NetNews Usenet Archive 1992 #31/NN_1992_31.iso/spool/sci/physics/21902

Internet Message Format | 1992-12-30 | 3.8 KB

Path: sparky!uunet!usc!rpi!gatech!destroyer!news.itd.umich.edu!pablo.physics.lsa.umich.edu!metzler
From: [e-mail] (Chris Metzler)
Newsgroups: sci.physics
Subject: Re: When your sun forges iron...
Date: 30 Dec 1992 20:47:44 GMT
Organization: University of Michigan Department of Physics
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In article <[e-mail]>, [e-mail] (Jim Carr) writes:
|> In article <[e-mail]> [e-mail] (Chris Metzler) writes:
|>
|>     < regarding whether Fe-56 fission drives a supernova >
|>
|> >In Supernovae of Type II, when the core burns to iron, it is no longer possible
|> >to support the mass above by nuclear processes, and the core collapses.
|> >This contraction results in very high temperatures and a lot of energetic
|> >photons.  For a Type II supernova, the iron core is above the Chandrasekhar
|> >limit, meaning that electron degeneracy pressure is not sufficient to
|> >support the mass of the star.  The matter can thus infall until the core is
|> >so hot and the photons so energetic that iron nuclei can photodisintegrate into
|> >He nuclei and neutrons.  This is not, strictly speaking, what we normally
|> >think of when someone says "fission," but the iron nonetheless does break
|> >down into smaller elements.
|>
|> Exactly, as I think I sketched in my post.  Certainly some of the iron
|> could photodisintegrate, some could even be struck by a very fast
|> neutron and be broken apart in a spallation reaction, but neither
|> would be called fission by a physicist.  I would also differ with
|> the implication in the original post that this is the dominant process
|> for driving the supernova, as I think your post makes clear also.
|> The dominant mechanism is loss of energy production, not loss due to
|> fission, followed by collapse if the mass exceeds the Chandrasekar limit
|> so that the electrons cannot hold it up against gravity.  At that
|> point, all sorts of interesting nuclear physics occurs, a lot of
|> which involves electron capture leading to a neutron excess -- very
|> important if we are to have the heavy elements that are formed
|> during this time.  Certainly some Fe does break down, but this is
|> irrelevant once the superclusters form just prior to the bounce.
|> If this was the dominant process, there would be no heavy nuclei.
|>
|>

Sorry it took me so long to reply; Xmas and all.

I would agree with you if you changed "no heavy nuclei" in that last
sentence to "a lot less heavy nuclei."  And in fact, that's exactly
what happens in a Type II SN.  Most of the heavy element production
occurs not in the core but in the envelope, and doesn't involve the
iron that once was the core.  This is why Type II's, which come from
more massive progenitors than Type I's, produce much less iron than
Type I's (0.07 solar masses on average for Type II's; 0.7 solar masses
on average for Type I's . . .progenitors' masses are an order of magnitude
or so different the OTHER direction).  For more on this, see Woosley
and Weaver's paper in IAU Colloq 89, Radiation Hydrodynamics in Stars
and Compaact Objects (Type II's); also see Nomoto, Thielemann, and
Yokoi (1984) ApJ 286, 644 (Type I's).


--
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