Scientists find dying star siphoned off by its unseen companion

Rodiano Bonacci
Ottobre 13, 2018

An worldwide research team discovered the first recorded "ultra-stripped supernova", a rare, faint type of supernova that is believed to play a role in the formation of binary neutron star systems.

"We saw this massive star's core collapse, but we saw remarkably little mass ejected". After the star's outer layers have been blasted away, all that remains is a dense neutron star-an exotic star about the size of a city but containing more mass than the Sun. Although the star was at least eight times the mass of our sun, it only ejected one-fifth of the sun's mass during the explosion.

Observations made by the Caltech team-including lead author Kishalay De and project principal investigator Mansi Kasliwal (herself a former-Carnegie postdoc)-suggest that the dying star had an unseen companion, which gravitationally siphoned away most of the star's mass before it exploded as a supernova.

Typically, the core would collapse inward before exploding outward in a powerful, violent blast.

"We call this an ultra-stripped envelope supernova and it has always been predicted that they exist", but, De added, "This is the first time we have convincingly seen core collapse of a massive star that is so devoid of matter".

A supernova occurs when a massive star runs out of fuel and its core collapses, causing the star to explode and appear temporarily, extraordinarily bright. So scientists began to investigate the mystery of the missing mass.

Stellar evolutionary sequence leading from a binary system of massive stars - starting from the top left - to a neutron star binary system.

"This is the first clear detection of a supernova which can result in the formation of a binary neutron star system", said co-author Dr. Takashi Moriya, an astronomer at the National Astronomical Observatory of Japan.

The star's companion also happens to be a dense, compact neutron star.

Because this new neutron star and its companion are so close together, they will eventually merge in a collision similar to the 2017 event that produced both gravitational waves and electromagnetic waves, according to the study.

Astronomers were surprised at how wimpy the explosion was from this dying star.

"A typical supernova is slow evolving", De said.

"Without data in its infancy, we could not have concluded that the explosion must have originated in the collapsing core of a massive star with an envelope about 500 times the radius of the Sun".

This event was what Piro and a team of astronomers from Carnegie and UC Santa Cruz saw back in August 2017 at Palomar Observatory as part of Palomar Transient Factory (iPTF). Because the iPTF survey keeps such a close eye on the sky, iPTF 14gqr was observed in the very first hours after it had exploded. As the earth rotated and the Palomar telescope moved out of range, astronomers around the world collaborated to monitor iPTF 14gqr, continuously observing its evolution with a number of telescopes that today form the Global Relay of Observatories Watching Transients Happen (GROWTH) network of observatories. But this supernova evolved extremely fast by comparison. In addition to De and Kasliwal, other Caltech co-authors are Gary Doran of the Jet Propulsion Laboratory; graduate student Gina Duggan; Shri Kulkarni, George Ellery Hale Professor of Astronomy and Planetary Science; and Russ Laher and Frank Masci of Caltech's Infrared Processing and Analysis Center.

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