[mesa-users] Weird HR diagrams.

Falk Herwig fherwig at uvic.ca
Thu Dec 3 18:02:53 EST 2015


Dear Felix,
Sam Jones pointed out to me that your simulation shows a very-late thermal pulse. He is right. The He-shell flash is being launched at model 4385. The He-shell flash convection zone will then draw H into the He-burning layer and burn it very rapidly. That’s what drives the further evolution. In your 1D stellar evolution model the H-shell burning luminosity reached log LH > 10 and the time step goes down to seconds or less. MESA let’s you do these runs, but clearly at this point your simulation violate 1D stellar evolution assumptions, such as the assumptions built into MLT that describe convection as time and spatial averages. Making time steps that are shorter than a few convective turn-over time scales is leading to a violation of the assumptions of the 1D code, and should be avoided. In fact I would propose to add a switch to MESA “dt_times_conv_MLT-time_min” that stops the code whenever dt is becoming less than t_conv * dt_times_conv_MLT-time_min.

At these high H-burning luminosities the spherical symmetry and convection assumptions of present 1D codes, such as MESA, are not very good anymore.

These H-ingestion events are well known in stellar evolution for a very long time, e.g.:

Iben, I. J., Kaler, J. B., Truran, J. W., & Renzini, A. (1983). On the evolution of those nuclei of planetary nebulae that experience a final helium shell flash. The Astrophysical Journal, 264, 605–612. http://doi.org/10.1086/160631


[…] many more [...]


Herwig, F., Blocker, T., Langer, N., & Driebe, T. (1999). On the formation of hydrogen-deficient post-AGB stars. Astronomy and Astrophysics, 349, L5–L8.

Herwig, F. (2001). Internal mixing and surface abundance of [WC]-CSPN, Astrophysics and Space Science 275, 15–26.

Herwig, F. (2001). The Evolutionary Timescale of Sakurai’s Object: A Test of Convection Theory? The Astrophysical Journal, 554(1), L71–L74. http://doi.org/10.1086/320911

[…] many more ….

These papers are guilty of applying the 1D approximation where it is not very good, or in fact wrong in at least some ways.

This paper starts to address the issue via 3D hydro simulations:

Herwig, F., Woodward, P. R., Lin, P.-H., Knox, M., & Fryer, C. (2014). Global Non-spherical Oscillations in Three-dimensional 4π Simulations of the H-ingestion Flash. The Astrophysical Journal, 792(1), L3. http://doi.org/10.1088/2041-8205/792/1/L3

Best, Falk.








On Dec 1, 2015, at 12:43 PM, Félix Blais <felix.blais.duff at gmail.com<mailto:felix.blais.duff at gmail.com>> wrote:

Hello, I did put okay_to_reduce_gradT_excess = .true. in the &controls of inlist_1.0. And i have a result that make me laugh a lot (looks like the star is dancing). I finally got the termination code : Log_L_lower_limit

stop because log_surface_luminosity <= log_L_lower_limit

First of all. i read what does this line do, but why do i have to put it in this case and not on the other? Why is this line not included in 1M_pre_ms_to_wd?

Enjoy the video. Would you say it's normal? This is the craziest one i have, because usually, the stars come back a little on the AGB phase and goes to WD after. I noticed that this last flash usually occurs with low metallicity (from my models), but now, at 0.03.... i dont know what to think.

Thank you

Félix B

2015-11-29 22:36 GMT-05:00 Jared Brooks <brooksjaredc at gmail.com<mailto:brooksjaredc at gmail.com>>:
Hey Felix,

Great video! It looks like your model underwent a pretty sizable last flash in the helium layer. I noticed the the test case you are working with (1M_pre_ms_to_wd) doesn't use MLT++, so you might want to try that by setting okay_to_reduce_gradT_excess = .true. in the controls section.

Hope this helps.

-Jared

On Sun, Nov 29, 2015 at 6:50 PM, Félix Blais <felix.blais.duff at gmail.com<mailto:felix.blais.duff at gmail.com>> wrote:
Hello, I am using the 1M_pre_ms_to_wd/ to check the evolution of stars depending on their metallicity (z) and Mass (M). I did plenty of simulations, but at 1.6M_z_0.03, I had a result that made no sense to me. There is an attachment of the video.

At first, I had this error

stopping because of convergence problems dt < min_timestep_limit



terminated evolution: convergence problems


with :


change_lnPgas_flag = .true.

new_lnPgas_flag = .true.


! check for retries and backups as part of test_suite

! you can/should delete this for use outside of test_suite

! max_number_backups = 200

! max_number_retries = 500


Then i commented change_lnPgas_flag and new_lnPgas_flag, but i got the result shown in the attachment. I had to stop it because the run had 10000 models done and the star wouldn't move.


What's happening? It doesnt do this with all of my other simulations.. (which inculde 1.6M at different metallicity, 1.8,1.4,1.2) Help!


Thank you


Félix B.

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<1.6M_z_0.03_.mp4>------------------------------------------------------------------------------
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--
Falk Herwig
Dept of Physics & Astronomy, U of Victoria
fherwig at uvic.ca<mailto:fherwig at uvic.ca>, tel: +1 (250) 721-7743



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