An international 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. These findings will advance our understanding of a wide variety of topics ranging from gravitational waves to the origin of precious metals like gold and platinum.
A collision between two neutron stars produces a variety of heavy elements, including precious metals. Such a collision also produces electromagnetic waves and gravitational waves like those observed in August 2017 . Although these collisions have been observed, it was unknown how two neutron stars could form close enough together to eventually spiral down into each other and collide.
A neutron star is a dense, compact object left behind when a massive star sheds most of its outer mass in a supernova explosion. Thus a neutron star binary system must have started out as a binary system of two massive stars. But this posed a problem because it was believed the second explosion would expel most of the remaining mass and make the system unstable rather than forming a neutron star binary system.
A research team including Takashi Moriya at the National Astronomical Observatory of Japan (NAOJ) proposed a new formation hypothesis where the gravitational force of the neutron star formed during the first supernova strips most of the outer layer of the remaining star. When this "ultra-stripped" star then explodes, the supernova has much less ejecta and the system can remain stable. The team named this second type of supernova "ultra-stripped supernovae."
Moriya simulated the light from ultra-stripped supernovae. His simulations predicted the explosion energy would be smaller than normal supernovae and that the brightness would peak at 5-10 days after the explosion. The team also speculated that there might be a diffuse helium envelope surrounding an ultra-stripped supernova.
Given these predictions, a team led by Kishalay De, a PhD student at California Institute of Technology, searched the data archive of the intermediate Palomar Transient Factory (iPTF) and found "iPTF14gqr," a peculiar supernova for which the brightness evolution and spectroscopic features closely matched Moriya's simulations, including hints of a diffuse helium envelope.
Moriya explains, "This is the first clear detection of a supernova which can result in the formation of a binary neutron star system. 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."
###
Moriya's simulations were performed on NAOJ's PC cluster and the supercomputer "ATERUI." This research was published in Science on October 12, 2018.
Science