Unlocking the Mystery of Hydrogen-Poor Supernovae

Thursday - December 28 2023, 19:11 UTC - 10 months ago

Astrophysicists have long been puzzled by the explanation of hydrogen-poor supernovae that seem to have materialized out of thin air. In order to better explain them, ISTA Assistant Professor Ylva Götberg and Maria Drout, an Associated Faculty Member of the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, Canada, proposed an explanation which uncovers a first-of-its-kind star population which bridges the knowledge gap and may be the precursor star to the phenomenon. This population consists of stars interlocked in binary star systems, where the mass stripping of the more massive star by its companion eventually leaves a hot and compact helium core.

Supernovae–stellar explosions as bright as an entire galaxy–have fascinated us since time immemorial. Yet, there are more hydrogen-poor supernovae than astrophysicists can explain. Now, a new Assistant Professor at the Institute of Science and Technology Austria (ISTA) has played a pivotal role in identifying the missing precursor star population. The results, now published in Science, go back to a conversation the involved professors had many years ago as junior scientists .

Some stars do not simply die down, but explode in a stellar blast that could outshine entire galaxies. These cosmic phenomena, called supernovae, spread light, elements, energy, and radiation in space and send galactic shock waves that could compress gas clouds and generate new stars. In other words, supernovae shape our universe. Among these, hydrogen-poor supernovae from exploding massive stars have long puzzled astrophysicists .

The reason: scientists have not been able to put their finger on their precursor stars. It is almost as if these supernovae appeared out of nowhere. “There are many more hydrogen-poor supernovae than our current models can explain. Either we can’t detect the stars that mature on this path, or we must revise all our models,” says ISTA Assistant Professor Ylva Götberg. She pioneered this work together with Maria Drout, an Associated Faculty Member of the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, Canada .

“Single stars would typically explode as hydrogen-rich supernovae. Being hydrogen-poor indicates that the precursor star must have lost its thick hydrogen-rich envelope. This happens naturally in a third of all massive stars through envelope stripping by a binary companion star,” says Götberg. Now, Götberg and Drout combined their areas of expertise in theoretical modeling and observation to hunt down the missing stars .

Their quest is successful: they document a first-of-its-kind star population that finally bridges a large knowledge gap and sheds light on the origin of hydrogen-poor supernovae. The stars that Götberg and Drout search for go in pairs: interlocked in a binary star system. Some binary systems are well-known to us Earthlings: these include the brightest star in our night sky, Sirius A, and its faint companion star Sirius B .

The Sirius binary system is located only 8.6 light-years away from Earth–a stone’s throw in cosmic terms. This explains Sirius A’s observed brightness in our night sky. Astrophysicists expect the missing stars to be initially formed from massive binary systems. In a binary system, the stars would orbit around one another until the more massive star’s thick, hydrogen-rich envelope expands .

Eventually, the expanding envelope experiences a stronger gravitational pull to the companion star than to its own core. This causes a transfer of mass to begin, which eventually leads the entire hydrogen-rich envelope to be stripped off, leaving the hot and compact helium core exposed–more than 10 times hotter than the Sun’s surface. This is precisely the type of star astronomers were searching for: the perfect candidate to eventually go supernova and explain the hydrogen-poor supernovae .