A team of astronomers led by Caltech’s Kishalay De has discovered the first recorded ‘ultra-stripped’ supernova — a faint type of supernova that is believed to play a role in the formation of compact neutron star binary systems. The discovery is described in a paper published in the journal Science.
The three panels represent moments before, when and after the faint supernova iPTF14gqr, visible in the middle panel, appeared in the outskirts of IV Zw 155, a spiral galaxy located 920 million light years away from us. Image credit: SDSS / Caltech / Keck.
A supernova occurs when a massive star exhausts its nuclear fuel, causing the core to collapse and then rebound outward in a powerful explosion. After the star’s outer layers have been blasted away, all that remains is a dense neutron star.
Usually, a lot of material — many times the mass of the Sun — is observed to be blasted away in a supernova.
However, the supernova that De and co-authors observed, called iPTF 14gqr, ejected matter only one fifth of the Sun’s mass.
“We saw this massive star’s core collapse, but we saw remarkably little mass ejected,” said co-author Dr. Mansi Kasliwal, also of Caltech.
“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.”
Also known as SN 2014ft, iPTF 14gqr was discovered by the intermediate Palomar Transient Factory (iPTF) on October 14, 2014.
It occurred in the outskirts — at a projected offset of 95,000 light-years from the center — of IV Zw 155, a tidally interacting spiral galaxy located approximately 920 million light-years from Earth.
Stellar evolutionary sequence leading from a binary system of massive stars — starting from the top left — to a neutron star binary system. Image credit: De et al, doi: 10.1126/science.aas8693.
“You need fast transient surveys and a well-coordinated network of astronomers worldwide to really capture the early phase of a supernova,” 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.”
The fact that the star exploded at all implies that it must have previously had a lot of material, or its core would never have grown large enough to collapse.
But where was the missing mass hiding? The astronomers inferred that the mass must have been stolen by a compact companion star.
The neutron star that was left behind from the supernova must have then been born into orbit with this compact companion.
Because this new neutron star and its companion are so close together, they will eventually merge in a collision.
“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.
“In addition to the observations of binary neutron stars by gravitational and electromagnetic waves, the detections of ultra-stripped supernovae will play an important role in understanding the birthplace of elements.”
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K. De et al. 2018. A hot and fast ultra-stripped supernova that likely formed a compact neutron star binary. Science 362 (6411): 201-206; doi: 10.1126/science.aas8693
