An international group of scientists has reported the discovery of a new class of stars that eventually collide with neutron stars, an inadequate link in the evolution of binary systems.

It is believed that during the explosion that accompanies such a collision, conditions for the formation of heavier iron elements such as silver, gold and platinum, which cannot occur during the kiloton explosion, in stellar cores. Therefore, the fusion of neutron stars is vital for the distribution of heavy elements in the universe. The missing link in the evolution of kilonovas are stars in binary systems whose outer layers of hydrogen have been absorbed by a companion star. The star that “suffers” from this process is left with exposed dense layers of hot helium, formed as a result of hydrogen synthesis.

Astronomers already know about the existence of small and conversely large bare stars (Wolff-Rayet stars), but they are too small or too large to be in kilonova-producing systems. Previously, it was not possible to detect helium stars with a mass of two to eight solar masses. This even led to the hypothesis of a “mass gap in helium stars”, raising questions about whether life cycle models of massive stars could be wrong. Now, an international group of scientists led by University of Toronto associate professor Maria Drout has managed to identify 25 possible examples of objects that represent this missing link in evolution.

Medium-mass naked helium stars begin their life cycles as giants with 8 to 25 solar masses. They are in binary systems with companions gradually capturing their outer layers. When such a star runs out of fuel for nuclear fusion, a supernova explosion occurs, expelling a relatively small amount of matter, but the core remains a neutron star. At this time, they swap places as a pair, and the new neutron star begins to absorb its companion, which at some point also produces a supernova.

Evolution of binary systems with nude stars

A binary system of neutron stars is formed, consisting of a pair of closely related “dead bodies” that emit gravitational waves as they rotate each other. These gravitational waves carry the angular momentum of the binary system with them, spinning faster and faster until the neutron stars collide and produce one kilowatt. But for the objects scientists have discovered, that scenario is still in the distant future.

Astronomers think there’s a reason why it’s so difficult to detect naked medium-mass stars. The light they emit in the visible field is interrupted by the light of hydrogen-burning comrades. To circumvent this limitation, researchers began searching for them in the ultraviolet range, and the search began with dwarf galaxies, the Large and Small Magellanic Clouds, located near the Milky Way. As a result, it was possible to discover 25 objects that would form supernova and neutron star pairs with subsequent fusion.

Such a star is very different from what scientists expect to see: it has not yet completely lost its outer hydrogen layer, and if a similar mechanism is characteristic of other medium-mass objects, then they may appear much larger and cooler than they really are. . This means that the stars of the new class may have been hidden from view all along.