[Mesa-users] Finding ZAMS

[Thomas Steindl]

Hello,

I have now tried your stopping condition. Therefore, I calculated 21  
models in a mass range of 0.5 to 6 solar masses (the 0.775 M_sun  
modelled would not converge). The stopping condition works well for  
the masses 1.05 - 5ish solar masses depending on where you define the  
cut off criterion. For the models with higher masses the ratio L_H/L  
never drops below 1 after it first rises above 1. I could avoid this  
problem by fitting a linear function in the points on the main  
sequence and determine the ZAMS by the first point which is below this  
linear fit. Unfortunately, this would not work for the 0.5 -1ish solar  
mass as the ratio converges from values above the main sequence value.  
What I could do is determing the ZAMS by finding the point which  
differs from the linear fit using the main sequence point by some  
value. Yet, I wonder if this stopping criterion would still be  
physical and not just some way to deal numerical issues?

I have again attached a zipped folder of the plots that show what I  
refer to. Here i have plotted log_L and log_LH in the upper pannel and  
LH/L in ther lower pannel. Also the green line shows the determined  
ZAMS timestep which is the point on which the ratio drops below 0.9995  
when looking at the inverse time.

Thank you in advance for any help!

Thomas Steindl


Zitat von Aaron Dotter <aaron.dotter at gmail.com>:

> Hi Thomas,
>
> [Josiah beat me to it!]
>
> The ZAMS is a hard thing to define precisely.  What does it mean for the
> project you're working on?
>
> You're seeing the different behavior because of the transition from
> H-burning dominated by the pp chain vs. the CNO cycle.  The basic net
> ignores several lesser species that equilibrate once the core gets hot
> enough for the reactions to proceed.
>
> If it's acceptable for you to evolve slightly past the ZAMS and then
> identify a previous timestep as the ZAMS, the following is the procedure I
> use (in post-processing a MESA history file).
>
> Stop once the central H mass fraction has been reduced by 0.0015 from its
> initial value.  Move backward from this point until you find the timestep
> for which the model first L_H/L_tot crosses 0.99, or whatever you want your
> ZAMS definition to be.
>
>
> Cheers,
> Aaron
>
>
>
>
> On Thu, Mar 14, 2019 at 5:30 PM Thomas Steindl <
> Thomas.Steindl at student.uibk.ac.at> wrote:
>
>> Hello,
>>
>> I am working on pre-MS Asteroseismology. Therefore, I am calculating
>> MESA models which I use as input for GYRE. In my analysis, I need to
>> know the timestep at which the star reaches ZAMS. Unfortunately, I
>> fear the stopping conditions implemented in MESA do not work. To show
>> this I have used three different inlists. A first, very simple inlist
>> (Basic_inlist), an inlist with additional varcontrol
>> (additional_varcontrol) and one with additional varcontrol and using
>> the ppandcnoextras net instead of the basic net
>> (additional_varcontrol_and_net). I have also attached the
>> corresponding inlists. In the corresponding png files, I have plotted
>> the values for log_L and log_Lnuc in the upper panel and the ratio
>> L/Lnuc in the lower panel. As you can see, the ratio seems to converge
>> to the value 1 for the first two inlists (for which this stopping
>> criterion works). But for the latter of the three inlists, Lnuc
>> exceeds L at some point in the evolution and the stopping criterion
>> fails.
>>
>> The zip folder 'ZAMS_second_stop_option' contains plots for 30 stellar
>> models computed with the 'additional_varcontrol_and_net' inlist and
>> different masses. In the 30 pngs in this folder I have plotted log_L
>> and log_Lnuc in the upper pannel and
>> abs(log(power_hburn)-log_surface_luminosity) in the lower panel. The
>> title shows the mass of a specific model and the value of
>> L_nuc_counter which states how many timesteps Lnuc/L has been between
>> 0.99 and 1.01. As you can see, the value for the second stop option
>> exhibits drops (mostly while log_Lnuc > log_L) which in terms makes
>> this stop option unreliable and I don't see how it can function as
>> global stop option for all masses.
>>
>> I have tried other options (gravitational energy, contraction speed,
>> etc.), yet I cannot find a criterion. Do you have any idea on which
>> stellar properties I could use? Any help would be greatly appreciated.
>>
>> Thank you in advance,
>> Thomas Steindl
>>
>> _______________________________________________
>> mesa-users at lists.mesastar.org
>> https://lists.mesastar.org/mailman/listinfo/mesa-users
>>
>>
>


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