[vsnet-campaign-sn2002ap 35] [resend] Hypernova article

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

Dear all,

  I happened to write an article about hypernovae, for the publication
of "2002 Special Bulletin of M 1 Group", on Jan. 24, 2002.  What a
coincidence!  It is now in press and will be published this February,
but by cortesy of the editor J. Ripero, I can introduce it here.  It
is very short article, so please have a fun.

  Next, I will write about the Galactic Supernova ;-).

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

---
Hypernovae: more energetic explosion than normal SNe

H. Yamaoka (Kyushu University, Japan)

  The search for the supernovae (SNe) has always a possibility of 
the discovery of a object in the other category.  It would be 
a comet (like C/1998 Y2 (Li) and C/1999 E1 (Li) by the Lick 
Observatory SN Search), or a Galactic cataclysmic variable 
(like KL Draconis, which was originally announced as SN 1998di).  
Hypernovae, however, are not in the other category but of the 
subclass of supernovae, which are the whole destruction of stars
at their end of the life.

  The most impressive example of hypernovae is SN 1998bw.  It was 
discovered by the ESO team, who were seeking the optical 
counterpart of the gamma-ray burst occured on Apr. 25, 1998.  
The spectrum of this object has some broad lines, which suggested 
that it is expanding with a high velocity, or, in the other word, 
exploding.  The expansion velocity of this object reached 30000 km/s, 
i.e., 10 % of the speed of light.  

  It does not resemble, however, to that of any known class of 
supernovae.  The hydrogen lines are absent, thus it would 
be classified as type I.  But no silicon line (typical for SN Ia) 
nor helium line (strong in SN Ib) are observed.  Such supernovae
are classified as SN Ic, but SN 1998bw did not resemble to the 
typical SNe Ic, such as SN 1994I in Messier 51.  Then, this 
"supernova" is now classified as "peculiar type Ic".  The expanding 
velocity and the light curve suggests that the explosion was occured 
sometime during Apr. 21-27, which supports the association of 
this SN with the gamma-ray burst.

  The other striking characteristics of SN 1998bw was its 
luminosity; it was more luminous than typical SNe Ia which are 
thought to be most luminous class among SNe.  The energy source 
of SNe after explosion is a decay of the radioactive nuclei (mostly 
Nickel 56) produced during the explosion, so the amount of such 
matters would be larger in this object than the normal SNe.  
The light curve shows a slow evolution, which suggests that the 
large amount of material is ejected.  

  On these basis, a "hypernova" model is constructed (Iwamoto et 
al., 1998, Nature).  Very massive star (having 40 solar mass or more 
at birth) evolved losing its hydrogen-rich envelope and helium layer, 
then finally the gravitational core collapse occured.  If the energy 
supplied by the collapse is 30 times larger than the typical 
core-collapse SNe, the amount of the produced radioactive nuclei are 
10 times larger than a typical core collapser, or slightly larger 
than in a typical SN Ia.  It also explains the higher expansion 
velocity.  The central remnant would be a blackhole, which may 
ejected the "gamma-ray burst" during the collapse.

  Such hypernovae were searched from the previously discovered SNe.  
Then, from the resemblance of the spectrum and the light curve, it 
is now considered that SN 1997ef is also a hypernova though the 
coincident gamma-ray burst was not detected.  This SN was 
discovered by Japanese amateur Yasuo Sano.  You may be the next 
discoverer of hypernova!  I hope your SN search and observation 
will reveal the new aspect of SN science.

							(end)

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