How Neutrinos help in the death of massive stars?

How Neutrinos help in the death of massive stars?

• 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.
• 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.