Predicted but unseen, these cannibal stars illuminate an astronomical mystery

Predicted but unseen, cannibal stars illuminate an astronomical mystery

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A star locked in close orbit around another will start to “eat” it, stripping hydrogen from its outer layers and leaving behind a hotter, denser star that is invisible to the human eye but glows hot in the ultraviolet spectrum, according to new research at the University of Toronto.

A search for these “hot helium” stars using both space and ground telescopes has identified two dozen in nearby galaxies called the Magellanic Clouds, and likely many more candidates whose physics are now being measured, proving long-held theory. The project was one of the first targeted searches of its kind. Its leaders expected to find lots of hot helium stars, and they did.

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Much astronomy is like this, based on the insight that some things must exist, even though scientists cannot see them, or have not yet seen them in the ways they have looked. Theories of how the universe works are so robust that even without direct observation, we know some things are there. They just have to be, otherwise the theory is wrong.

Dark energy, for example, makes up most of the universe and drives its expansion, as far as we understand it, but is otherwise not just unseen but unseeable, hence the name. Dark matter and black holes, likewise, are not things you can see in any normal sense, and yet their gravity is plain as day. So they must also be there in real life, not just in theory. At the other end of the size spectrum, at the very smallest scale of physics, a similar dynamic led to the discovery a decade ago of the Higgs Boson, long predicted by theory before an experiment blasted one into existence for proof.

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Further down this mysterious list are hot helium stars between two and eight times the mass of our sun. They may not sound as evocative, but they similarly illustrate the productive tension between theory and observation.

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The problem is that there should be lots of them, and unlike black holes, we should be able to see them. They are stars, after all.

But they have not been observed, other than a single candidate.

Maria R. Drout, an expert on the evolution of large stars, and assistant professor in astronomy and astrophysics at the University of Toronto, calls this a “big, glaring hole.”

“If it turned out that these stars are rare, then our whole theoretical framework for all these different phenomena is wrong, with implications for supernovae, gravitational waves, and the light from distant galaxies,” Drout said.

This new discovery by Drout and colleagues reveals a population of these hot helium stars. It proves their existence, in other words, and takes the first step toward observing, rather than just predicting, their physical properties. Their report is newly published in the journal Science.

It illustrates the potential of space telescopes and ground telescopes to reveal different crucial aspects of the same star, and it is relevant to the developing science of gravitational wave astronomy, a new way to observe some of the greatest cataclysms the universe has produced since the Big Bang — the merging of black holes.

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Most stars are unlike our own solitary sun. Most big stars are in pairs, two stars in orbit around each other.

“Especially at very massive stars, we think essentially all of them have binary companions,” Drout said in an interview.

This must be some consequence of how stars are formed. These pairs have not met by coincidence in the vastness of space, and captured each other by gravity. They have always been paired. They were formed together, orbiting each other. That’s the theory, anyway. But why heavier stars should be more likely to be paired off this way remains what Drout calls an “open question.”

The theory of star formation roughly goes like this. In the Big Bang, about 14 billion years ago, the universe expanded out from a single point, rapidly cooling as it became less dense. At first, this infant universe seems to have been uniformly smooth, but wrinkles appeared. Once one area is more dense with matter than another, gravity takes charge. Matter clumps together. The simplest elements, primarily hydrogen, are drawn under their own gravity into spheres, densest in the centre, and when they get big enough, the pressure and heat in their cores causes hydrogen atoms to fuse into heavier helium, releasing nuclear energy in the process that keeps the star burning. Eventually, the star uses up enough of its fuel that it collapses into itself, then explodes in a supernova.

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That first generation of stars thus becomes the raw material for the second, in which heavier elements were formed, and then scattered again as supernova shrapnel, and then formed again out of this stardust into newer stars, each time creating heavier elements, including the carbon and oxygen crucial for life, and all manner of metals and rocks that make up planets.

It is not clear why this process should favour the creation of large stars in binary pairs, but it seems to.

As a star starts to burn out, it also gets bigger. Not hotter, not denser, just wider across. It becomes what’s known as a red supergiant, the last phase before going supernova. Our sun will do this, theory predicts, but not for quite some time.

drout
Lead co-author Maria Drout of the University of Toronto with the Magellan Telescope at Las Campanas Observatory. Photo by Tom Holoien/Maria Drout

If one member of a binary pair starts doing this, Drout explains, the outer bit of the expanding star can become more closely bound gravitationally to the other star. So, as Drout describes it, the other star basically “eats” away at its partner’s shell, drawing it into itself.

