Missing stars might answer supernovae riddle

Astronomers have long wondered why there are so many hydrogen-poor supernovae in the universe.

Current models can’t explain it but a bold new theory involving hidden stars could provide the answer.

Supernovae are the explosive deaths of large stars. They can be as bright as entire galaxies.

But more low-hydrogen supernovae from massive stars exist than physicists have been able to explain. The precursor giant stars that should have made them don’t seem to be numerous enough to have produced the supernovae.

It’s as if the supernovae are exploding into existence out of nothing.

“Either we can’t detect the stars that mature on this path, or we must revise all our models,” says Ylva Götberg an assistant professor at the Institute of Science and Technology Austria (ISTA).

But Götberg has led research published in Science which has now identified the missing precursor stars.

 “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.

Using theoretical modelling and observation the team successfully hunted down a first-of-its-kind star population that puts to bed this decades-old riddle.

The stars to look for come in pairs: a binary system.

As the two stars orbit each other, the hydrogen envelope of one expands before being stripped off by the gravitational pull of its companion. This leaves the hot, compact helium core of the star – 10 times hotter than the surface of the Sun – exposed.

“Intermediate-mass helium stars stripped through binary interaction are predicted to play important roles in astrophysics. Yet, they were not observed until now,” says Götberg. “Yet, the stars that follow this path have such a long lifetime that many must be scattered all over the observable universe.”

Götberg and her colleague Maria Drout, from the Dunlap Institute for Astronomy & Astrophysics, at the University of Toronto, Canada, have now found 25 stars that fit the bill. They are in the nearby Large Magellanic Cloud and the Small Magellanic Cloud.

In those galaxies the stars shone bluer than a single star could. Normally, single stars become redder as they mature.

“A star only shifts in the opposite direction if its outer layers are removed–something that is expected to be common in interacting binary stars and rare among single massive stars,” explains Götberg.

Strong spectral signatures of ionised helium verified that the objects they found were intermediate-mass helium stars.

“This work allowed us to find the missing population of intermediate-mass, stripped helium stars, the predicted progenitors of hydrogen-poor supernovae. These stars have always been there and there are probably many more out there. We must simply come up with ways to find them,” says Götberg. “Our work may be one of the first attempts, but there should be other ways possible.”