Astronomers have discovered a group of stars that represent the “missing link” in the evolution of binary systems that eventually became colliding neutron stars.
These explosive fusion events, called “kilonovas”, are thought to generate violent conditions that lead to the formation of elements heavier than iron, including silver, gold and platinum, that cannot be created in the cores of stars. The merging of neutron stars is therefore vital to the scattering of heavy elements throughout the universe.
The missing piece of the kilonova’s evolutionary story comes in the form of stars in binary systems whose outer layers of hydrogen have been stripped away by a companion star. This leaves behind an exposed star the dense, hot outer layers of helium that were formed deeply with the star via hydrogen fusion.
Astronomers have known that low-mass stars (sub-dwarfs) and barren high-mass stars (Wolf-Rayet stars) of this type exist for some time, but these types are either too young or too large to give rise to systems that can evolve into kilonovas. However, the average mass that stripped helium stars with masses between two and eight times the mass of the Sun, which is well-suited for such an event, was a long way off. Failure to discover these missing-ring stars led to the so-called “helium star mass gap” that made scientists wonder if life-cycle models of massive stars could be wrong.
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In a new study, a team led by University of Toronto Assistant Professor Maria Drutt discovered 25 possible examples whose brightness and temperatures correspond to these missing stellar connections.
“The naked stars identified here are thus valuable for constraining the physical properties of this important but elusive group,” the authors wrote in the study. With masses estimated at about two to eight solar masses, it fills the helium star mass gap, associates sub-dwarfs with Wolf-Rayet stars and represents the first sample of the most likely progenitor of supernovae that can be studied. in detail.”
kilonova evolution
Bare helium stars of medium mass begin life as giants between eight and 25 times the mass of the Sun. It exists in a binary system with another star gradually pulling back its outer layers.
When a naked star runs out of nuclear fusion fuel, it undergoes a type of cosmic explosion called an abstract supernova, which ejects little material but leaves behind a neutron star. At this point, the tables turn, and that newly born neutron star begins feeding on its companion star, causing its companion to also experience an extremely abstract supernova explosion.
The result is a neutron star binary system consisting of two dense, tightly bound stellar bodies that emit gravitational waves as they orbit each other. These gravitational waves, ripples in space-time first predicted by Albert Einstein in his 1915 theory of gravity, general relativity, constantly carry angular momentum from the neutron star binary, causing the stellar remnants to swirl together faster and faster until they collide and merge. in kilonova.
Most of this process is in the distant future of the medium-mass helium star systems found by Drouet and her colleagues.
Illustration of the evolution of a binary star system into a neutron star merger. (Image credit: Varsha Ramachandran, ZAH/ARI)
How astronomers found the missing link
The team suspects that the reason these missing stellar bonds are so hard to spot is because the light they emit in the optical spectrum is dominated by emissions from companion stars that are currently burning hydrogen.
The researchers worked around this nuisance by searching for these stars in ultraviolet light instead, and targeted two of our Milky Way’s neighbors, the dwarf galaxies the Large Magellanic Cloud and the Small Magellanic Cloud, in data collected by T.V.It is the observatory of Neil Gehrels SwiftMagellanic cloud survey. This work revealed 25 sources that are expected to eventually erupt into stripped-core supernovae, resulting in a neutron star binary that will spiral together and merge.
This detailed study of these mass gap stars may help solve a mystery regarding an intermediate-mass helium star found in June this year by a team led by Heidelberg University researcher Varsha Ramachandran, in the form of a giant light star in the Small Magellanic Cloud.
“With our discovery, we prove that a long-lost population of these stars does indeed exist!” Ramachandran said in a statement last month. “But our findings also suggest that it may look very different from what we expected.”
The star, Ramachandran and her team, didn’t completely lose its outer layers, instead retaining a small but sufficient amount of hydrogen above its helium core. If this is the case with other medium-mass helium stars, it could make them appear much larger and cooler than they really are. So it’s likely that these partially naked stars have been hiding in plain sight all along.
The new study has been accepted for publication in the journal Science. A preprint version of the paper is currently available on the Paper Depository website arXiv.
