[Mesa-users] Inquiry about Units in profile.data

[Guo Li]

Hi Meng and Diogo




Thank you for your guidance！




I am currently using MESA version r12778 and have modified the example from the test_suit, specifically based on the agb example. I have identified that the profile column 'log_rate_r_c12_ag_o16' is defined in the src/run_star_extras.f file.

My goal in examining the reaction rates is to calculate the timescales for specific reactions, such as the CNO cycle, and to compare them with the timescales for convective mixing. I have considered whether the appropriate units might be dy/dt (grams/(mole sec)), dn/dt (cm^-3/s), or simply the reaction rate units (cm^3/(mole sec)). However, my calculations have not yielded consistent results, and I have not found any documentation specifying the units for the profile column 'log_rate_r_c12_ag_o16'.

Any guidance or references would be greatly appreciated.

Best regards,
Guo Li
National Astronomical observatories,CAS













At 2025-02-17 18:13:16, "Diogo Capelo" <diogo.capelo at tecnico.ulisboa.pt> wrote:

Greetings,

That will depend on the version of MESA you're using. In the newer versions of MESA (23 and 24), the reaction rates you can output directly from profile_columns.list are in units of (s^-1). However, the default names of the columns don't match the one you use.

The name you use suggests to me (but I may be mistaken) that you are using an older version (pre-23) of MESA with a custom piece of code in run_star_extras to read out the reaction rates. In this case, the documentation on the “rate_screened” variable (which is the one likely being read and printed) is defined in “$MESA_DIR/net/public/net_def.f90”, where you can read:

“The units here depend on the number of reactants. In all cases, the rate_screened times as many molar fractions as there are reactants gives a number with the same units as dy/dt. So for a 2body reaction, there are 2 Y factors, each with units [moles/gram] and the rate_screened units for such a reaction are [grams/(mole sec)], which when multiplied by [moles/gram]^2 gives the same units as dydt. For a 1body reaction (e.g., a decay), there is only 1 Y factor, so the units are [1/second]. Similarly, a 3body reaction will have rate_screened with units of [gram^2/(mole^2sec)].”

I would advise checking whether you're writing the rates into the profile directly or whether you're outputting them via run_star_extras and, in the latter case, checking which variable is being passed onto the profile, and then tracking down that variable in the MESA files.

Hope this helps!

--

Diogo

A 2025-02-17 09:25, Guo Li via Mesa-users escreveu:

Hi all,

I hope this message finds you well.

I am currently analyzing the profile.data file and would like to inquire about the units for the reaction rate column, such as log_rate_r_c12_ag_o16. 

Thank you for your support!

 
Best regards,
Guo Li
National Astronomical observatories,CAS
 


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