[mesa-users] Add my whinging to the discussion

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I see that there's been a lot of kvetching here lately. I might as well
jump right in.<br>
<br>
I've been revisiting the question of the evolutionary status of Type II
Cepheids, prompted by a suggestion from Alfred Gautschy that this
should be looked at with a modern evolution code and up-to-date
physics. The Conventional Wisdom is that the short-period Type II
Cepheids are stars evolving from the horizontal branch to the
asymptotic giant branch, while longer-period Type II Cepheids are
making blue excursions due to shell helium flashes. And by
"Conventional Wisdom" I mean Wallerstein (2002) and sources therein.<br>
<br>
So I've been exploring the relevant parameter space. To keep things
tractable, I assume 13.8 GYr as the Hubble time, Y=0.2485 as the
current accepted Big Bang helium fraction, 0.07 as the overshoot scale
height fraction, and to get to a reasonable starting place, I'm trying
to reproduce the morphology of M14 since it's a cluster with an unusual
number of Type II Cepheids; that means [Fe/H] around -1.5 and an age
close to the Hubble time. Unsurprisingly, it takes a fair amount of
mass loss on the red giant branch to produce a reasonable horizontal
branch with reasonable RR Lyrae masses in a Hubble time. So far so good.<br>
<br>
Now, what's puzzling me is that just about every model in this region
undergoes hydrogen ingestion either in the core helium flash or on the
first shell flash. This leads to some spectacular mixing and the model
almost immediately goes to a cooler, brighter carbon star. There isn't
a prayer of getting a blue loop out of any of these models. But before
reaching sweeping conclusions about the incorrectness of the
Conventional Wisdom for explaining longer-period Type II Cepheids, I
thought a sanity check was in order. Anything I should/should not be
doing that might explain this alacrity for ingesting hydrogen? <br>
<br>
And, yeah, what Bill said earlier. If I'm stupid enough to be working
in stellar evolution, I'm probably too stupid to be working in stellar
evolution. Or something. <br>
<br>
--Kent G. Budge<br>
<br>
david arnett wrote:
<blockquote
 cite="mid:CAGpgfQFgxZXp+ra-V=UjU...@ma..."
 type="cite">Hi Bill and Tuguldur,
  <div>This is an "interesting" region, which, imho, is based on
algorithms not physics. I plan to add to Mesa a physics based algorithm
(with Bill's aid and permission) in October when I am at Kavli ITP. I
am now testing it in TYCHO. It is based on the work that Casey Meakin
and I have done, analyzing 3D turbulent simulations. I think that Bill
and I will not stop until this becomes sane.</div>
  <div>Dave Arnett</div>
  <div><br clear="all">
  <div><br>
  </div>
-- <br>
David Arnett<br>
Regents Professor<br>
Steward Observatory<br>
University of Arizona<br>
  </div>
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