[mesa-users] off-center Ne and O burning (again…)

[Tuguldur Sukhbold]

Dear MESA users,

I am getting 2 different results with the exact same inlist for two different versions of the code: 4798 and the current 4906. Since MESA is constantly being developed, I understand that we should expect changes between the versions. However I am little surprised to see such a 'big' change in two releases came out almost one after the other.

The attached plot shows the central T-Rho plane evolution for 15Msun model with the following simple inlist:

&star_job

      mesa_dir = '' 
      create_pre_main_sequence_model = .true.

      set_rate_c12ag = 'Buchmann'
      num_special_rate_factors = 1
      reaction_for_special_factor(:) = 'r_c12_ag_o16'
      special_rate_factor(1) = 1.2

      eos_file_prefix = 'mesa'
      kappa_file_prefix = 'gs98'
      
/ !end of star_job

&controls

      initial_mass = 15
      initial_Z = 0.02
      initial_Y = 0.25

      mixing_length_alpha = 2

      RGB_wind_scheme = 'Nieuwenhuijzen'
      AGB_wind_scheme = '' 
      RGB_to_AGB_wind_switch = 1d-4
      Nieuwenhuijzen_wind_eta = 1.0d0

      photostep = 50
      profile_interval = 25
      history_interval = 1
      terminal_interval = 10
      write_header_frequency = 10

!hign
!      xa_central_lower_limit_species(1) = 'h1'
!      xa_central_lower_limit(1) = 0.72
!hdep
!      xa_central_lower_limit_species(1) = 'h1'
!      xa_central_lower_limit(1) = 0.01
!heign
!      xa_central_upper_limit_species(1) = 'c12'
!      xa_central_upper_limit(1) = 0.01
!hedep
!      xa_central_lower_limit_species(1) = 'he4'
!      xa_central_lower_limit(1) = 0.01
!cign
!      log_center_temp_limit = 8.69897 !0.5d8K
!cdep
      log_center_temp_limit = 9.07918 !1.2d9K
!odep
!      xa_central_lower_limit_species(1) = 'o16'
!      xa_central_lower_limit(1) = 0.05

/

If you guys remember, I have previously brought up the issue of off-center O burning, in the case of alpha_mlt = 1, in version 4798. This behavior was gone if I set this parameter to alpha_mlt = 2, as it creates a larger CO core. However, when I try to recreate the older results with the current version of MESA - 4906 - I am facing the off-center burning issue again with alpha_mlt = 2. 

In the response to my previous email, Bill has reported that he got the same result (no off-center burning) with alpha_mlt=2 in his version of MESA - 4917.

MESA version	alpha_mlt=1	alpha_mlt=2
4798			y			n
4906			y			y
4917			y			n

(yes/no to off-center Ne and O burning)

I haven't seen anything in the release email from Bill for 4906, that might be relevant to what I see here. 

In general, 15Msun is a funny mass in MESA. It is sitting on the border of two different paths of evolutions of CO core: (1) degeneracy is important and off-center burnings of Ne and O as we've seen (2) degeneracy effects are no longer important and everything ignites in the center first. In other words, 15Msun model essentially with all the default settings, is producing a CO core mass that borderlines these two paths. This is why changing parameters here and there are producing these rather big changes for this 15Msun case. I don't think one will see this behavior in bigger mass models in any version of the code with any alpha_mlt, simply because the CO core will be big enough.

From my experience, this funny mass is usually much lower in other codes, more like in the 8-12Msun range.

Tuguldur



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