Supernovae - explosions of stars as bright as the entire Milky Way - have fascinated us since ancient times. However, 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 key role in identifying the missing precursor cluster. The results, now published in the journal Science, can be traced back to a conversation between two professors who were junior scientists many years ago.
Some stars don't simply die, but explode in stellar explosions that can exceed the power of entire galaxies. These cosmic phenomena, called supernovae, spread light, elements, energy and radiation through space and create galactic shock waves that compress gas clouds and create new stars. In other words, supernovae shaped our universe. Among them, hydrogen-poor supernovae produced by the explosion of massive stars have always puzzled astrophysicists. The reason: scientists have been unable to find their progenitor stars. These supernovae appear almost as if they appear out of thin air.
"There are many more hydrogen-poor supernovae than our current models can explain. Either we cannot detect stars that mature on this path, or we have to modify all our models," says ISTA Assistant Professor Ylva Götberg. She pioneered the work with Maria Drout, an associate professor at the Dunlap Institute of Astronomy and Astrophysics at the University of Toronto, Canada.
"Single stars typically explode as hydrogen-rich supernovae. Hydrogen poverty indicates that the precursor star must have lost its thick hydrogen-rich envelope. This occurs naturally in one-third of massive stars as the binary companion strips off its envelope," Gottberg said.
Now, Gottberg and Drout are combining their expertise in theoretical modeling and observations to search for the missing stars. Their search was successful: they documented a first-of-its-kind stellar population, finally filling a huge knowledge gap and revealing the origin of hydrogen-poor supernovae.
Double stars and capsular peeling
The stars Gottberg and Drout were looking for were in pairs: crossed with each other in a binary system. The Sirius binary star system is only 8.6 light-years away from Earth - from a cosmological point of view, it is literally a stone's throw away. This explains the observed brightness of Sirius A in our night sky.
Astrophysicists expect that the missing stars originally formed from massive binary star systems. In a binary system, the stars orbit each other until the more massive star's thick, hydrogen-rich envelope expands. Eventually, the expanding envelope's gravitational pull on the companion star will be greater than the gravitational pull on its own core.
This causes mass to begin to shift, eventually causing the entire hydrogen-rich envelope to be stripped away, leaving behind a hot and compact helium core - more than 10 times hotter than the sun's surface. This is exactly the type of star Gottberg and Drout are looking for.
"Intermediate-mass helium stars stripped off through binary interactions were thought to play an important role in astrophysics. However, they have not been observed until now." In fact, there is a huge mass gap between the known types of helium stars: the more massive Wolf-Rayet (WR) stars are more than 10 times the mass of the sun, while low-mass subdwarfs may be only about half the mass of the sun. However, according to model predictions, the mass of the hydrogen-poor supernova precursor after stripping is between 2 and 8 solar masses.
Not looking for a needle in a haystack
Prior to Gottberg and Drout's study, only one star had been found to meet the expected mass and composition criteria, and was dubbed a "quasi-wolf ray" (or "approximate wolf ray").
"However, stars that follow this path live so long that many of them must be scattered throughout the observable universe," Gottberg said. Don't scientists "see" them at all? So Gottberg and Drouter used their complementary expertise. With the help of ultraviolet photometry and optical spectroscopy, they identified a cluster of 25 stars that matched expectations for intermediate-mass helium stars. These stars are located in two well-studied neighboring galaxies, the Large Magellanic Cloud and the Small Magellanic Cloud.
"Our study shows that these stars are bluer than the star birth line, which is the bluest stage in a single star's life. The maturation process of a single star is towards the red region of the spectrum. The star moves in the opposite direction only when its outer layers are removed - a situation expected to be common in interacting binaries but rare in single massive stars," Gottberg explains.
The scientists then used optical spectroscopy to verify the candidate star population: they found that these stars had strong spectral signatures of ionized helium.
"The strongly ionized helium lines tell us two important things: first, they confirm that the outermost layers of stars are mostly helium, and second, their surfaces are very hot," Gottberg said. "After stripping off, the star's core is exposed, compact and helium-rich, and that's what happens to stars."
However, both stars in a binary system contribute to the observed spectrum. This technique therefore allows researchers to classify clusters of candidate stars based on which star contributes the most to the spectrum.
"This work has led us to the missing population of intermediate-mass helium-depleted stars, the predicted ancestors of hydrogen-poor supernovae. These stars have always been around, and there may be more. We have to figure out how to find them," Gottberg said. "Our work is probably one of the first attempts, but there should be other possible approaches."
From graduate student to astrophysics leader
The idea behind the project was sparked by a discussion after Gottberg and Drout attended a conference during their graduate studies at which Gottberg gave a talk. Both scientists were early-career researchers heading for the stars and are now leaders in their fields.
Gottberg, who worked as a NASA Hubble Postdoctoral Fellow at the Carnegie Observatory in Pasadena, California, joined ISTA in September. At the International Institute for Astrophysics, Gottberg joins the institute's growing ranks of young astrophysics group leaders and leads her own group focused on studying binary interactions of stars.
Compiled source: ScitechDaily