[extropy-chat] will the sun rise?

Amara Graps Amara.Graps at ifsi.rm.cnr.it
Sun Jan 2 11:07:45 UTC 2005


Eugen Leitl:
>Sun is harder because if you trap the radiation it will heat up
>and bloat up and simultaneously reduce the fusion rate. Perhaps
>you can blow off chunks of photosphere, by periodic/asymmetric
>feedback of the solar output.

>I don't see how this is a controlled disassembly process, though. 
>Ditto Jupiter.

It seems to me that the convection process (usually assumed in
the outer layers of a main sequence star) for transporting energy
would make star-lifting very hard to control.

I wonder if anyone has studied a scenario of asymmetric p-p
chain(s) nucleosynthesis in a main sequence star like our Sun. In
the later stages of a star's life, asymmetric burning is a
standard feature in its evolution. The unprocessed hydrogen in a
star's shell ignites when the star's helium core contracts, and
this swells the star into a giant.

The people who have written the old nucleosynthesis code (going
back to the 1950s) usually assume spherical symmetry for
main sequence stellar evolution and even with that, it was a 
hard problem because you have four first-order partial differential 
equations (PDEs) :

(not writing out the equations.. 
see: http://www.amara.com/ftpstuff/fusionzone.txt)

1) <Conservation of Mass>
2) <Conservation of Momentum (hydrostatic equil.)>
3) <Energy Balance>
4) <Energy Transport>
	(stable layer- radiative) 
	(unstable layer- convective)

and four constitutive equations:
A) <Equation of State>
B) <another Equation of State>
C) <Nuclear Reactions>
D) <Opacity>

Because our PDE's, we need four boundary conditions - Two
are imposed at the center, and two are applied at the surface. If
the star is in a steady state, and close to thermal equilibrium,
then the quantities should depend on the density, temperature and
chemical composition of the star, where all of them are functions
of the radius r. If any of the equations (say, energy generation)
are asymmetric, then our variables should track all of the three
dimensional spatial variables too. In the 'old' days, the problem
was computationally intractable, but today, it should be doable.

Something along these lines was studied in a simulation of
massive late evolution stars on their way to supernova. The
simulation looked at off-center triggering of a star going
supernova, which sent a very asymmetrical blob of hot material
rising up through the star. At the point where the bubble
surfaces, outer stellar layers are strongly accelerated, fly over
the star surfaces, and then come together on the opposite side of
the star. That focus then forms the center of the resulting
explosition (November 2004, Physics Today :Kadanoff:
Computational Scenarios, pg. 10) The bubble was started in just a
slightly off-center position, but in a spherically symmetrical
environment.

--------------------------------------------------------------

http://www.arXiv.org/abs/astro-ph/0405162

From: Alan C. Calder [view email]
Date: Fri, 7 May 2004 23:27:35 GMT   (306kb)

Type Ia Supernovae: An Asymmetric Deflagration Model
Authors: A. C. Calder, T. Plewa, N. Vladimirova, D. Q. Lamb, J. W. Truran
Comments: 11 pages, 4 figures, some figures degraded to reduce
size. Submitted to Astrophysical Journal Letters

We present the first high-resolution three-dimensional
simulations of the deflagration phase of Type Ia supernovae that
treat the entire massive white dwarf. We report the results of
simulations in which ignition of the nuclear burning occurs
slightly off-center. The subsequent evolution of the nuclear
burning is surprisingly asymmetric with a growing bubble of hot
ash rapidly rising to the stellar surface. Upon reaching the
surface, the mass of burned material is $\approx 0.075 M_\sun$
and the kinetic energy is $4.3 \times 10^{49}$ ergs. The velocity
of the top of the rising bubble approaches 8000 km s$^{-1}$. The
amount of the asymmetry found in the model offers a natural
explanation for the observed diversity in Type Ia supernovae. Our
study strongly disfavors the classic central-ignition pure
deflagration scenario by showing that the result is highly
sensitive to details of the initial conditions.

--------------------------------------------------------------
Amara Graps, PhD          www.amara.com
Istituto di Fisica dello Spazio Interplanetario (IFSI)
Istituto Nazionale di Astrofisica (INAF),
Adjunct Assistant Professor Astronomy, AUR,
Roma, ITALIA     Amara.Graps at ifsi.rm.cnr.it







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