[vsnet-chat 1592] Re: (fwd) Type II supernovae

[Hitoshi YAMAOKA <yamaoka@rc.kyushu-u.ac.jp>]

Dr. Richmond wrote:

>  This is a complicated topic!  There are several complications which
>make it difficult to predict the brightness of a supernova explosion,
>based on the mass of the progenitor.

I agree these difficulties that Dr. Richmond pointed out, but I will
attempt to do some simplification.  

For the first, you should use the word "gravitationally collapsed SN"
instead if "type II SN".  The massive stars who lost their hydrogen
envelope by stellar winds or binary interaction will also explode as
supernovae driven by gravitational collapse, but they will not show
the H lines, which are charactaristic for type II SN.  It is thought
that type Ib/Ic SNe are of this case. 

>     a. a star which starts out its life with, say, 20 solar masses
>            of material will LOSE some of that mass during its 
>            lifetime.  In general, the more massive a star is,
>            the greater a fraction of its initial mass it expels
>            as a stellar wind before it reaches the supernova state.
>            So a star which is born with 20 solar masses might have
>            only 16 solar masses of material left when it reaches
>            the supernova stage.
>
>     b. we don't understand how and why stars expel some of their
>            mass via stellar winds during their lifetimes, so we 
>            cannot predict accurately how much material will be
>            left when they are ready to explode.

So we should use the mass at birth of the star (zero-age main-sequence
mass = ZAMS mass) for the discussion.  The mass loss process is very
enhanced in the later (supergiant) phase of the life of the massive
stars, when its core have been developed.  The stars whose ZAMS masses
are equal are thought to have nearly the same core region at their
explosion no matter with mass loss or not.  

>     c. the brightness of a supernova depends on the exact density
>            structure of its outer layers when it explodes; we don't
>            know enough of the details of stellar evolution to predict
>            that structure precisely.
>
>     d. the brightness of a supernova can depend strongly on the amount
>            of gas and dust immediately surrounding it; this may be
>            material shed by the star via a stellar wind before it 
>            explodes.  Once again, our models of stellar evolution
>            don't permit us to predict this circumstellar environment
>            very accurately.

c. and d. are true in detail; but roughly saying, the brightness of
the gravitationally collapsed supernovae (and also of the type Ia =
thermonuclear runaway supernova) is determined by the amount of the
synthesized radioactive nuclei, mainly 56Ni.  

However, we cannot predict with today's understanding how large amount
of 56Ni are produced in the gravitational collapse of a star with
certain ZAMS mass.  SN 1987A case, a star of 20 solar mass at ZAMS
produced 0.07 solar mass of 56Ni.  SN 1993J's progenitor, which are
thought to be 15 solar mass at its birth, produced about 0.1 solar
mass of it.  And, SN 1994I case, we guess the produced 56Ni mass is
about 0.07 solar mass from a star of 13 solar mass at ZAMS.  These
examples tell us that there would be no simple relationship between
the ZAMS mass and produced 56Ni mass.  

> It's possible that there _is_ a simple relationship -- but I don't
> know it. 

Also, I don't know it, but want to know ;-) .

Sincerely Yours,
Hitoshi Yamaoka, Kyushu Univ., Japan
yamaoka@rc.kyushu-u.ac.jp

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