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

[Chris Mankovich]

>
> In my plot, the red line shows s(r) = s_central + int_0^R (ds/dr) dr,
> where ds/dr = -{c_p/H_p) (gradT_sub_grada), and is what I expected the
> entropy profile to look like for a fully convective model.


Ah, very good.

I am not familiar with what SCvH tables are, could you briefly explain what
> these are please? So, if I understood correctly, it could be the
> interpolation that MESA performs that could cause this strange artefact of
> specific entropy?
>

SCvH (Saumon, Chabrier, & van Horn 1995
<http://adsabs.harvard.edu/abs/1995ApJS...99..713S>) is one of the
equations of state in MESA for low temperatures. Whether zones in your
model have eos info coming from SCvH just depends on their rho and T.  From
mesa1 <http://adsabs.harvard.edu/abs/2011ApJS..192....3P>:
[image: Inline image 1]

Again you can plot these regions on the pgstar TRho_profile diagram
using show_TRho_Profile_eos_regions
= .true. .

So, if I understood correctly, it could be the interpolation that MESA
> performs that could cause this strange artefact of specific entropy?


It could be.  It's not unlike things I've seen in the entropy profile in
lower-mass objects.


Chris

On Fri, Oct 30, 2015 at 2:47 AM, Ireland, Lewis <lgi201 at exeter.ac.uk> wrote:

> Hi Chris,
>
> Thank you for your response. In my plot, the red line shows s(r) =
> s_central + int_0^R (ds/dr) dr, where ds/dr = -{c_p/H_p) (gradT_sub_grada),
> and is what I expected the entropy profile to look like for a fully
> convective model.
>
> I am not familiar with what SCvH tables are, could you briefly explain
> what these are please? So, if I understood correctly, it could be the
> interpolation that MESA performs that could cause this strange artefact of
> specific entropy?
>
> Thanks again!
>
> Lewis
>
> On 29 Oct 2015, at 19:30, Chris Mankovich <cmankovich at ucsc.edu> wrote:
>
> 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
>>
>> <entropy_v_radius.png>
>>
>>
>> ------------------------------------------------------------------------------
>>
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>> mesa-users at lists.sourceforge.net
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>>
>>
>
>
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