Discovery of the first binary star system destined to explode in kilonova
artist's impression of the kilonova progenitor star system. | ctio/noirlab/nsf/aura/j. da silva/spaceengine/m. zamani
Still searching for the origin of the Universe and our solar system, astronomers have confirmed the first detection of a binary star system that will one day form a kilonova — an explosion producing heavy elements (gold and platinum) , caused by the merger of neutron stars . These systems are so rare that experts estimate that there are only about ten in the entire Milky Way.
A kilonova occurs when two neutron stars , which are among the densest objects in the universe, merge together. It is distinguished from classic supernovae of massive stars by the fact that there is little or no ejecta during the explosion. It therefore appears as a faint supernova.
Scientists believe these kilonovae occur in massive binary systems after the exploding star loses its surface through interactions with its companion, gradually stripping it of the dark matter that stabilizes it and holds it together. These supernovae lead to the formation of a neutron star without loss of the binary companion, which itself can also evolve into another neutron star. The merger of these two stars results in a kilonova.
Recently, a team of astronomers from the Embry-Riddle Aeronautical University (USA) discovered the first example of an extremely rare type of binary star system, which brought together all the conditions to possibly trigger a kilonova. Such an arrangement is so rare that only about ten such systems are thought to exist in the entire Milky Way . Their findings are published in the journal Nature .
A rare binary system from a failed "bang"
This unusual system, known as CPD-29 2176, is located about 11,400 light-years from Earth. It was first identified by NASA's Neil Gehrels Swift Observatory. Subsequent observations with the 1.5-meter SMARTS telescope at the Cerro Tololo Observatory (in Chile), allowed astronomers to deduce the orbital characteristics and types of stars that make up this system.
Clarissa Pavao, a student at Embry-Riddle Aeronautical University and co-author, plotted the spectra of a Be star. It is a B star, very bright and hot, which shows emission lines in its spectrum. The emission lines are thought to originate from a disk of material that surrounds the star.
Concretely, C. Pavao first had to clean the data of visual noise. She specifies in a press release : “ The telescope observes a star, and it captures all the light possible in order to visualize the elements that make up the latter. But Be stars tend to have discs of matter around them making direct observation difficult .
With her mentor, Dr. Noel D. Richardson, assistant professor of physics and astronomy at Embry-Riddle, she found a simple line that came from the star and was not influenced by the disc around it. After quickly integrating Pavao's data into a computer program, Richardson realized that they had identified a viable orbit for the star, but it was different than expected. Further analysis of the data revealed that a star did indeed circle around the former every 60 days or so.
The round orbit was a key clue allowing researchers to identify the binary system's second star as a depleted or ultra-stripped supernova. Usually, after a star has consumed all of its nuclear fuel, its core collapses before exploding into space as a supernova.
But Dr. Noel D. Richardson, assistant professor of physics and astronomy at Embry-Riddle, explains: " The star was so exhausted that the explosion did not even have enough energy to give the orbit the more typical elliptical shape seen in similar binaries ”.
Then with Jan J. Eldridge, of the University of Auckland and co-author, they managed to schematize the life cycle of the two stars of the binary system. Concretely, the evolution takes place in 9 stages:
Stage 1: Two massive blue stars form in a binary star system.
Stage 2: The larger of the two stars is nearing the end of its life.
Stage 3: The smaller of the two stars absorbs material from its larger, more mature companion, stripping it of much of its outer atmosphere.
Stage 4: The largest star forms an ultra-stripped supernova, the explosion at the end of a star's life with less force than a typical supernova, "similar to a failed firecracker" according to scientists.
Stage 5: As currently observed by astronomers, the neutron star resulting from the previous supernova begins to absorb matter from its companion, reversing the roles in the binary system.
Stage 6: With the loss of much of its outer atmosphere, the companion star also experiences an ultra-stripped supernova. (This stage will occur in this case in about a million years.)
Stage 7: The binary system now contains a pair of neutron stars in close mutual orbit.
Stage 8: The two neutron stars rotate towards each other, giving up their orbital energy as weak gravitational radiation.
Stage 9: The two neutron stars collide, producing a powerful kilonova, "the cosmic factory of heavy elements in our universe".
summary of the stages in the evolution of the star system cpd-29 2176.
ctio/noirlab/nsf/aura/p. marenfeldOur origin and our place in the Universe
As mentioned earlier, researchers estimate that there are probably only about ten such star systems in the galaxy right now. By studying it, they hope to uncover new clues about the origin of the Milky Way and de facto solar system.
Clarissa Pavao says, “ When we look at these objects, we are looking back in time. We learn more about the origins of the universe, and the future evolution of our solar system ”.
Richardson added that without binary systems like CPD-29 2176, life on Earth would look very different. Indeed, according to him, such systems have the possibility of evolving into binary neutron stars, which end up merging and forming heavy elements which are projected into the universe. He specifies: “ These heavy elements allow us to live as we do. For example, the majority of gold [and platinum] was created by stars similar to the supernova relic or neutron star in the binary system we studied. Astronomy deepens our understanding of the world and our place in it .
A future satellite for the study of kilonovae
The Roman Space Telescope is a NASA observatory designed to address critical questions in the fields of dark energy, exoplanets, and infrared astrophysics. The telescope has a primary mirror 2.4 meters in diameter — the same size as the Hubble Space Telescope's primary mirror . The telescope will be equipped with two instruments, the Wide Field Instrument with a field of view 100 times larger than the Hubble infrared instrument and the Coronagraph instrument.
After its launch no later than May 2027, this telescope will allow researchers to use this data to identify kilonovae and the frequency with which these events occur, the amount of energy they give off and their proximity.
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