[mesa-users] Add my whinging to the discussion

Mon Aug 29 10:49:39 EDT 2011

Dear Kent,
Hi. I hope I understood your message well. In case you are in favor of
blue looping models to produce HB stars, and let them evolve up the AGB
branch, I spent some days exploring this with MESA.

My first initial suggestion is to use Ledoux criterion to mix all
surroundings of burning zones with partially mixed material, enlarge their
extent and mass, and postpone their evolution till they start burning He
just before their first RGB excursion.
There remains the sensitivity of occurrence of blue loops to the initial
semi-convection and overshooting coefficients. I used to explore this with
models of 15 to 20 Msun.
However, I still cannot comprehend the central role of Ledoux criterion on
this.
Moreover, I found rotational mixing repulsive towards creating blue loops.
This is in contrast to the very recent work of Mayenet et al. (2011) who
have blue loops in massive star models including rotation. Yet to be
explored ...

Maybe I totally misunderstood you, and I am not worried, since I am for
sure stupid enough to stay in stellar physics.

Best wishes.
Ehsan.

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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=UjU0J3D69vcYH4fks5t6wCA0ucCM8LiQ at mail.gmail.com"
>  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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