These binary stars are so close together that their year is only 20.5 hours long: ScienceAlert

A team of astrophysicists has discovered a binary pair of ultracool dwarfs so close together that they look like a single star.

They are remarkable because they only take 20.5 hours to orbit each other, meaning their year is less than one Earth day. They are also much older than similar systems.

We cannot see ultracool dwarf stars with the naked eye, but they are the most numerous stars in the galaxy. They have such low masses that they only emit infrared light, and we need infrared telescopes to see them.

They are interesting objects because theory shows that stars so close together should exist, but this system is the first time astronomers have observed this extreme proximity.

A team of astronomers presented their findings at the 241st meeting of the American Astronomical Society in Seattle. Chih-Chun “Dino” Hsu, an astrophysicist at Northwestern University, led the research. The system is called LP 413-53AB.

“It’s exciting to discover such an extreme system,” said Chih-Chun “Dino” Hsu, a Northwestern astrophysicist who led the study. “In principle, we knew these systems should exist, but no such systems had been identified yet.”

Nature’s extremes play an important role in the calibration of our theoretical models, and this is true for low-mass binaries. Before this discovery, astronomers knew of only three short-period, ultracool binaries.

The research team found the pair in archival data. They sifted through the data using an algorithm Hsu wrote that models stars based on their spectral data.

However, in the earlier images, the stars were accidentally aligned, so they appeared as a single star. The chance of that happening is high for a close binary pair like this.

But Hsu and his colleagues thought the data was odd, so they took a closer look at the star with the Keck Observatory. The observations showed that the light curve changed so quickly that there must be two stars.

Eventually they realized they had found the closest binary pair ever found.

“When we did this measurement, we could see things change over a couple of minutes of observation,” said UC San Diego professor Adam Burgasser. Burgasser was Hsu’s advisor while Hsu was a doctoral student.

“Most of the binaries we follow have orbital periods of years. So you get a measurement every few months. Then, after a while, you can put the puzzle together. With this system, we could see the spectral lines move apart in real time. It’s amazing to see something happen in the universe on a human time scale.”

To emphasize how close the stars are to each other, Hsu compared them to our own solar system and another well-known system.

The pair are closer together than Jupiter and one of its Galilean moons, Callisto. It is also closer than the red dwarf star TRAPPIST-1 is to its nearest planet, TRAPPIST-1b.

The stars are much older than the other three similar systems known to astronomers. While these three are relatively young at up to 40 million years old, LP 413-53AB is several billion years old, like our Sun.

Their ages are a clue that the stars did not start out so close to each other. The researchers believe they could have started in an even tighter orbit.

“This is remarkable because when they were young, something like 1 million years old, these stars would have been on top of each other,” Burgasser said.

Or the stars may have started as a pair on wider orbits and then become closer over time.

Another possibility is that the stars started out as a triple star system. Gravitational interactions could have simultaneously ejected one star and pulled the remaining two into a tighter orbit.

More observations of the unique system may help answer that.

Astronomers are interested in stars like these because of what they can tell us about habitable worlds. Since ultracold dwarfs are so faint and cool, their habitable zones are tight spaces.

It’s the only way they can heat the planets enough to sustain liquid surface water. But in LP 413-53AB’s case, the habitable zone distance is the same as the stellar orbit, eliminating the possibility of habitable exoplanets.

“These ultracool dwarfs are neighbors of our sun,” Hsu said. “To identify potentially habitable hosts, it is useful to start with our nearest neighbors. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found.”

Now that astronomers have found one system as close as this one, they want to know if there are more. It is the only way to understand all these different scenarios.

It’s hard to even come close to any conclusions when you only have one data point.

But astronomers don’t know if they’ve only found one because they’re so rare or because they’re so hard to spot.

“These systems are rare,” said Chris Theissen, study co-author and chancellor’s postdoctoral fellow at UC San Diego.

“But we don’t know if they’re rare because they rarely exist or because we just don’t find them. That’s an open question. Now we have one data point that we can start building on. This data had been sitting in the archive for a long time . Dino’s tools will enable us to look for more binaries like this.”

This article was originally published by Universe Today. Read the original article.

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