[Mesa-users] Fwd: other_momentum subroutine

sheyda najafi sh.najafi8590 at gmail.com
Fri Oct 20 07:07:56 EDT 2017


---------- Forwarded message ----------
From: sheyda najafi <sh.najafi8590 at gmail.com>
Date: Fri, Oct 20, 2017 at 1:57 PM
Subject: other_momentum subroutine
To: mesa-users at lists.sourceforge.net


Hi MESA users

I am trying to find the evolution of the RGB star in the presence of an
extra force ((-Beta) * dx/dm), which is added to the other_momentum
subroutine as follows

dp/dm = - G m/4 pi r^4 - (Beta) * dx/dm

where Beta is an arbitrary constant.

I have run mesa for different Betas and as you can see in the attached plot
(Tip of the RGB.png), the temperature of the Tip of the Red Giant Branch
increases for (0<Beta<300) but for ( 300<Beta<400) there are some
instabilities in the evolutionary track of the star and for (450<Beta<500)
the instabilities increase in such a way that  the star leaves the standard
track which is shown in the plot (HR.png) for Beta=450, and the code
terminates therefore the evolution stops after RGB phase.
 Also for (Beta>500) the code does not converge even in zams phase.

My question is whether we can have physical interpretation for the
instabilities in the evolutionary track of the star or it is just the
instability in the numerical code?

In other words, is it admissible that the extra force becomes so large for
beta>300, and makes the star unstable so the star can not live any longer
and these models are not physical, or it is just a large quantity that
makes the numerical code unstable?

Thank you in advance



Sheyda Najafi
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