Many stars, towards the end of their lifetimes, form supernovas – massive explosions that send their outer layers shooting into the surrounding space. Most of the energy of the supernova is carried away by neutrinos – tiny particles with no charge and which interact weakly with matter. Researching the mechanisms of the so called Type II supernovas, a team from IIT Guwahati has come up with new insights into the part played by neutrinos in this dramatic death of massive stars. The collaboration includes astrophysicists from Max Planck Institute, Munich, Germany; Northwestern University, Illinois and University of California, Berkeley, in the U.S.

Fate of the star

All stars burn nuclear fuel in their cores to produce energy. The heat generates internal pressure which pushes outwards and prevents the star from collapsing inward due to the action of gravity on its own mass. But when the star ages and runs out of fuel to burn, it starts to cool inside. This causes a lowering of its internal pressure and therefore the force of gravity wins; the star starts to collapse inwards. This builds up shock waves because it happens very suddenly, and the shock wave sends the outer material of the star flying. This is what is perceived as a supernova. This happens in very massive stars.

In stars that are more than eight times as massive as the Sun, the supernova is accompanied by a collapsing of the inner material of the dying star – this is also known as core collapse supernova or Type II supernova. The collapsing core may form a black hole or a neutron star, according as its mass. “Our work is on these core-collapse events of type II supernova,” says Sovan Chakraborty of the physics department of IIT Guwahati, in an email to The Hindu.

Three flavours

Neutrinos come in three ‘flavours’, another name for ‘types’, and each flavour is associated with a light elementary particle. For instance, the electron-neutrino is associated with the electron; the muon-neutrino with the muon and the tau-neutrino with the tau particle.

As they spew out of the raging supernova, the neutrinos can change from one flavour to another in a process known as neutrino oscillations. As Dr. Chakraborty explains, due to the high density and energy of the supernova, several interesting features emerge as this is a nonlinear phenomenon: “This [phenomenon] may generate neutrino oscillations happening simultaneously over different energies (unlike normal neutrino oscillation), termed collective neutrino oscillation. The oscillation result may dramatically change when one allows the evolution with the angular asymmetry, the oscillations can happen at a nanosecond time scale, termed fast oscillation.”

Models of this process, dubbed the effective two-flavour models, have only taken into account the asymmetry between electron neutrino and the corresponding antineutrino. In a paper published in Physical Review Letters, the researchers from IIT Guwahati claim that a three-flavour model is needed to predict well the dynamics of the supernova.

Fast oscillations

The fast oscillations are important because the researchers find that these can decide the flavour information of the supernova neutrinos.

So far, this has not been done, and models have only kept terms involving a neutrino and its corresponding anti-neutrino. “We find that fast nonlinear oscillations of neutrinos are sensitive to three flavours, and neglecting the third flavour may yield the wrong answer,” says Dr. Chakraborty. “Thus, the presence of …[asymmetry between] the muon neutrinos and antineutrinos will be crucial for the neutrino oscillations, in turn influencing the supernova mechanism.”

Understanding this is important when one wants to measure the influence of neutrinos and their oscillations on supernova mechanism and heavy element synthesis in stellar environments.