[mesa-users] Fwd: semiconvection (typo)

[Tuguldur Sukhbold]

Hi Aaron,

> What you find in the default settings for some parameter, such as the "typical" value of alpha _semiconvection, should not be taken as an endorsement of that value by the MESA developers.  Nor does it mean that that value has any physical significance, as David says.

of course. I was just curious about why the value in the default setting has changed recently, and also how far people have tested this implementation, especially on massive stars. I imagine it is not straightforward to test it observationally, but we can compare with other calculations. As far as I know the results I get with MESA are significantly different than what we get in our group using KEPLER, and models by Meader & Meynet. But obviously the 'quality' of the code is not tested in terms of how long it has been around or how well it is agreeing with others, so you're right that we don't have an expected outcome to compare with. 

I don't mean to say which code is right/wrong, just there are very obvious differences in the results, and I want to understand them.

thanks,
Tuguldur





> 
> On Monday, April 1, 2013, Tuguldur Sukhbold wrote:
> Dear MESA users,
> 
> just noticed that I said following backwards in my yesterday's email:
> 
>> For example, it would be really hard to explode 1.d-3 model, as the CO core is too tiny, even 1.d-1 case is still very degenerate.
> 
> I meant to say the opposite, too easy to explode. Basically I wanted to say that all of these 15Msun cases are developing really degenerate cores that one would 'usually' expect at much lower mass (~maybe around 10Msun or so). Sorry I was writing at 4am in the morning ;-)
> 
> Tuguldur
> 
> 
> 
> 
> 
> 
> Begin forwarded message:
> 
>> From: Tuguldur Sukhbold <sukhbold at ucolick.org>
>> Subject: semiconvection
>> Date: March 31, 2013 4:12:51 AM PDT
>> To: "mesa-users at lists.sourceforge.net" <mesa-users at lists.sourceforge.net>
>> Cc: Stan Woosley <woosley at ucolick.org>
>> 
>> Dear MESA users,
>> 
>> I have a sort of general questions about setting up the semi-convection in MESA. I would be very grateful if anybody shares their experience using it especially on massive stars.
>> 
>> 1) According to star/defaults/controls.defaults, we only need these two in our controls:
>> 
>> 	use_Ledoux_criterion = .true   
>> 	alpha_semiconvection = some value
>> 
>> is this really it? am asking this because I have found this document on the forum:
>> 
>> http://mesastar.org/documentation/tutorials/zams-test-cases/semiconvection/view
>> 
>> here it mentions these two extra lines:
>> 
>> 	use_gradmu_alt_for_mlt = .true
>> 	radL_comp_term_logRho_switch = 3
>> 
>> says it is used to control computations of 'certain' gradients. I couldn't find any information on these in my version (4798), probably because this document is made for 4299..? Here I just want to make sure that I am not missing anything in the setup.
>> 
>> 2) Now more importantly, what is the value of some value? According to the control.defaults:
>> 
>> alpha_semiconvection = 0 ! 0.01 ! determines efficiency of semi convective mixing
>> 
>> but I remember in previous versions of the code (and also in the document mentioned above) it used to claim that 1.d-3 is a typical value. Can someone tell me where exactly these numbers are coming from? If one checks the actual Langer's paper:
>> 
>> http://adsabs.harvard.edu/full/1985A%26A...145..179L
>> 
>> it says the value is more like 0.1 (see section 3).
>> 
>> Attached figure shows the result of a quick survey I did in the past couple of hours. I took 15Msun models and run it from ZAMS until when the central temperature reached T_c=5.d8K, this is roughly the condition when the carbon starts igniting in the center. I have calculated the same model with bunch of different values of alpha_semiconvection [0.00001-100] (anything over 1 is crazy), plus the same model with just pure Schwarzschild criterion. In all figures dashed curve/line represent result from this pure Schwarzschild criterion. The red line in the last plot shows the rough red-supergiant size limit.
>> 
>> First of all, varying the parameter from 1.d-3 to 1.d-1 produces quite significant changes. But the crazier thing is that in all cases the CO core I get is uncomfortably small. For example, it would be really hard to explode 1.d-3 model, as the CO core is too tiny, even 1.d-1 case is still very degenerate.
>> 
>> Please let me know if you find I made an error, also I would like to know how far people have tested this Langer's prescription in MESA?
>> 
>> thanks,
>> Tuguldur
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