[Mesa-users] Finding ZAMS

[Aaron Dotter]

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
>
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