[mesa-users] Weird jumps in radius for rotating 8.1Msun model

[Falk Herwig]

On 2012-06-08, at 6:51 AM, Aaron Dotter wrote:

> 
> You're delving into the murky depths of science now!  It has been appreciated for many years that some amount of convective core overshoot is required to match the observed CMDs of open clusters.  It is common to assume an overshoot mixing height of 0.2 H_p for stars more massive than 2 Msun, but this 0.2 H_p is in terms of a fully-mixed region rather than the exponential mixing that you're using now.  To my knowledge, no one has "calibrated" the exponential overshoot with open clusters.
> 

We did calibrate exponential H-core convection against observations in an approximate way. Here is a figure from my thesis (Herwig, 1998):

Of course these things depend a bit on input physics, etc. With the modern input physics we use today f=0.014 is approximately reproducing the observed width of the main sequence. Of course it is known that rotation may change the width of the main sequence as well. Therefore we can only view this value as an order of magnitude guess.


As far as the jumps go and their relation to overshooting I have the following comment. In real stars there is no such thing as a Schwarzschild boundary. Any convective boundary has a finite width. Many meaningful stellar evolution results can be obtained by not resolving this boundary and adopting a Schwarzschild boundary that implies a discontinuity at a grid boundary. Results that are shown to be sensitive to this assumption need to adopt a convective boundary model that allows for a smooth transition between the convective and radiative zone. The width of this zone can be made as small as it can be computationally afforded, and as large as it is physically viewed to be, but it has to be resolved in space and time. The exponential convective boundary mixing algorithm is one simple way for a numerically resolvable model (concerning species mixing), that can produce numerically (better) converged results. Whether they are more accurate is another question.

Falk.
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