[mesa-users] Overshooting

[David Arnett]

Sorry for being too terse; I am packing  for the summer and I must be jumpy
;-(

1. I know of no error in the programming/math. Bill is not at fault, except
for listening to astrophysicists.

2. The physics of "overshoot" is wrong, and the algorithm does not work
smoothly for large values of overshooting. It gives He cores that are
smaller by a factor 40 percent or so for sub-dwarf B stars, and similar
errors for the cores of red giants, as deduced from asteroseismology by
Kepler.

3. We have done well-resolved 3D simulations of turbulent flow, and find
that boundary layers develop, enclosing convective zones.  These are not
accounted for in mixing-length theory (MLT). These boundary layers modify
the connection between convective and radiative regions. We are sorting out
how this changes between neutrino-cooled stages (the easier case) and
photon-cooled stages (the more complex case); they are not the same.

We have the answer numerically for a few cases, and are working on how to
fit it into a stellar evolution code like MESA in an easy, accurate and
general way.

Advice for now: do not believe overshooting in detail, especially for large
values of the parameters. If your problem requires it, be wary. Predicted
core sizes may be suspect; for H and He burning the present predictions
probably underestimate core sizes.

Hope this helps,
Dave


On Tue, Jun 2, 2015 at 1:37 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:

> Hi Dave,
>
> Could you perhaps give us your opinion and clarify your email in the
> process -- your mention errors in the present implementation of
> overshooting in MESA: do you think these are result of a software bug in
> the MESA implementation, or are they the result of a "bug" in our basic
> understanding of the physical processes taking place near the boundaries of
> convective regions?  In other words, is the fix you mention a correction to
> a software bug, or is it a fix to our understanding of how to model the
> relevant physical processes in a 1D code that might lead to a new
> implementation?
>
> Thanks,
> Bill
>
> On Jun 2, 2015, at 11:39 AM, David Arnett wrote:
>
> The present implementation of overshooting in MESA causes large errors in
> some cases;
> see  (Schindler et al 2015 ApJ accepted). We are working on a fix.
> 2014arXiv1410.8204S
> <http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=2014arXiv1410.8204S&db_key=PRE&link_type=ABSTRACT&high=555a90a0bb02864>
>
>
> <http://cdsads.u-strasbg.fr/cgi-bin/nph-data_query?bibcode=2014arXiv1410.8204S&db_key=PRE&link_type=ABSTRACT&high=555a90a0bb02864>
> David Arnett
> Regents Professor
> Steward Observatory
> University of Arizona
>
> Facts are stubborn, but statistics are more pliable.  Mark Twain
> Facts do not cease to exist because they are ignored.  Aldous Huxley
>
> ------------------------------------------------------------------------------
> _______________________________________________
> mesa-users mailing list
> mesa-users at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/mesa-users
>
>
>


-- 
David Arnett
Regents Professor
Steward Observatory
University of Arizona

Facts are stubborn, but statistics are more pliable.  Mark Twain
Facts do not cease to exist because they are ignored.  Aldous Huxley
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.mesastar.org/pipermail/mesa-users/attachments/20150602/256890d8/attachment.html>