A spectacular new way for stars to die

[SF]

Meet pair-instability supernova SN2006GY, the most extraordinary explosion in the cosmos. Unlike its smaller, regular supernova cousins, which blast off the outer layers of a star and pack what remains into a neutron core or a black hole, the pair-instability supernova is a much more violent celestial finale. These events happen only in stars that are at least 150 times as large as our sun and result in total annihilation of the star. Astrophysicists contend that this type of eruption helped seed the cosmos with heavy metals like iron, a process that ultimately allowed planets to form.

http://www.popsci.com/military-aviation-space/article/2008-05/biggest-boom-universe

 

[g33kski11z]

From that article {#3 I believe}, "gamma rays born from nuclear fusion in the core spontaneously morph into pairs of electrons and their antimatter twins, positrons".

.. is that possible?

 

[lzkelley]

Yes. If you've heard of anti-matter annihilations (electron + positron = photons), this is just the reverse, which is totally allowed.
Generally when photons change into a e- e+ pair, they'll soon collide reforming the photons. Sometimes another event (like a strong electric of magnetic field) will prevent them from annihilating, and you'll be left with a brand new electron and positron.

 

[marcus]

As I recall we had some discussion of this at PF about a years ago---in May 2007.

The paper referred to was this one posted December 2006 and published round that time.
Proud to say I know one of the authors---a teacher of mine years back.

http://arxiv.org/abs/astro-ph/0612617
SN 2006gy: Discovery of the most luminous supernova ever recorded, powered by the death of an extremely massive star like Eta Carinae
Authors: Nathan Smith, Weidong Li, Ryan J. Foley, J. Craig Wheeler, Dave Pooley, Ryan Chornock, Alexei V. Filippenko, Jeffrey M. Silverman, Robert Quimby, Joshua S. Bloom, Charles Hansen
14 pages, 4 color figs
2007, Astrophysical Journal, 666, 1116
(Submitted on 21 Dec 2006 (v1), last revised 22 May 2007 (this version, v3))

Abstract: (abridged) We report our discovery and observations of the peculiar Type IIn supernova SN2006gy in NGC1260, revealing that it reached a peak magnitude of -22, making it the most luminous supernova ever recorded. It is not yet clear what powers the total radiated energy of 1e51 erg, but we argue that any mechanism -- thermal emission, circumstellar interaction, or 56Ni decay -- requires a very massive progenitor star. The circumstellar interaction hypothesis would require truly exceptional conditions around the star probably experienced an LBV eruption like the 19th century eruption of eta Carinae. Alternatively, radioactive decay of 56Ni may be a less objectionable hypothesis. That power source would imply a large Ni mass of 22 Msun, requiring that SN2006gy was a pair-instability supernova where the star's core was obliterated. SN2006gy is the first supernova for which we have good reason to suspect a pair-instability explosion. Based on a number of lines of evidence, we rule out the hypothesis that SN 2006gy was a ``Type IIa'' event. Instead, we propose that the progenitor may have been a very massive evolved object like eta Carinae that, contrary to expectations, failed to completely shed its massive hydrogen envelope before it died. Our interpretation of SN2006gy implies that the most massive stars can explode earlier than expected, during the LBV phase, preventing them from ever becoming Wolf-Rayet stars. SN2006gy also suggests that the most massive stars can create brilliant supernovae instead of dying ignominious deaths through direct collapse to a black hole.

=====================

the pair instability business is an interesting mechanism!

Here is something I wrote about it last November which tries to make it intuitive. Wallace or others please correct me if there is something wrong with this intuitive explanation:

Pair-production-instability is a very beautiful runaway (positive feedback) mechanism that involves gammaray photons so energetic they can produce electron+antielectron pairs!

In the core of an ordinary star like the sun it is 15 million kelvin and that produces XRAY photons which gradually percolate out towards the surface getting scattered and divided into more and more lower energy photons as they percolate out.
like cashing a 20 dollar bill into pennies---by many successive transactions
then the pennies get to the surface and fly off to warm the earth, and do other good stuff.

Very roughly, when you convert kelvin to eevee it is about 10,000 kelvin counts as one eevee, so 15 million kelvin is only 1500 eV. Your DENTIST machine can do something like that. those Xrays are no big deal.

