Re: [mesa-users] instability strip definition (no MESA content)

[Earl B. <bel...@mp...>]

Dear Aaron,

I am probably not going to say anything you don't already know, but
here are my two cents on the matter...

>From a theoretical point of view, the instability strip is found by
determining when stars become unstable to fundamental (or
low-overtone) radial oscillations. More specifically, this occurs when
the imaginary component of radial mode eigenfrequencies change sign.

This happens at several stages during evolution: on the main sequence
(delta Scuti and other types of stars), during a "blue straggle" (SX
Phoenicis variables), during the Hertzsprung gap in the sub-giant
phase (first-crossing Cepheids), during blue loops after ignition of
core helium burning in the star's ascent up the giant branch
(Cepheids), on the horizontal branch with double-shell burning (RR
Lyraes), etc...

On the red edge, the location of the sign change depends on the
details of pulsation-convection interaction which, as far as I know,
is neglected in pulsation codes such as GYRE and ADIPLS, and as such
is typically regarded as being unreliable. The location of the blue
edge is more reliable but depends for example on one's choice of the
mixing length parameter. So a theoretical definition of the
instability strip in terms of Teff, log g, or L will depend on
potentially arbitrary assumptions. That being said, one can find
empirical instability strips from real stars in the literature.

I hope what I have said is correct. If there are any errors, I hope
someone will please correct me.

Best regards,
Earl Bellinger

---
www.earlbellinger.com
Department of Astronomy, Yale University
Stellar Ages & Galactic Evolution Group, Max Planck Institute


On Thu, Jun 1, 2017 at 11:13 AM, Aaron Dotter <aar...@gm...> wrote:
> Hi all,
>
> (Sorry for the non-MESA request.  Too much expertise on this mailing list to
> resist asking here!)
>
> Does anyone have a favorite definition of the realm of the instability strip
> in the H-R diagram?  There are lots of cartoons out there, most of which are
> vague and/or conflicting.  I'm not aware of a solid theoretical or empirical
> definition of the bounds of the IS in terms of Teff, surface gravity, and/or
> luminosity.
>
> Thanks,
> Aaron
>
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