ARI research helps explain exploding stars puzzle
New research published in the journal Nature, using highly-detailed radio-telescope images, has pinpointed the locations where a stellar explosion called a nova emitted gamma rays, the most energetic form of electromagnetic waves. The discovery, together with models originally developed at LJMU’s Astrophysics Research Institute (ARI), revealed a probable mechanism for the gamma-ray emissions - a phenomenon that has mystified astronomers since first observed from a nova in 2012.
A nova typically comprises a close system of two stars, one like the Sun but a bit cooler and smaller, the other a White Dwarf - the core of a star like the Sun left behind when it runs out of fuel and the outer layers of gas are lost into space.
But the two stars in a nova system are in fact so close together that hydrogen rich material from the outer layers of the cooler star is being transferred via an 'accretion disc' onto the surface of the White Dwarf. Ultimately, the conditions are such at the base of the accreted envelope that a runaway thermonuclear explosion occurs, the object brightens by maybe 100,000 times in less than a day, and roughly a mass equivalent to that of 100 Earths may be ejected into space.
Astronomers did not expect this scenario to produce high-energy gamma rays. However, in June of 2012, NASA's Fermi spacecraft detected gamma rays coming from a nova in the constellation of Monoceros (‘The Unicorn’ in Greek) called V959 Mon, some 6500 light-years from Earth.
At the same time, observations with the Karl G. Jansky Very Large Array (VLA) in New Mexico, USA, indicated that radio waves coming from the nova probably were caused by subatomic particles moving at nearly the speed of light interacting with magnetic fields. The high-energy gamma-ray emission, the astronomers noted, also required such fast-moving particles.
Later observations with the extremely sharp radio "vision" of the Very Long Baseline Array (VLBA) of radio telescopes in the US and the European VLBI Network (EVN) revealed two distinct knots of radio emission. These knots then were seen to move away from each other. This observation, along with studies made with the e-MERLIN radio array in the UK, and another round of VLA observations in 2014, provided the scientists with information that allowed them to put together a picture of how the radio knots, and the gamma rays, were produced.
Professor Mike Bode, Director of the ARI and a member of the international team of scientists conducting the work, explains:
“What's needed to give rise to the gamma rays and radio waves we detect is thought to be the violent interaction of material travelling at several thousand km per second setting up shock systems in which electrons can be accelerated to near the speed of light in a magnetic field and thus emit via the 'synchrotron' mechanism.
“Although novae eject material at high enough velocities, the problem is that there is not thought to be anything much for the ejecta to slam into in the immediate surroundings of the nova. However, the work described in this paper relies on a new twist to a model of interactions within the ejecta itself. This was originally developed at the end of the 1990s at LJMU by Professor Tim O'Brien (now Associate Director at Jodrell Bank), the late Dr John Porter, and myself, and still remains the most sophisticated model of its kind.”
In the first stage of this scenario, the White Dwarf and its companion give up some of their orbital energy to boost some of the explosion material, making the ejected material move outward faster in the plane of their orbit. Later, the white dwarf blows off a faster wind of particles moving mostly outward along the poles of the orbital plane. When the faster-moving polar flow hits the slower-moving material, the shock accelerates particles to the speeds needed to produce the gamma rays, and the knots of radio emission.
"By watching this system over time and seeing how the pattern of radio emission changed, then tracing the movements of the knots, we saw the exact behaviour expected from this scenario," project leader Dr Laura Chomiuk of Michigan State University said.
Since the 2012 outburst of V959 Mon, the Fermi spacecraft has detected gamma rays from three additional nova explosions.
"This mechanism may be common to such systems. The reason the gamma rays were first seen in V959 Mon is because it's close," Chomiuk said.
Mike Bode added: “The detection of high energy gamma rays from novae came as a real surprise. Now with a combination of observations from a new generation of facilities, coupled with new twists on old models, we may have begun to solve this specific mystery. In turn, our work may be able to give astronomers new insights into the evolution of what’s known as the ‘common envelope stage’ which all close binary stars are thought to go through.”
The ARI has also recently welcomed a research student from the Institut d' Estudis Espacials de Catalunya, (IEEC-CSIC), Barcelona to work on gamma ray emission from novae. This is funded by the Spanish Government for a two month period through its programme of research student mobility in research and development.
Caption for images:
Artist’s impression of the central system in a nova. Gas flows from the cool (red) companion star through a disk onto the White Dwarf that is hidden inside the central region of the disk. As the gas flows ever closer to the white dwarf, it gets increasingly hot, as indicated by the change in colours from yellow to white. When the explosion occurs on the White Dwarf’s surface, it rapidly engulfs the disk of gas and the red companion star. (Image credit: NASA/CXC/M.Weiss).
Model of the outburst of V959 Mon showing the denser ejected material in the equatorial plane of the central binary (yellow doughnut) and the fast wind from the White Dwarf (blue cones). Shocks arise between the two flows (orange lines) giving rise to gamma ray and radio emission, and at the edges of the main ejecta the shocked material yields compact radio knots (red blobs). (Image credit: NRAO/B.Saxton).
The paper is available online: Binary orbits as the driver of gamma-ray emission and mass ejection in classical novae
