A year in a day: Ultracool dwarf star system breaks record
Thanks to the vast distances between objects in our galaxy, astronomers often have to wait months or years to observe how celestial bodies like stars and planets move around each other. Recently though, astrophysicists at Northwestern University and the University of California San Diego (UC San Diego) saw the light from a pair of ultracool dwarf stars change within minutes. This led to the discovery that what was once thought to be just one star was actually two. What's more, the two stars were found to be so close to each other that they broke a record for being the tightest ultracool binary system ever observed, revolving around each other in less than an Earth day.
Ultracool dwarf stars are difficult to see with the human eye because they emit most of their energy in the infrared spectrum. In other words, they don't burn bright enough to give off light we can see with our naked eyes. Still, they are extremely common in the universe and astronomers can observe them with specialized tools.
Chih-Chun “Dino” Hsu, a postdoctoral researcher in physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences decided to train such tools – in the form of the telescopes at the W.M. Keck Observatory in Hawaii – on a star system that is now known as LP 413-53AB. He did so after using an algorithm he developed to analyze archival data regarding the light emitted from various systems. His analysis detected that in one measurement, the spectral lines created by the light from the two stars overlapped but in another, the lines diverged. The first measurement would lead an observer to believe LP 413-53AB was just one star; the second would mean two were present.
Sure enough, thanks to his own observation of the system, which is located in the constellation Taurus, Hsu was able to see the shift for himself.
“When we were making this measurement, we could see things changing over a couple of minutes of observation,” said Hsu's advisor, Adam Burgasser. “Most binaries we follow have orbit periods of years. So, you get a measurement every few months. Then, after a while, you can piece together the puzzle. With this system, we could see the spectral lines moving apart in real time. It’s amazing to see something happen in the universe on a human time scale.”
Hsu determined that the distance between the two dwarf stars is approximately one million kilometers (about 621,370 miles), putting the pair much closer than the distance between Jupiter and its moon Callisto. Put another way, the space between the stars is just 1% of the space between the Earth and the Sun. The companion stars are so close that it takes just 20.5 hours for them to revolve around each other, making a year on each star less than a day in Earth time.
This is only the fourth ultracool dwarf binary system discovered by astronomers. In addition to having the shortest period of all the others, LP 413-53AB is also much older, clocking in at billions of years old versus around 40 million years old for the other systems. Also, whereas there was hope that previous binary dwarf systems could have habitable planets around them, Hsu says that won't be the case with LP 413-53AB as the planets would simply exist too close to the surfaces of the stars to sustain life.
“These ultracool dwarfs are neighbors of our Sun,” Hsu said. “To identify potentially habitable hosts, it’s helpful to start with our nearby neighbors. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found.”
Alien life aside, Hsu's algorithm does provide researchers with a new tool to seek out other ultracool dwarfs found in binary systems which have been historically hard to spot.
“These systems are rare,” said Chris Theissen, study co-author and a Chancellor’s Postdoctoral Fellow at UC San Diego. “But we don’t know whether they are rare because they rarely exist or because we just don’t find them. That’s an open-ended 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 tool will enable us to look for more binaries like this.”
The findings on LP 413-53AB were presented at the 241st meeting of the American Astronomical Society in Seattle, Washington on January 10.
Source: Northwestern University