Yamaoka is happy to write an article of the Galactic SN. Requests for writing new articles in magazines, books, etc. on supernovae should be directed to email@example.com.
(Published in "2002 Special Bulletin of M 1 Group", see also introduction in SN 2002ap list)
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.
[Link to GRB Japanese page]
[On Japanese expression]
>Would it be possible to more explain the >supposed origin of such narrow lines in SNe spectra? Some recent SNe IIn >are known to have higher luminosities than ordinary SNe II. Is there >any explanation for this?I have little to say about them.... SNe spectra triggered by gravitational collapse differ from one to another, because of the difference of the outer envelope region, not of the central core. The brightness of supernovae is determined by the central nucleosynthesis, so it is not well-known how to connect the spectral details and its brightness.
It may be a hint that some recent peculiar SNe Ic (1997ef, 1998bw) are also very luminous. They are thought to be an explosion of highly mass-lossed massive star.
Sincerely Yours, Hitoshi Yamaoka, Kyushu Univ., Japan firstname.lastname@example.org
SN 2001bb (VSOLJ news, in Japanese)
SN 2001bl (VSOLJ news, in Japanese)
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