Antimatter made in stars?

[GordonCopestake]

http://space.newscientist.com/article.ns?id=dn11563&feedId=online-news_rss20

[Black Shuck]

Very interesting.

The interaction of higher energy gamma rays with the electrostatic field of an atomic nucleus can cause the gamma to form an electron-positron pair. As the electron and positron are equal mass of 511keV then the initiating gamma ray must be > 1022keV. I wonder if what they are saying is that the "unstable" star is producing more than normal high energy gamma radiation interacting with the heavy nuclei which is producing this matter-antimatter reaction???

Bill£

[Kaptain Klevtsov]

So what happens when the matter and antimatter collide (in the abscence of dilithium crystals ), would you get a new 1022keV gamma ray? Enough for it to go on and on until enough matter and antimatter are manufactured to go up in one big bang initiated by a pressure wave?

Where would you get hold of such a gamma ray? Extreme temperatures or exotic fusion reactions?

Captain Chaos

[Black Shuck]

Plot a course Capt' - Maximum Warp!

Please don't ask me to do the Maths :?!!! From what I recall (i'm not an academic Physicist) when the positron and electron annihilate, in order to conserve momentum and energy, 2 gamma rays are produced (2 X 511keV) and are emitted in opposite directions. If there is a lot of kinetic energy then you can get particles produced (Mesons, etc) as well as photons of energy? I am probably barking up the wrong tree but I would have thought if this was going on in a large scale (stellar proportions) this would produce a huge release of energy but whether it would be self sustaining and/or result in the phenomenon described, then I am purely speculating. It would be interesting to see the full article rather than an abstract - it might enlighten us further? One of my colleagues subscribes so I will see if he has any further info.

Bill£ :?

P.S - It's not unusual to get gammas > 1022keV in "normal" conditions. Potassium-40, which is a naturally occuring radioactive element, has one at 1460keV. There are undoubtedly many processes of atomic bombardment and excitation going on in a star that would lead to the production of high energy gamma rays.

[Astroman]

I'm a bit skeptical of this interpretation, as 1022keV is not particularly energetic as gamma rays go. Also, the supernova they mention was not detected as a gamma ray burst by SWIFT, but observed after the fact in the ultraviolet. One would think the anihilation of electron and positron pairs would emit more in the hard x-ray region, due to their mass, and indeed they do around black hole candidates, but the core of a high mass star is not hospitible to the existance of positrons due to the pressure.

Just a thought.

[Black Shuck]

That's interesting Astroman. Have you read the full article? I was just postulating based on the abstract from the link above. The lack of gamma rays blows my thoughts out the window.

However, wouldn't a positron-electron annihilation as they are describing produce 2 X 511keV gamma photons (i.e 2 x the rest mass of the electron/positron)? I suppose it is very likely that weak gammas such as these would be readily attenuated by the surrounding medium and not actually make ot out of the star.

As you say, 1022keV isn't particularly energetic for a gamma ray. In reality the energy has to be quite a bit higher to initiate pair production but 1022keV is theoretically the threshold energy for pair-production (2x511keV). I would have thought that there were some pretty heavy duty gammas flying about in a star, although as you say pair production may be inhibited due to the hostile conditions within the star?

It was nice to get the brain working again over Easter

Bill£

P.S (I deal mainly with gamma spectroscopy down in the sub 2MeV range so my thoughts are probably a bit polarised).

[Astroman]

I didn't read the whole paper, (is it published anywhere besides New Scientist? Science or Nature?), but I did read the article Gordon cites, plus did a bit more digging into GRB's and such. Your energies for pair anhiliation are correct, but my understanding of the electromagnetic spectrum is gamma rays start around 1200keV, which puts the reaction well into the hard x-ray, maybe soft gamma range. Gamma rays emmitted in stars, due to fusion reactions generally have a higher energy, since the reactions are between heavier particles and nucleii with more massive stars fusing heavier elements. This is part of why I'm skeptical. The energies released in a supernova would need to be much higher than the hard x-ray region, due to the temperature within the star, and even though not all SNe beget GRB's, the fact that SWIFT observed it in the UV tells a tale of lower energy studies. Even if there were electron-positron pairs anhiliated, their lower energy would be swamped by the higher enery gamma rays when the star exploded.

Tough call.

[Black Shuck]

Tough call.

Definitely!! Way out of my league but it's always good to speculate though. Keeps the brain ticking over

As far as I recall X-rays and Gammas overlap to a certain extent between 120 - 1200keV. The main difference between them being that gammas originate from the atomic nucleus and X-rays from changes in the electron orbits.

My main thought was if very high energy gammas were causing pair production then the excess energy (ie. original gamma energy minus 1022keV) would go into the kinetic energy of the pair which go off and annihilate. However this would only liberate 2x511keV photons + the kinetic energy which would probably be absorbed by the surrounding media. Therefore this is obviously not the process. I think the articles mention of positrons and my dealings with gamma spectroscopy got me "excited" and I started off on that track.

Well i've got clear skies here and I am getting some good seeing at the moment and some really nice observations of Saturn. New focuser and time spent on collimation working nicely. I am off back out to see if my neighbours security light has gone off yet!!!

Bill£