“We call this stripping,” Drout said.

This tends to remove the lightest outer layer of the first star, which is mostly hydrogen, leaving behind underlayers of heavier elements, mostly helium.

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This, in theory, is how a “hot helium” star is formed. Its binary partner has eaten its hydrogen envelope. Drout does not actually call these partner stars cannibals, but that is the idea. They eat their own.

One effect is that the hot helium star now burns much hotter, and as a result most of its light is emitted in the ultraviolet range, with shorter wavelengths than visible light.

“We think this process should happen,” Drout said. “We think that about one in three massive stars, things that will eventually explode, should have their hydrogen envelope stripped off.”

There are many good reasons to think this. Many supernovae are observed to be poor in hydrogen, suggesting they are the end result of this process. But until now, astronomers had not found any, just a single candidate.

“We hadn’t actually seen stars that looked like that,” Drout said. “If we don’t find them, something’s wrong.”

reasearch team
Study authors Bethany Ludwig, Anna O’Grady, Maria Drout and Ylva Götberg observing on the Magellan telescopes at Las Campanas Observatory in Chile, where they gathered data for this research. Photo by University of Toronto

So this was a search for something she knew would be there. For Drout, it began in 2016. That was a big year in astronomy. That was the year an American experiment called LIGO made its first detection of gravitational waves. This was proof of a key component of Albert Einstein’s theory that space and time are part of the same cosmic fabric, called spacetime, in which gravity is a distortion caused by mass. It proved that major cataclysms like the merging of two black holes or neutron stars can cause waves to ripple through spacetime, just like waves rippling across water when a stone falls in. And if humans can build a detector sensitive enough, we can observe the ripple caused by this ancient merger as it passes through the spacetime around planet Earth.

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Earlier this year, Drout points out, LIGO was switched on again for a new observational run. Having already made the definitive discovery and won its leaders the Nobel Prize, however, some of the pressure is off.

It’s the same with hot helium stars. Now that they are discovered, the work continues, but the stakes are lower.

The second author on the new paper about hot helium stars is a theorist who worked on predictions of what these stars should look like in the ultraviolet spectrum. So, in their search, the team was looking for what appeared to be a single large star visible with normal light, but which was also emitting a lot of ultraviolet light in the predicted way, suggesting it is in fact two stars in close orbit that have gone through this process.

That turns out to be tricky.

NASA
Surveys conducted by NASA’s Swift-UVOT telescope provide the most detailed overviews ever captured in ultraviolet light of the Large and Small Magellanic Clouds, the two closest major galaxies to our own. Photo by NASA/Swift/S. Immler (Goddard) and M. Siegel (Penn State)

Earth’s atmosphere blocks ultraviolet light, so you need a telescope in space. Drout and colleagues used archival ultraviolet images of stars in nearby galaxies called the Magellanic Clouds by NASA’s Swift space telescope, launched in 2004, to look for a few hundred candidates in a few million potentials. Out of half a million sources of UV radiation, they selected 25.

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Then they consulted images of the same candidate stars taken with a large ground telescope in Chile, this time looking at their optical spectra to see what elements they were burning. This was the proof. They were hot helium stars, finally observed.

“We found these and we’re able to show that that’s what these stars are. They do absolutely exist, and that’s great because if they didn’t a lot of things would have to change,” Drout said. “Now that we’ve found them, we’re going to be able to study the properties of these stars in detail.”

It was a pilot project, seeking only the clearest examples, and with its success, they are now going after some of the other potential candidates.

Not all of these pairs of stars will merge into one, in such a way that earthbound gravitational wave detector such as LIGO might one day pick it up. But later on in their evolution, if they have become heavy and ultra dense neutron stars, some might. Drout thinks some of the pairs they discovered are stripped helium stars with a companion that is either a neutron star or a black hole, which is approaching the last stage before a merger. That’s why it was so important to understand this intermediate stage in their lifespan, the “stripping” of hydrogen.

“If you were going to have a merger, you would have had to do this first,” Drout said.

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