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An Unusual Supernova May Be a Missing Link in Stellar Evolution Research

Image of SN 2005cz
Figure 1. Supernova (SN) 2005cz and its host galaxy NGC 4589. The star indicated by the arrow is the SN. This image was taken by Subaru Telescope and FOCAS instrument on 2005 August 10. The field-of-view is 3 arcmin by 2 arcmin. The SN was about 19 th magnitude in R-band.

An international astronomer team including K. Kawabata (Hiroshima Univ), K. Maeda and K. Nomoto (Tokyo Univ) revealed the nature of unusual Type Ib Supernova (SN) 2005cz (Fig. 1), which was discovered in an elliptical galaxy NGC 4589 by Mr. Koichi Itagaki in Japan.

This SN was very peculiar: It was intrinsically faint, quickly faded, and showed much weaker [O I] 6300, 6364 line than [Ca II] 7291, 7323 in the late-phase spectrum (Fig. 2). The appearance in elliptical galaxy was also peculiear, since a genuine elliptical galaxy contains only old pupulation stars, that is, low-mass stars and cannot produce any core-collapse supernova.

The team proposed that these peculiarities are well explained by rather `normal' core-collapse supernova from a less massive progenitor star (main-sequence mass of approximately 10 solar mass) than other Type Ib SNe. This mass corresponds to the low-mass end of the range of massive stars that explode. Such stars die as supernova about a few ten milion years after their birth, which is consistent with the recent studies suggesting that the host galaxy probably underwent a galactic merger and would stimulate a global star formation 10-100 milion years ago. This is the first evidence of the theoretical prediction that stars in the 8-12 solar mass range can explode as supernovae. Such stars might have a very special abundance pattern in the ejecta and play important roles in the chemical evolution of galaxies.

This study is published in Nature on 2010 May 20.

It is noted that SN 2005E is very similar to SN 2005cz. However, any mark of recent star formation has not been found at the location of this SN (Perets et al. 2010, in the same issue of Nature). Therefore, the core-collapse scenario might not be plausible for SN 2005E.

Late-time spectrum of SN 2005cz

Figure 2. Late-time spectrum of SN 2005cz, shown together with spectra of some supernovae. Large [Ca II] / [O I] line ratio is distinct compared with other type Ib/c/IIb supernovae (=envelope-stripped core-collapse supernova).

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Update: 2010-05-20