[mesa-users] Specific entropy odd increase in fully convective star

[Valery Pipin]

Hi Lewis,

You should get the expected result if you will use the MLT expression 
for heat transport where the gradient entropy is involved.
It depends on your goals. Take the  convective flux as known.
I computed it recently for my purpose as input parameter for my models 
of the  mean-field anisotropic heat transport in rotating fully
convective 0.3 MS star  of 1Gyr age (not ZAMS). As expected, the 
gradient is negative and rather small in the bulk of the star
and it become steep at the very near surface layers.

Best regards!
Valery
> Hi Lewis,
>
> Is this larger than the deviation from constant entropy that MLT 
> predicts? Near the surface I'd expect a larger gradT-grada, which 
> would be consistent with some steepening of the entropy profile.  You 
> might use gradT_sub_grada to check this.
>
> My other thought: does your model get sufficiently cool that it dips 
> into SCvH territory?  You can use
>
> show_TRho_Profile_eos_regions = .true.
>
> to find out. The SCvH tables themselves from 1995 are quite sparse and 
> so their implementation in MESA required some interpolation.  As a 
> result the entropy is pretty wonky even for cases where gradT-grada is 
> tiny, as in giant planets and brown dwarfs.  Thankfully entropy is a 
> secondary variable as far as MESA is concerned-- the solver doesn't 
> work with entropy directly; it's generally computed post-hoc in an EOS 
> call.  Thermodynamic derivatives like grad_ad, critically, are OK. See 
> this thread 
> <http://sourceforge.net/p/mesa/mailman/mesa-users/thread/CAD995WOyF7NC16JvKeyBT6NZBzGgN3Hoqu63wXMVXn6TnNpa%2Bw%40mail.gmail.com/#msg32094236>.
>
> Maybe your model is safely outside this range of T-Rho space.
>
>
> Chris
>
> On Thu, Oct 29, 2015 at 9:44 AM, Ireland, Lewis <lgi201 at exeter.ac.uk 
> <mailto:lgi201 at exeter.ac.uk>> wrote:
>
>     Hi all,
>
>     I am modelling a 0.3 solar mass star using the (condensed) following:
>
>     &star_job
>
>       ! begin with a pre-main sequence model
>         create_pre_main_sequence_model = .true.
>
>     / !end of star_job namelist
>
>     &controls
>
>       ! starting specifications
>         initial_mass = 0.3d0 ! in Msun units
>
>     mixing_length_alpha = 2.0d0
>
>       ! stop when the star nears ZAMS (Lnuc/L > 0.99)
>         Lnuc_div_L_zams_limit = 0.99d0
>         stop_near_zams = .true.
>
>       ! stop when the center mass fraction of h1 drops below this limit
>         xa_central_lower_limit_species(1) = 'h1'
>         xa_central_lower_limit(1) = 1d-3
>
>     / ! end of controls name list
>
>     For the fully convective case, I expect the entropy to be roughly
>     constant throughout up until the photosphere, but I get an
>     artefact of ds/dr > 0 after about half way through the stellar
>     interior out to the surface (please view attached image - the
>     constant entropy profile (red) is from a formulation that
>     calculates what I should be getting). What is causing this
>     increase in specific entropy at this point? I expect there to be
>     some increase near the photosphere when radiative transfer takes
>     over from convection, but why is it starting to increase so early
>     in the radial profile? Is there another test case I can use to
>     model this kind of star?
>
>     Kind regards,
>
>     Lewis
>
>
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