[Mesa-users] BD deuterium burning

Thu Aug 7 20:27:26 UTC 2025

Dear Jared,

Thank youso much for your elaborate answer. You are correct, I get to log Tc=5.5 , for a shortperiod of time.  

>From what Isee the nuclear activation energy is too high  for my example and ifthere is deuterium burning at this temperature it is small.

I found this reference by Chabrier and Baraffe2000: https://www.annualreviews.org/content/journals/10.1146/annurev.astro.38.1.337

Their figure 2, indicate (although not exactly similar to my example),no D – burn. From what I read in their page 357, I need a higher mass forD-burn. I will work on it.

Thank you so much for your help,

Ealeal 

    On Thursday, August 7, 2025 at 05:34:47 PM GMT+3, Jared Goldberg <jgoldberg at flatironinstitute.org> wrote:  

 Hi Ealeal,
I suspect this is real, that a bit of D burning is going on. We don't need much D burning to supply the faint brown dwarf with its luminosity. 
In your models, what is the power produced by deuterium burning (or the pp chain broadly, which is dominated by deuterium burning at these temperatures)? Is it comparable to the total luminosity leaving at the surface? 
Here's a few history plots I see when I try to reproduce your results by modifying the test_suite. Note how the luminosity from nuclear reactions starts to match the luminosity from the star: we've reached a main-sequence where nuclear burning provides the stellar luminosity. You should verify for yourself in your models if this is what's happening.

This makes sense to me. Here's my logic: 
Our intuition for the required T_c = 10^6 K comes from stars of about a solar mass. Deuterium burning is quite sensitive to T. The luminosity is pretty sensitive to the mass. 
Very loosely, in gas-pressure-dominated stars, your luminosity L scales like M^3. For D burning, L scales like T^12 (check me on this -- I found it with a quick google, but the lack of weak reactions make me think it should be somewhere between pp sensitivity to T which is shallower, and cno-cycle sensitivity which is steeper). If D is supplying of-order the stellar luminosity then you can set those equal to eachother, in which case we get that the required T_c \propto M^1/4. 
If that's the case, and the brown dwarf is 0.01 solar masses, then to supply the luminosity with deuterium burning we only need central temperature T_c ~ 0.3 times the T_c required for 1 solar mass (10^6K), or ~10^5.5K. That's a bit higher than 5.3 as you report, but my estimate was quite crude.  (Also, in the above plot you do start to achieve 10^5.5 K temperature when you start contracting... then as you do fusion, you slow the contraction, run shorter on fuel, and the central temperature drops as you add heat.)
So if you *start* to get a tiny bit of D burning even earlier than T_c ~ 10^6 K, then perhaps the star decides that a bit of burning is enough; if you were burning D at 10^6 K your nuclear luminosity may exceed the luminosity of the brown dwarf itself. 
Now, there's a separate question: Is it possible to fuse any deuterium at these low temperatures? Or is the "nuclear activation energy" too high? I'll let you do the fun calculation of the probability of quantum tunneling through the coulomb barrier for a proton and an h2 nucleus in a thermal distribution at 10^5.3K and brown dwarf densities. It does appear that MESA thinks it's possible.
I hope this helps! And others should feel free to chime in if I made an error in my logic. 
Cheers,~Jared
On Thu, Aug 7, 2025 at 4:21 AM Bear Ealeal <ealeal44 at yahoo.com> wrote:

Dear Jaredand Matías,

Thank you very much for your answers.

What I did is, I enabled the initialcomposition (the net was already ok) to:

set_uniform_initial_composition= .true.

                              initial_h1  = 0.70

                              initial_h2  = 2d-5

                              initial_he3 = 1d-5

                              initial_he4 = 0.28
and now I getan increase in He3 and a decrease in h2, in the center, however the center temperatureis still about logT_c \approx 5.3, which surprises me since  I expected it to be 10^6K.
The raw_rate_r_h2_pg_he3is between 10^27  - 10^32 reactions / (g·s).  The value of raw_rate_r_h2_pg_he3 increases up to 10^32 andthen decreases.

Do you knowif these values are indicative to deuterium burning?  I do not think I  have deuteriumburn because of the center T.  I will tryto play with the initial composition but any other suggestions will be great.

Thank youvery much,

Ealeal

    On Wednesday, August 6, 2025 at 11:40:49 PM GMT+3, Jared Goldberg <jgoldberg at flatironinstitute.org> wrote:  

 By default, MESA uses a very simplistic nuclear reaction network called "basic.net" which does not include deuterium or its burning pathways. (See https://docs.mesastar.org/en/latest/net/nets.html)
Even when using a network including deuterium, it does not initialize stars with any deuterium by default, so you will need to explicitly set the initial abundances including deuterium, in addition to making sure that your nuclear reaction network contains deuterium burning. 
In practice, this means that in the &star_job section of your inlist, you will want to switch to a net that includes the appropriate reactions, for example: 

change_net = .true.
change_initial_net = .true.
new_net_name = 'pp_and_cno_extras.net'

You will then need to add deuterium by setting the initial composition or relaxing the composition, e.g. 

set_uniform_initial_composition = .true. (documentation: https://docs.mesastar.org/en/latest/reference/star_job.html#set-uniform-initial-composition)
initial_h2 = 1d-5 ! or however much deuterium you want

and set the other initial abundances and abundance ratios as desired. 
Cheers,~Jared

On Wed, Aug 6, 2025 at 2:13 PM Matias Jones via Mesa-users <mesa-users at lists.mesastar.org> wrote:

Hi Ealeal,

Probably you need to modify the default rate_list.txt file (I guess is under data/rates_data/rate_tables/ dir) to include D burning. This worked for me.

I am using an older MESA version, but probably this is also the case for v23.5.1.

Cheers, 

--------------------------------------------------------------------
Dr. Matías Jones F.
VLT Operations Staff Astronomer
ESPRESSO Instrument Scientist SPHERE Instrument Scientist (IS2)European Southern Observatory
Alonso de Córdova 3107, Vitacura, Santiago, Chile
--------------------------------------------------------------------De: Mesa-users <mesa-users-bounces at lists.mesastar.org> en nombre de Bear Ealeal via Mesa-users <mesa-users at lists.mesastar.org>
Enviado: miércoles, 6 de agosto de 2025 12:26:50
Para: mesa-users at lists.mesastar.org
Asunto: [Mesa-users] BD deuterium burning 
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Dear Mesa-users,

I am working on MESA 23.5.1 and I run the example from test -suite named: "make_brown_dwarf" with no changes.

mass and radius are:

mass_in_gm_for_create_initial_model = 1.9892d31 ! 0.01 Msun; m_jupiter = 1.8986d30; msun = 1.9892d33

   radius_in_cm_for_create_initial_model = 3.5d10 ! 5 r_jupiter; r_jupiter = 6.9911d9. ok for most brown dwarfs.
The age when the inilist stops is 2Gyr. 
However, I do not get deuterium burning although I getraw_rate_r_h2_pg_he3 up to 1e18, the temperature at the center is too low also and there is no increase even not in the core in He3.Does someone have experience with BD in MESA, especially how to make the deuterium burn ?
Thanks,

Ealeal

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