The Loneliest Supernovae in the Universe Created by Collisions of White Dwarfs and Neutron Stars

The loneliest supernovae in the universe are likely created by the collisions of white dwarf stars and neutron stars--at least, that's what a new study suggests. Scientists have taken a closer look at these collisions and have found these compact binary star systems have been thrown far from their host galaxy.

"Our paper examines so-called 'calcium-rich' transients," said Joseph Lyman, one of the researchers, in a news release. "These are luminous explosions that last on the timescales of weeks, however, they're not as bright and don't last as long as traditional supernovae, which makes them difficult to discover and study in detail."

Calcium comprises about half of the material thrown off in calcium-rich transients as opposed to regular supernovae, which only contain a tiny fraction of calcium. This means that these events may very well be some of the main producers of calcium in our universe.

Calcium-rich transients are usually located tens of thousands of parsecs away from any potential host galaxy. In fact, a third of these events are located at least 65 thousand light-years from a potential host galaxy. That's why scientists decided to take a closer look at these transients with the Very Large Telescope in Chile and the Hubble Space Telescope.

With these new observations, the scientists ruled out the presence of faint dwarf galaxies or globular star clusters at the locations of these nearest examples. In addition, they ruled out massive stars as the progenitors of calcium rich transients.

"It was increasingly looking like hypervelocity massive stars could not explain the locations of these supernovae. They must be lower mass longer lived stars, but still in some sort of binary systems as there is no known way that a single low mass star can go supernova by itself, or create an event that would look like a supernova," said Lyman. "What we therefore propose is these are systems that have been ejected from their galaxy. A good candidate in this scenario is a white dwarf and a neutron star in a binary system. The neutron star is formed when a massive star goes supernova."

The findings reveal a bit more about these calcium-rich transients and show that it's likely that these may indeed be some of the loneliest supernovae in the universe.

The findings are published in the journal Monthly Notices of the Royal Astronomical Society.