Free Astronomy Magazine July-August 2023

50 JULY-AUGUST 2023 ASTRO PUBLISHING T his infographic illustrates the evolu- tion of the star system CPD-29 2176, the first confirmed kilonova progenitor. Stage 1, two massive blue stars form in a binary star system. Stage 2, the larger of the two stars nears the end of its life. Stage 3, the smaller of the two stars siphons off material from its larger, more mature companion, stripping it of much of its outer atmosphere. Stage 4, the larger star forms an ultra-stripped supernova, the end-of-life explosion of a star with less of a “kick” than a more normal super- nova. Stage 5, as currently observed by astronomers, the resulting neutron star from the earlier supernova begins to siphon off material from its companion, turning the tables on the binary pair. Stage 6, with the loss of much of its outer atmosphere, the companion star also un- dergoes an ultra-stripped supernova. This stage will happen in about one million years. Stage 7, a pair of neutron stars in close mutual orbit now remain where once there were two massive stars. Stage 8, the two neutron stars spiral into to- ward each other, giving up their orbital energy as faint gravitational radiation. Stage 9, the final stage of this system as both neutron stars collide, producing a powerful kilonova, the cosmic factory of heavy elements in our Universe. [CTIO/NOIRLab/NSF/AURA/P. Marenfeld] would also need to explode as an ultra-stripped supernova so the two neutron stars could eventually col- lide and merge.” As well as representing the discovery of an incredibly rare cosmic oddity, finding and studying kilonova pro- genitor systems such as this can help astronomers unravel the mystery of how kilonovae form, shedding light on the origin of the heaviest ele- ments in the Universe. “For quite some time, astronomers speculated about the exact condi- tions that could eventually lead to a kilonova,” said NOIRLab astronomer and co-author André-Nicolas Chené. “These new results demonstrate that, in at least some cases, two sib- ling neutron stars can merge when one of them was created without a classical supernova explosion.” Producing such an unusual system, however, is a long and unlikely process. “We know that the Milky Way contains at least 100 billion stars and likely hundreds of billions more. This remarkable binary system is essentially a one-in-ten-billion sys- tem,” said Chené. “Prior to our study, the estimate was that only one or two such systems should exist in a spiral galaxy like the Milky Way.” Though this system has all the right stuff to eventually form a kilonova, it will be up to future astronomers to study that event. It will take at least one million years for the mas- sive star to end its life as a titanic su- pernova explosion and leave behind a second neutron star. This new stel- lar remnant and the pre-existing neutron star will then need to grad- ually draw together in a cosmic bal- let, slowly losing their orbital energy as gravitational radiation. When they eventually merge, the re- sulting kilonova explosion will pro- duce much more powerful gravitational waves and leave be- hind in its wake a large amount of heavy elements, including silver and gold. “This system reveals that some neu- tron stars are formed with only a small supernova kick,” concluded Richardson. “As we understand the growing population of systems like CPD-29 2176 we will gain insight into how calm some stellar deaths may be and if these stars can die without traditional supernovae.” !