[mesa-users] Overshooting

Tue Jun 2 23:59:12 EDT 2015

Ok. Just to wrap up this interesting overshooting tread and avoid any further confusion (please correct me if this is wrong):

1) Overshooting is implemented correctly in MESA (in the sense there are no bugs, as far as we can tell).
2) The physics of overshooting is complex and not fully captured by the simplified models adopted in stellar evolution codes. More work is currently done to improve the situation (thanks to brave souls like David Arnett and collaborators)
3) Asteroseismology can help from the observational sides of things by providing a best fit to the stellar models (thanks to the work done by e.g. Ehsan Moravveji and collaborators). 
4) The original problem raised by  Kenny  arises because of an inaccurate initialization of overshooting (thanks to Rob and Chris for spotting this).

Cheers,
-M

Matteo Cantiello  |  http://matteocantiello.com/ 
Associate Specialist in Astrophysics  | Kavli Institute for Theoretical Physics
Chief Scientist and Board Member, Authorea  |  https://www.authorea.com/

On Jun 2, 2015, at 5:58 PM, Chris Mankovich <cmankovich at ucsc.edu> wrote:

> Hi Kenny and everyone else,
> 
> I think Rob identified the reason why M_cc looks so ratty as a function of time.  With f=f0 you're effectively not adding an overshoot region.  You can plot the mixing D's to see this, and in fact with the f=f0=0.1 case you end up with a lot of isolated, borderline-stable zones that tend to merge with each other and sporadically with the convective core.
> 
> If I run f0=0.1, f=0.2 so that there is an overshoot region extending 0.1 H_p beyond the standard boundary, things behave more like you'd expect.
> 
> 
> 
> Chris
> 
> On Tue, Jun 2, 2015 at 3:25 PM, Ehsan Moravveji <e.moravveji at gmail.com> wrote:
> 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 
> 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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>>> mesa-users mailing list
>>> mesa-users at lists.sourceforge.net
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