Pair-Instability Supernovae & No Black Hole Remnant

[ian2012]

I recently got interested in reading about Pair-Instability Supernovae. What I don't understand is: "...triggering a collapse that vaporises the star. This pair-instability supernova creates a larger quantity of elements heavier than helium ("metals") than in other types of supernova and not leaving a black hole remnant." - Wikipedia (http://en.wikipedia.org/wiki/Supernova#Pair-instability_type)

How is there no remnant left behind (i.e. black hole)? I always thought the greater the mass of the star, the greater the certainty of leaving a black hole.

 

[Agent M27]

It is true for massive stars to leave behind black holes or neutron stars as a result of a supernova explosion. Pair-Instability supernovae differ however in their constiuent elements. They are between 150&250 solar masses and contain mostly light elements. The cores of such a large star is incredibly energetic so that the gamma rays produced via nuclear fusion have shorter mean free paths. During these shorter paths the gamma ray is rapidly interacting with other atomic nuclei resulting in a loss of thermal pressure which let's gravity take the upper hand momentarily. During this decrease in pressure results in a collapse igniting a more vigorous nuclear reaction stage. It is this nuclear reaction phase which produces a thermal pressure that far out weighs the force of gravity resulting in a supernova. It doesn't leave anything behind because the thermal pressure was so great that it scattered all of the material before any gravitational effects between those particles could take over. These supernova would be known as hypernova but the core is so energetic that the gamma rays turn into electron-positron pairs which adds to the instability and is somewhat a catalyst to the immense increase in thermal pressure that resulted in nothing remaining. There have been speculation that quark material might be the only remnants but there is no observational data to support that. To my knowledge these were only theory until one was observed in ~2009. Hope that helps.

Joe

 

[ian2012]

Thanks for your response.
I read about this in a New Scientist article called: "Primordial Giant: The star time that time forgot". And yes, it was observed sometime in 2009 (SN 2009bi).
I was reading the paper: A., Gal-Yam et al, "Supernova 2007bi as a pair-instability explosion", Vol 462, 3rd December 2009, nature.
What I didn't understand is the process by which the luminosity of the explosion is powered by.
In the New Scientist article it says the afterglow is powered by radioactive decay of heavy elements generated during nuclear fusion - Ni-56. In the paper by Gal-Yam he states the total illuminated mass of the nebular emission comes out as >50 solarmasses. 'However this falls well below the total mass derived from photometry, indicating that even the unprecedented amount of radioactive Nickel produced by SN 2007bi was not sufficient to energize the entire mass ejected by this extreme explosion.'
What does this mean exactly?

 

[twofish-quant]

1) The relationship between supernova and black hole creation is not very simple and very uncertain. In the case of pair-production supernova, what happens is that the whole core gets disrupted, and so it's unlikely to leave behind a black-hole. In your standard core collapse supernova, the core just sits there and stuff basically bounces off of it, and that may leave a black hole

One other way of thinking about is that in your ordinary type II supernova, the explosion starts at the edge of the star, whereas in a pair instability one, it starts in the center and blows the whole star up.

2) You can think of the supernova as a big giant flurorescent light bulb. You have radioactive elements which decay, and these create gamma rays which get absorbed by the material in the star, which reemits them as light. You know that the original source is something radioactive since you can see the brightness fall off over time.

What Gal-Yam is saying is that we calculate that the amount of bright stuff and you come up with 50 solar mass, and the explanation is that there really is 150 solar mass of stuff there, but there is only enough radioactive material to make 50 solar mass of that stuff glow in the dark.

 

[ian2012]

okay cool. Is radioactive decay of Ni-56 the main process for the afterglow? Are there any other processes?