April 25th was an important day for astronomers as it marked the release of a massive set of data from the European Space Agency’s Gaia probe. The aim of the mission, launched in December 2013, is to create the most comprehensive map of the billions of stars our galaxy. The data dump, the second since the mission began, saw the release of information regarding the velocities, positions, brightness, colours and compositions 1.3 billion stars. But for Ken Shan and his team, the data presented a unique opportunity to gather evidence to support a long-standing hypothesis regarding the cause of Type Ia Supernovas.
These supernovas are the most consistently bright objects in the universe, always displaying the same luminosity. They are so consistent in fact, that astronomers can use these explosions as ‘standard candles’ to measure distances to other objects. These objects are also responsible for dispersing heavy elements throughout the universe, making their study vital to building models of cosmic evolution. Unfortunately, as of yet, no one quite knows why these cataclysmic explosions occur.
The most persistent theory thus far about the origin of type Ia Supernovae is that they begin with white dwarfs, the remnants of low to medium mass stars such as our Sun. The question is what disturbs these stable remnants? Astronomers believe that the key lies in binary systems containing two stars orbiting each other, one or more of which is a white dwarf, speculating that the supernova is caused by the two bodies smashing together.
But Ken Shen an astrophysicist at the University of California, Berkeley has a different theory. Shen believes that the partner star in a binary system, be it a white dwarf or giant star, may destabilise the white dwarf by donating mass to its companion via stellar winds or through a process known as Roche Lobe overflow. This material hits the surface of the white dwarf increasing the body’s mass substantially until it is large enough to trigger the nuclear fusion of carbon in the star’s core. Ultimately, this triggers a second explosion, the type Ia supernova explosion at the star’s core. This massive thermonuclear blast should send the white dwarf’s neighbour star hurtling away at a velocity of thousands of kilometres per second. Theoretically, this should mean these liberated objects should be observable as some of the fastest moving astronomical bodies in the universe.
These high-velocity stars seem to be exactly what Shen and his team have found in the Gaia data.
The team examined seven possible candidates of high-velocity stars, eventually eliminating four as stars travelling at ordinary speeds, leaving them with three viable candidates which they identified as white dwarfs. These three possess other characteristics that seem to support Shen’s hypothesis, they are larger than white dwarfs typically tend to be, possibly due to their outer layers being expanded by the energy imparted in the explosive event that launched them on their trajectory. The three white dwarfs also seem to display a lack of the lighter elements, hydrogen and helium, which would be expected if these elements had comprised the donated matter that triggered the type Ia Supernova explosion.
Shen and team checked their high-velocity candidates with ground-based telescopes in California, the Canary Islands and South Africa to confirm their radial velocities and managed to trace the path of at least one of the candidates back to faint supernova remnant discovered three years ago within the Pegasus constellation. They found one of the objects travelling at approximately 2400 km/s making it one of the fastest objects in the galaxy.
The team published their findings at the online paper repository on April 30th and it was later submitted to the Journal of Astrophysics for peer-review.
Other astrophysicists concurred Shen’s discovery and its origins in principle but urged caution. “This is a great result, observationally,” astrophysicist Kris Stanek at The Ohio State University in Columbus told Science. “Theorists still need more time to consider Shen’s scenario but the white dwarf is probably is a part of a supernova explosion.”
The Gaia probe may well have already achieved an element of its mission if Shen’s theory proves to be true and a long-standing cosmological mystery has been solved. And this may well just be the cusp of the discoveries that are made with this data. Other teams have already obtaining interesting results from the data, including observations of high-velocity stars which may have originated from other galaxies. In fact, it’s remarkable when we consider it has only been available for approximately two weeks and it may have already solved such a lingering conundrum.