the r22.11.1 model from your work directory, and it doesn't look like there's a breathing pulse as far as I can tell. I just edited the Kipp plot to zoom in on the end and changed Kipp_mix_interval to 1 to make sure to catch any fluctuations in the convection, but I don't see any. That rules out a breathing pulse, right?

[evbauer]

Same thing with just the Kipp panel so it's easier to see:

[rjfarmer]

Maybe not quite a pulse but that dip and expansion of the convection zone at model ~ 1130 seems to keep the convection zone going for longer than it should and depletes the c12.

[evbauer]

Ah, yes. That dip seems to be where there's basically a transition from He to C burning. But something is continuously feeding in small amounts of He into the convection zone so that "C burning" is basically all alpha captures throughout this phase? That does seem pretty weird. Seems like overshoot must be involved?

[Debraheem]

I get the same-ish result without overshoot, as far as c-alpha burning depleting the core, though the convection kind of dies out. The dip in L_he at model number 1030 is when carbon burning begins. The dip is less pronounced but still there.
(edited with Kipp_mix_interval = 1)

[fxt44]

maybe a sanity check, how do the temperature and density profiles compare between r15140 and an r22* at core helium depletion?

[evbauer]

Thanks for the check on overshoot Eb. I guess it turns out we don't have he4 being slowly fed into the core by mixing.

I've been digging deeper into this, and I suspect it's a rates issue after all. Specifically, even after the convective core vanishes, you can see that all of the burning is still dominated by alpha captures, yet he4 is depleted much more slowly than it seems like it should be. It turns out that this is because there's quite a significant rate of disintegration of c12 back into he4 via r_c12_to_he4_he4_he4. See attached plot.

Should the following be calling do1_reverse instead of do1?

mesa/rates/private/raw_rates.f90

Lines 133 to 134 in 04e4de4

	case(ir_c12_to_he4_he4_he4) ! c12 to 3 alpha
	call do1(rate_tripalf_jina)

[Debraheem]

Woah, nice catch Evan! I think you've nailed it? I changed 'do1' to 'do1_reverse' for the reverse rate 'r_c12_to_he4_he4_he4' in $MESA_DIR/rates/private/raw_rates.f90, recompiled and ran the model again in r22.11.1.

The model behavior is now identical to r15140, and he4 depletes normally 'quickly' as Evan noted it should, with center_c12 = 0.284 at he4 depletion. I think we can call this resolved?

[fxt44]

nice evan!!

.. and oof, what a bug.

[evbauer]

Gold star to Eb for a very important bug report, just in time to get this fixed before the next release.

[Debraheem]

For the record, as Evan and Rob noted: This bug does not effect r21.12.1. This bug only applies to r22.05.1 and r22.11.1.

(My previous r21.12.1 model I ran earlier in this thread was run with a stale rates_cache from a previous run, which contaminated my results.)