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

[Ireland, Lewis]

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<mailto: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<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

<entropy_v_radius.png>

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