Ordinary Meeting, 2002 March 16
Supernova Secrets; Infrared Insights
To open, Prof. Meikle showed a number of images of supernovae, illustrating that their intensity could outshine even the brightest of foreground stars. With reference to 1987A, it was demonstrated that the spectra of such objects show a number of broad peaks, indicating very substantial temperatures. The speaker also demonstrated that supernovae show much more structure when imaged in the radio band. SN1993J was used to illustrate how a featureless expansion in the optical region showed complex structure at radio wavelength.
The speaker moved on to ask why some stars explode in this way. The speaker estimated the energy release of such events to be of the order of 1044J, which can be visualised as 1028 megatons of TNT or 200 times the rest mass energy of the Earth. Main sequence stars do not explode as they have negative specific heat and are consequently gravitationally bound. White dwarf stars differ in that they are composed of a degenerate state of matter. This is a quantum mechanical state, which can be visualised by modelling the star as a finite potential well. Only a discrete set of bound states are allowed, and as electrons are fermions, the Pauli exclusion principle limits the occupation of each state to two electrons. Consequently, in a dense white dwarf the highest energy electrons are in relatively high energy states since all lower energy states are occupied. These fast moving energetic electrons create a pressure known as degeneracy pressure which is sufficient to support the white dwarf against its own gravitational attraction.
It has been shown, however, that at the critical Chandresehkar mass, 1.4 solar masses, relativistic effects involving the variation of electron mass with velocity limit the degeneracy pressure to be weaker than the gravitational attraction. The speaker suggested that the result would be the collapse of the iron-rich core of the star from the size of the Earth to that of a lemon in around 100ms, raising the temperature to 8×109K. The collapse would at this point be abruptly halted and bounced by neutrinos trapped in the core. These neutrinos are formed by the particle interaction:
p + e- → n + νe
Although neutrinos only interact via the nuclear weak force and are renown for their lack of interaction, in the dense core of such a collapsed star interactions would be plentiful. The speaker demonstrated this bounce effect by dropping two spheres on top of each other, and the upper one rebounded to the ceiling. This also demonstrated Prof. Meikle's next point concerning the rebounding material surging out of the core. The speaker suggested that in essence the same would happen as had happened in the lecture theatre – the material hits the outer stellar gas which acts as a ceiling. A number of computerised models of increasing complexity have been proposed, initially one dimensional, and now two dimensional, and the speaker showed such a model in action. Such theoretical models predicted that fingers of iron would be observed in supernova remnants, and the speaker suggested that were these to be observed, it would provide a strong indication of the validity of the theory. Furthermore, he suggested that such fingers had already been seen in 1987A.
Not all supernovae, however, form by this mechanism. The speaker went on to describe Type 1A supernovae, which are formed by the accretion of matter onto a white dwarf from a binary partner. Such accretion could push the white dwarf above the critical Chandresehkar limit. In this case, the core collapse would cause sufficient temperature rise for carbon and oxygen in the core to undergo nuclear fusion to form 56Ni. The energy released could surpass 200 times the rest mass energy of Earth.
The speaker commented that supernovae are unlike other explosions in that they take a long period of time to fade. The typical light-curve for a supernova rises to maximum brightness within hours, but takes many months to fade. The speaker proposed that plentiful supplies of 56Ni would form in a Type 1A supernova, and this undergoes radioactive γ decay to 56Co and then 56Fe. The halflife is 77 days, and so could explain the afterglow, but only in the γ region of the spectrum, not optical photons. The speaker suggested the existence of complex mechanisms for the interconversion of γ and optical photons, including the absorption of γ radiation by excited gas and subsequent reemission at optical wavelength. Estimates for the cobalt/iron mass ratio from the spectrum of 1987A closely matched theoretical predictions from this model, but matched more closely if the slower 57Ni decay route was also taken into account.
Moving onto the ongoing puzzles, Prof. Meikle showed an image of SN1987A which has two outer rings which remain unexplained. For SN1998S, the speaker explained that the light curve had remained brighter for much longer than any current theory predicted – another mystery. The spectrum is interesting as it appears to contain a 1200K blackbody, possibly indicating dust particles. If verified, supernovae could explain the origin of the dust in the universe, another puzzle. Examination of supernovae in the Hubble deep field has revealed curious anomalies in their magnitudes, supporting the need for Einstein's serendipitous cosmological constant. Finally, the speaker referred to type 1C supernovae, which are thought to be a source of the curious phenomenon of gamma ray bursts.
To close, Prof. Meikle thanked all of the amateur astronomers in the audience who had helped search for supernovae, as this is a field where professional resources are very limited indeed. Following the applause for Prof. Meikle's highly informative talk, the President adjourned the meeting for tea, after which Mr. Martin Mobberley returned in lively style to give his Sky Notes.