Astronomers report two new space oddities

Astronomers have described not one but two rather rare sights in the Monthly Notices of the Royal Astronomical Society.

In the first, an international team led by the University of Birmingham, UK, reports spotting a blast of light emitted by a star as it is sucked in by a supermassive black hole just 215 million light-years from Earth.

The phenomenon, known as a tidal disruption event, is caused when a star passes too close to a black hole and the extreme gravitational pull from the black hole shreds the star into thin streams of material – a process with the wonderful name “spaghettification”.

As the process occurs, some of the material falls into the black hole, releasing a bright flare of energy which astronomers can detect. It’s usually not easy, but this flare was found just a short time after the star was ripped apart.

Matt Nicholl and colleagues also had access to some of the big guns of the telescope world, including the ESO’s Very Large Telescope (VLT) and New Technology Telescope (NTT), the Las Cumbres Observatory global telescope network, and the Neil Gehrel’s Swift Observatory.

They monitored the flare, named AT2019qiz, for six months as it grew brighter then faded away. Their paper is available on the pre-print server arXiv.

“The idea of a black hole sucking in a nearby star sounds like science fiction, but this is exactly what happens in a tidal disruption event,” says Nicholl. “We were able to investigate in detail what happens when a star is eaten by such a monster.”

Prompt and extensive observations in ultraviolet, optical, X-ray and radio light revealed, for the first time, a direct connection between the material flowing out from the star and the bright flare emitted as it is devoured by the black hole.

“The observations showed that the star had roughly the same mass as our own Sun, and that it lost about half of that to the black hole, which is over a million times more massive,” Nicholl says.

In the second paper, an Australian-led team confirms and explains a new member of an elite club that breaks all the rules.

One in a hundred million stars is a Wolf-Rayet: hot and very bright but doomed to imminent collapse in a supernova explosion leaving only a dark remnant, such as a black hole. Rare even among Wolf-Rayets are elegant binary pairs that, if the conditions are right, are able to pump out huge amounts of carbon dust driven by their extreme stellar winds.

As the two stars orbit one another, the dust gets wrapped into a beautiful glowing sooty tail. Just a handful of these sculpted spiral plumes has ever been discovered.

Researchers led by Peter Tuthill, from the University of Sydney, found one 8000 light-years from Earth two years ago and called it Apep. They were stumped, however, by the expansion of its dust spiral.

“The dust seems to have a mind of its own, floating along much slower than the extreme stellar winds that should be driving it,” says Yinuo Han, lead author of the new paper that describes the strange physics.

Using the VLT, he and colleagues produced a model that matches the intricate dust spiral structure for the first time, revealing that it is expanding at only a quarter the speed of the measured stellar winds, something they say is unheard of in other systems.

The leading theory to explain this behaviour, they suggest, makes Apep a strong contender for producing a gamma-ray burst when it does finally explode, something never before witnessed in the Milky Way.

“There has been a flurry of research into Wolf-Rayet star systems: these really are the peacocks of the stellar world,” says co-author Joe Callingham, from Leiden University in the Netherlands.

“Discoveries about these elegantly beautiful, but potentially dangerous objects, [are] causing a real buzz in astronomy.”

The numbers reveal Apep’s extreme nature. The two stars are each about 10 to 15 times more massive than the Sun and more than 100,000 times brighter. They orbit each other about every 125 years at a distance comparable to the size of our Solar System.

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