Loneliest Supernovae Created by White Dwarf, Neutron Star Collisions

A team of astronomers and astrophysicists has found that some of the Universe's loneliest supernovae are likely created from collisions between white dwarf and neutron stars, according to a recent study.

Researchers from the University of Warwick focused on calcium-rich transients, luminous explosions that can last for weeks. But what made these specific explosions difficult to study was the fact that they aren't as bright and don't last as long as traditional supernovae.

Calcium-rich transients observed to date can be seen tens of thousands of parsecs (equal to about 3.26 light years) away from any potential host galaxy, with a third of these events at least 65 thousand light years from a potential host galaxy.

The researchers used the Very Large Telescope in Chile and Hubble Space Telescope observations of the nearest examples of these calcium rich transients to attempt to detect anything left behind or in the surrounding area of the studied explosion.

"One of the weirdest aspects is that they seem to explode in unusual places," lead researcher Dr. Joseph Lyman explained in a university news release. "However, a large fraction of these are exploding at huge distances from their galaxies, where the number of stellar systems is miniscule."

"What we address in the paper is whether there are any systems underneath where these transients have exploded," he added.

Their observations ruled out the presence of faint dwarf galaxies or globular star clusters at the locations of these explosions, as well as massive stars.

Researchers then compared their data with what is known about short-duration gamma ray bursts (SGRBs), which occur when two neutron stars collide, or when a neutron star merges with a black hole. These are also often seen to explode in remote locations with no coincident galaxy detected.

Although the combination of a black hole and neutron star could not explain these calcium-rich explosions, the research team suggested that white dwarf stars could be involved.

Professor Andrew Levan, who was involved in the study, explains: "The mechanism of the supernova explosion causes the neutron star to be 'kicked' to very high velocities (100s of km/s). This high velocity system can then escape its galaxy, and if the binary system survives the kick, the white dwarf and neutron star will merge causing the explosive transient."

The study's findings were published in the journal Monthly Notices of the Royal Astronomical Society.