A new, bigger kind of boomPosted: January 12, 2010
Firstly apologies that this post may seem a bit behind the times given that it’s based on an article that was in Nature a month ago. I don’t read Nature too often as I’m too cheap to buy my own subscription and the only other way to read it would be going into the institute’s library which is full of grad students who make me feel old. However the article in question appeared on the arXiv this week.
As you probably know, stars sometimes blow up completely. These massive explosions are known as supernovae. They rarely happen in our Galaxy (maybe one a century), but look at enough galaxies and the chances are you’ll see one. Most supernovae fall into two main types, Type Ia supernovae and core collapse supernovae. In the former a white dwarf explodes, probably after gaining mass from a companion in a binary system. In the latter a massive star fuses elements in its core until it has an iron centre. Fusing iron nuclei together won’t produce energy, so the star must have some other way of supporting it. However if the star is sufficiently massive (above about 8 solar masses) as stars that have iron cores are, it has no other way to support itself and hence its core collapses, leading to an explosion of the star. There is however a third possibility.
A very massive star is supported against collapse by high energy photons (particles of light) which provide a pressure to push again gravity. However if the star is hot enough this mechanism can go horribly wrong. In a strange quirk of particle physics when photons are produced with a high enough energy, they can turn into an electron and its antiparticle, a positron. This process leads (with a few complex steps) to the star’s core becoming unstable and collapsing. This happens long before the star would have time to form an iron core. Such an event is called a Pair Instability Supernova (PISN) and only occurs with a star more massive than about 140 solar masses.
And so this article claims that a recent supernova (SN 2007bi) found in a dwarf galaxy has all the characteristics of a PISN. The authors followed the supernova for 18 months and also took spectral observations. Supernovae get bright and then fade away with time. The authors note that this event closely matches the expected light curve and maximum brightness for a very high mass progenitor with a core mass of about 100 solar masses and a total mass double that. They also examined the object’s spectrum and found that the event produced an extremely high amount of radioactive nickel. In fact the authors believe the explosion produced more than seven solar masses of radioactive nickel. This is another pointer that the event was a PISN.
It isn’t 100% certain that this was a PISN, another paper on the subject suggests the galaxy this explosion occurred in was not the type of environment PISN supernovae can happen in. However given the maximum brightness and spectrum of this event it is possible this is the first observation of a very big kind of boom.
Gal-Yam A, Mazzali P, Ofek EO, Nugent PE, Kulkarni SR, Kasliwal MM, Quimby RM, Filippenko AV, Cenko SB, Chornock R, Waldman R, Kasen D, Sullivan M, Beshore EC, Drake AJ, Thomas RC, Bloom JS, Poznanski D, Miller AA, Foley RJ, Silverman JM, Arcavi I, Ellis RS, & Deng J (2009). Supernova 2007bi as a pair-instability explosion. Nature, 462 (7273), 624-7 PMID: 19956255