However try this on google: put in "electron mass*c^2/eV"
that will give you the energy equiv. of an electron, divided by one eV------which is the number of eV it takes to make one electron!
It turns out to be about 500 thousand eV.

So if you have an angry swarm of gamma photons which are not just 1500 eV (like in the sun) but 500 thousand eV, then these photons begin to get distracted by the process that they keep turning into PARTICLEANTIPARTICLE PAIRS! and then turning back again and back and forth. Two photons meet and form an electronpositron pair, and then those proceed to annihilate and make two photons again. SO THE GAMMA IS SCATTERING SO MUCH THAT IT DOESNT PERCOLATE OUT OF THE CORE AS WELL as if the core itself had become more opaque and they get DETAINED in the core.

The whole point is this opaqueness. When the radiation gets hot enough the photons get trapped and don't percolate out as easily.

And then, because energy is not flowing out as efficiently as before, paradoxically the OUTER LAYERS START TO COOL and they loose some of the heat that was holding them up before so they SINK DOWN and compress the core so the CORE GETS STILL HOTTER
so the gammaray in the core is even more energetic and able to produce pairs.

It's positive feedback because once the core gets up to that 500,000 ev temperature the outerlayers will cool and compress it still further and so the core gets even hotter. Then something has to give.

I suspect that the ability of a star to become that massive, so that its core can reach that threshold temperature, without blowing the outer layers off too early, has to do with purity.

It might happen if the material out of which the start condensed was almost metal-free----no carbon, nitrogen, oxgen to catalyze efficient fusion and provide the energy to blow off excess mass. Because of its relative purity the star isn't fighting its own condensation so much. Anybody clear me up on this?
How do EtaCarinae type stars manage to coagulate?

Kudos to SF the O.P. who brought this up. Interesting topic!

 

[marcus]

More stuff.
A November 2007 UC SantaCruz press release with some more detailed explanation
http://www.ucsc.edu/news_events/press_releases/text.asp?pid=1742

Some PF discussion where at the end of the thread George Jones mentions pair instability supernova
https://www.physicsforums.com/archive/index.php/t-169612.html
George gives link to a UC Berkeley press release
http://www.berkeley.edu/news/media/releases/2007/05/07_supernova.shtml

there is also a Wikipedia article about pair-instability supernova, I think

 

[PanTheory]

From that article {#3 I believe}, "gamma rays born from nuclear fusion in the core spontaneously morph into pairs of electrons and their antimatter twins, positrons".

.. is that possible?

g33,

This has been duplicated in the lab long ago. Apposing beams of gamma radiation contact each other at twice the speed of light (collision speed). Gamma rays are the highest frequency waves (shortest waves). Upon contact with each other they produce numerous pairs of stable electrons and positrons. Conversely, when electrons and positrons come in contact they annihilate each other producing gamma rays. Within a star according to theory the same process occurs. The primarily unanswered theoretical question is: why are gamma rays somehow different in that they can produce stable particles? They are EM radiation but what does pure energy really mean? There are only a few theories concerning this question and little controversy.

 

[SF]

The primarily unanswered theoretical question is: why are gamma rays somehow different in that they can produce stable particles? They are EM radiation but what does pure energy really mean? There are only a few theories concerning this question and little controversy.

So what, non-gamma rays are "impure" energy?

 

[robertm]

Hey two points for public radio! They had a short report on this discovery on NPR yesterday afternoon!

 

[Chronos]

One issue not well discussed in the literature is massive binaries of similar size and histories. What if a massive pop III star went SNc while its neighbor was at the brink? I would expect to see 'camel' humps in the light curve. Detonation of the first would set off its companion. The primary and secondary spectrums in such an event would be novel.

 

[chemisttree]

It might happen if the material out of which the start condensed was almost metal-free----no carbon, nitrogen, oxgen to catalyze efficient fusion and provide the energy to blow off excess mass. Because of its relative purity the star isn't fighting its own condensation so much. Anybody clear me up on this?
How do EtaCarinae type stars manage to coagulate?

Ah! Good question! Is it true that a metal-rich protostar ignites at a lower temperature (mass?)? Is there data suggesting that much more massive stars than our sun are all metal poor?