In the paper, they reported observations of a peculiar Type Ib supernova 2005cz, and concluded that this is a supernova whose progenitor mass at its birth was about 10 times the Sun. Such a star represents a boundary between stars that end their lives with the gigantic supernova explosion and those without explosions.

Supernovae from stars that were originally about 10 solar mass should occupy a large fraction of supernova explosions taking place in the whole universe. However a supernova whose progenitor mass lies just above the boundary has not been identified.

This is a reason why astronomers have been seeking for an explosion in this mass range. This study thus finally provides a solid confirmation on the stellar evolution theory. Having identified properties of the resulting supernova explosion, this study also serves as an important step forward to understand roles of supernovae in evolution of the universe.

The standard theory of stellar evolution tells us that a life of a star is determined when it is born - the mass at the birth is a main function. Stars whose initial masses are above 8 to 10 solar masses experience a violent death - at the end of their lives, their inner core should suffer from a gravitational collapse, which then leads to the gigantic explosion known as a supernova explosion.

This phenomenon is believed to be the origin of "apparently new stars" ("supernovae") which suddenly appear on the night sky (note that there is another channel leading to a supernova explosion, a Type Ia supernova, that is a thermonuclear explosion of a white dwarf. (In this paper, we hereafter call the core-collapse supernova explosions from massive stars as supernovae.)

The number of stars in the Universe decreases as a function of their mass: Namely, there are more "less-massive" stars than "more-massive" stars. Therefore, it has been believed that stars that are about 10 solar masses at the birth are the largest population among stars that end their lives as supernovae.

However, a supernova from this lower boundary in the progenitor mass had not been identified. It seemed that all core-collapse supernovae for which the research group estimated the progenitor masses originated from stars whose initial masses were at least 12 solar masses - sometimes above 40 solar masses.

The researchers wondered why they did not identify the explosion from the "lower-limit-mass" stars - Is there anything wrong in the theory of stellar evolution? If it is the case, it is a disaster for astronomy: In many fields of astronomy, it has been assumed that these stars are the predominant population of supernovae.

The research group observed a peculiar Type Ib supernova SN 2005cz, using several telescopes, including 8.2m Subaru Telescope of NAOJ. They found various puzzling properties of this supernova:

(1) it showed up in an elliptical galaxy that usually lacks massive stars to become Type Ib supernovae,

(2) it was faint, reaching only 20% of typical luminosity of other Type Ib supernovae, and

(3) it faded very quickly. On top of these properties, a late-time spectrum taken by the Subaru telescope at about 200 days after the explosion was most striking.

An emission line from oxygen which is the strongest in Type Ib supernovae is nearly missing, while there is a very strong emission line from calcium.

This property is indeed a unique feature expected for an explosion of a star with about 10 solar masses. The research group concluded that all the peculiarities that SN 2005cz showed can now be understood consistently by this scenario of the explosion of the "least-massive-star".

Note: In the same volume, Perets et al., reported observations of SN 2005E which look similar to SN 2005cz in many aspects, and suggested that it is a new type of the explosion that is an explosion within the surface layer of a white-dwarf. Kawabata and collaborators on the other hand argued that the core-collapse explosion scenario is more likely at least for SN 2005cz. Further study is necessary to understand a possible link between these two supernovae.

"Our study has rescued the standard theory of stellar evolution," mentioned Koji Kawabata from Hiroshima University, continuing, "This supernova was faint and gone quickly. This is probably a main reason why we have not got this kind of supernovae before". "This type of supernova should be intrinsically abundant in the Universe", mentioned Keiichi Maeda, an assistant professor at IPMU.

"They are important as origins of various things which we see in the Universe today. For example, these stars are believed to be main contributors of some elements including carbon and nitrogen which are essential ingredient of the life on Earth."

Another example includes the diffuse neutrino background in the Universe. IPMU researchers have been trying to detect a signature of past supernovae explosions by looking into the "sum" of neutrino emissions from multiple past supernovae. Supernovae of the type discovered by this research may well be a predominant population here.

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