[Mesa-users] BD deuterium burning

Thu Aug 7 14:34:30 UTC 2025

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.

[image: image.png]

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 Jared and Matías,
>
> Thank you very much for your answers.
>
> What I did is, I enabled the initial composition (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 get an increase in He3 and a decrease in h2, in the center,
> however the center temperature is still about logT_c \approx 5.3, which
> surprises me since  I expected it to be 10^6K.
>
> The raw_rate_r_h2_pg_he3 is between 10^27  - 10^32 reactions / (g·s).  The
> value of raw_rate_r_h2_pg_he3 increases up to 10^32 and then decreases.
>
> Do you know if these values are indicative to deuterium burning?  I do not
> think I  have deuterium burn because of the center T.  I will try to play
> with the initial composition but any other suggestions will be great.
>
> Thank you very 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 v
> 23.5.1.
>
>
>
> Cheers,
>
>
>
> *--------------------------------------------------------------------*
> *Dr. Matía**s 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
>
> This email was sent from outside of ESO from the address *[mesa-users-bounces at lists.mesastar.org
> <mesa-users-bounces at lists.mesastar.org>]*. If it looks suspicious, please
> report it to phishing at eso.org.
>
>
> 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 get *raw_rate_r_h2_pg_he3
> up to 1e18, t*he 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
>
>
>
> _______________________________________________
> mesa-users at lists.mesastar.org
> https://lists.mesastar.org/mailman/listinfo/mesa-users
>
>
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