KAILUA-KONA — The W. M. Keck Observatory on Maunakea assisted a team of Caltech-led researches in observing the peculiar death of a massive star that exploded in a surprisingly faint and rapidly fading supernova.
The observations suggest that the star had an unseen companion whose gravity siphoned away from the star’s mass to leave behind a stripped star that exploded in a quick supernova, according to the California Institute of Technology (Caltech).
The explosion is believed to have resulted in a dead neutron star orbiting around its dense and compact companion, suggesting that, for the first time, scientists have witnessed the birth of a compact neutron star binary system.
The research was led by graduate student Kishalay De and is described in a paper appearing in the Oct. 12 issue of the journal Science. The work was done primarily in the laboratory of assistant professor of astronomy Mansi Kasliwal, who is the principal investigator of the Caltech-led Global Relay of Observatories Watching Transients Happen (GROWTH) project.
When a massive star — at least eight times the mass of the sun — runs out of fuel to burn in its core, the core collapses upon itself and then rebounds in a powerful explosion called a supernova. After the explosion, all of the star’s outer layers have been blasted away, leaving behind a dense neutron star — about the size of a small city but containing more mass than the sun, according to Caltech.
Normally, during a supernova event, the dying star blasts away all of the material in its outer layers, which usually equates to a few times the mass of the sun. However, Caltech said the event that Kasliwal and her colleagues observed, dubbed iPTF 14gqr, ejected matter only one-fifth of the mass of the sun.
“We saw this massive star’s core collapse, but we saw remarkably little mass ejected,” Kasliwal said. “We call this an ultra-stripped envelope supernova and it has long been predicted that they exist. This is the first time we have convincingly seen core collapse of a massive star that is so devoid of matter.”
From the observation, the researchers inferred that the mass must have been stolen — the star must have some kind of dense, compact companion, either a white dwarf, neutron star, or black hole — close enough to gravitationally siphon away its mass before it exploded, according to Caltech. The neutron star that was left behind from the supernova must have then been born into orbit with that dense companion.
Thus, observing iPTF 14gqr was actually observing the birth of a compact neutron star binary, according to Caltech. Because this new neutron star and its companion are so close together, they will eventually merge, producing both gravitational waves and electromagnetic waves similar to a collision that occurred in 2017, the researchers predicted.
The event was first seen at California’s Palomar Observatory as part of a nightly survey of the sky for transient, or short-lived, cosmic events like supernovae. The observatory kept tabs on iPTF 14gqr during the first hours after it had exploded, and as the Earth rotated, astronomers around the world collaborated to monitor iPTF 14gqr, continuously observing its evolution with a number of telescopes, including Keck Observatory, that form the GROWTH network of observatories.
The researchers utilized Keck I’s Low Resolution Imaging Spectrograph to characterize the astrophysical nature of the event, providing important clues about the type of supernova that was observed, as well as its environment, specifically iPTF 14gqr’s host galaxy and the galaxy group.
Also used was the DEep Imaging and Multi-Object Spectrograph on Keck II for follow-up spectroscopy of the supernova as it was fading.