Astronomers study faint supernova explosions from isolated stars

Many of the heaviest stars in the universe end their lives in a bright explosion, known as supernovae, which briefly transcend the rest of the galaxy in which they reside, allowing us to see these rare events far away. At the lower end of this mass range, a supernova explosion would squeeze the star's core into a dense neutron sphere with a much greater density than can be reproduced in the lab. Therefore, scientists must rely on theoretical models and astronomical observations to study these objects, called neutron stars.

At the very low end of this range, supernova explosions are considered weaker and fainter, but even with the most advanced supernova simulations, testing this hypothesis is challenging. In our recently published study, we found a new way to test these weaker supernovae: By linking weaker supernova explosions to slow-moving neutron star remnants, neutron stars can accurately estimate weaker supernovae without the need for expensive simulations.

Instead of shining like other stars, neutron stars produce a very narrow beam of radio waves that may (if we're lucky) pointing at Earth. As the neutron star rotates, the beam of light seems to flicker on and off, creating a lighthouse effect. When this effect is observed, we call it a pulsating star, or pulsar. Recent advances in radio telescopes allow precise measurements of the speed of pulsars. We combined the measurements with simulations of millions of stars and found that typical high pulsar velocities do not allow many weak supernovae to appear.

However, there is one fact to note: many massive stars that produce neutron stars are born in stellar binary stars. If a normal supernova occurs in a stellar binary, the remnants of the neutron star will experience a massive recoil, like a shell rushing out of exploding gunpowder, and it will most likely be ejected from its companion star, where it may be observed as a single pulsar. But if the supernova is weak, the neutron star may not have enough energy to escape the gravitational pull of its companion star, while the stellar binary star system will remain intact. This is a necessary step in the formation of neutron star doubles, so the existence of these binary stars proves that some supernova explosions must be weak.

We found that, to explain the existence of neutron binary stars and the absence of slow pulsars, weak supernovae can only occur in very close star diplogues, rather than in single, isolated stars. This is useful for simulating supernovae and complements a growing body of research suggesting that weak supernovae may only occur in binary stars that have previously interacted. Studies like this, simulating many stars in relatively low detail, are key to understanding the effects of uncertain physics on star populations, which is not feasible in highly detailed simulations.