[mesa-users] Overshooting

Tue Jun 2 18:25:44 EDT 2015

Along the line of Falk’s reasoning, I would like to point out that very recently, we studied the so-far richest main sequence B-type pulsator, KIC 10526294, which shows 19 consecutive dipole g-modes. We tested several input physics, among which, the choice of step-function versus exponential overshooting prescription, and examined which set of models can reproduce the observation better; indeed, our intuition dictates that the more smooth exponential presciption should be preferred over the steep step-function. However, we tried to be unbiased and let this star guide us through.
What comes out is that the models with exponential prescription — as expected — outperform the other with a factor 2 to 3 in chi-square sense. We hope to keep testing this for our other B-type dwarfs too. 
More information can be found here: http://adsabs.harvard.edu/abs/2015arXiv150506902M <http://adsabs.harvard.edu/abs/2015arXiv150506902M> 
Indeed, we all hope that a more physically grounded model, as Dave preaches, would soon liberate us from this shortcoming of MLT.

Best regards
Ehsan.

> On 02 Jun 2015, at 19:58, Falk Herwig <fherwig at uvic.ca> wrote:
> 
> Dear Kenny, 
> 
> step overshooting is provided in MESA as an option to compare with historical computations that 
> adopted this kind of overshooting treatment, usually at a time when it was numerically not practical
> to treat convective mixing as a time dependent process (say, in the 1970’s). Now, I would say that we 
> have no evidence that overshooting is step-like. Any simulation of convective boundary mixing, 
> including overshooting that has ever been made shows some gradual decline of the mixing efficiency 
> in the transition from unstable to stable stratification (I admit that too much convective boundary mixing 
> is the easy and wrong answer to obtain with too low grid resolution, a continuing challenge of such 
> simulations). And any terrestrial example of overshooting or convective boundary mixing would support 
> this notion. Whether or not the exponential decay of mixing model is the only one or even the best 
> one can be debated. It does have some support from hydro simulation though. You may adopt 
> another mixing model, but in any case it should allow for a smooth transition between stable 
> and unstable layer. You can make this smooth transition as thin as you can computationally afford, but
> I am confident that it will note be the diffusion length-scale! One reason for a smooth and numerically resolved 
> transition to be desirable comes from purely numerical considerations that it is impossible to converge 
> in the presence of grid face discontinuities that are not somehow modelled. This may contribute to 
> your results to not “seem very good” to you. 
> 
> Best, Falk.
> 
> --
> Falk Herwig
> Dept of Physics & Astronomy, U of Victoria
> fherwig at uvic.ca, tel: +1 (250) 721-7743
> 
> 
> 
>> On Jun 2, 2015, at 8:34 AM, Kenny Van <kvan at ualberta.ca> wrote:
>> 
>> Hi, I was trying to produce some results using overshooting and it seems as though regardless of timestep the plot produced doesn't seem very good. I have attached the three plots I created using my data. The overshooting and changes in timestep are said in the title. But just in case I will list all the properties of the star.
>> 
>> &star_job
>> 
>>    mesa_dir = '/home/kvan/mesa_svn'
>> 
>>    create_pre_main_sequence_model = .true.
>> 
>> / !end of star_job namelist
>> 
>> 
>> &controls
>> 
>> ! initial parameter
>> 
>>  ! initial mass of star
>>    initial_mass = 20.0
>> 
>>  ! initial metallicity of star, MESA only goes from 0 to 0.04
>>    initial_z = 0.014
>> 
>>  ! initial helium mass fraction of star
>>    initial_y = 0.266
>> 
>>  ! changing the MLT to match that in Meynet
>>    mixing_length_alpha = 1.6
>> 
>> ! overshooting
>> 
>>  ! all exponential overshooting is by default zero
>>  ! setting step overshooting
>>    step_overshoot_f_above_burn_h = 0.1
>>    overshoot_f0_above_burn_h = 0.1
>>    step_overshoot_f_below_burn_h = 0.1
>>    overshoot_f0_below_burn_h = 0.1
>>    step_overshoot_f_above_burn_he = 0.1
>>    overshoot_f0_above_burn_he = 0.1
>>    step_overshoot_f_below_burn_he = 0.1
>>    overshoot_f0_below_burn_he = 0.1
>>    step_overshoot_f_above_burn_z = 0.1
>>    overshoot_f0_above_burn_z = 0.1
>>    step_overshoot_f_below_burn_z = 0.1
>>    overshoot_f0_below_burn_z = 0.1
>> 
>> ! Mass loss
>> 
>>  ! Changing mass loss so system acts like that of Ekstrom et al.
>>  ! Use Vink mass loss rate
>>    RGB_wind_scheme = 'Vink'
>>    Vink_wind_eta = 1
>> 
>> ! timestep limits
>>  ! Limit magnitude of max change in log10 total pressure in any cell
>>    delta_lgP_limit = 0.2
>> 
>>  ! Limit magnitude of max change in log10 temperature in any cell
>>    delta_lgT_limit = 0.2
>> 
>> ! when to stop
>> 
>>  ! max timestep size
>>    max_years_for_timestep = 500
>> 
>>  ! stop when we reach AGB to produce similar results to Maynet et al.
>>    stop_at_TP = .true.
>> 
>>  ! max age
>>    max_age = 1.0d10
>> 
>> / ! end of controls namelist
>> <20M_Low_Lim_noOS_5d5.png><20M_Low_Lim_OS_5d5.png><20M_Low_Lim_OS_500.png>
>> 
>> Thanks
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