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

[Chris Mankovich]

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> 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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