Astronomers have shown what separates real stars from the wannabes. Not in Hollywood, but out in the universe.
“When we look up and see the stars shining at night, we are seeing only part of the story,” said Trent Dupuy of the University of Texas at Austin and a graduate of the Institute for Astronomy at the University of Hawaii at Manoa. “Not everything that could be a star ‘makes it,’ and figuring out why this process sometimes fails is just as important as understanding when it succeeds.”
Dupuy is the lead author of the study and is presenting his research today in a news conference at the semi-annual meeting of the American Astronomical Society in Austin.
He and co-author Michael Liu of the University of Hawaii have found that an object must weigh at least 70 times the mass of Jupiter in order to start hydrogen fusion and achieve star-status. If it weighs less, the star does not ignite and becomes a brown dwarf instead.
How did they reach that conclusion? The two studied 31 faint brown dwarf binaries (pairs of these objects that orbit each other) using W. M. Keck Observatory’s laser guide star adaptive optics system (LGS AO) to collect ultra-sharp images of them, and track their orbital motions using high-precision observations.
“We have been working on this since Keck Observatory’s LGS AO first revolutionized ground-based astronomy a decade ago,” said Dupuy. “Keck is the only observatory that has been doing this consistently for over 10 years. That long-running, high-quality data from the laser system is at the core of this project.”
Their goal was to measure the masses of the objects in these binaries, since mass defines the boundary between stars and brown dwarfs.
The research team also used the Hubble Space Telescope to obtain the extremely sharp images needed to distinguish the light from each object in the pair.
However, the price of such zoomed-in, high-resolution images from Hubble and Keck Observatory is that there is no reference frame to identify the center of mass. Wide-field images from the Canada-France-Hawaii Telescope (CFHT) containing hundreds of stars provided the reference grid needed to measure the center of mass for every binary.
The result of the decade-long observing program is the first large sample of brown dwarf masses.
“It’s the synergy between Keck Observatory and CFHT that really gets us the full power of the results,” said Dupuy.
“As they say, good things come to those who wait. While we’ve had many interesting brown dwarf results over the past 10 years, this large sample of masses is the big payoff. These measurements will be fundamental to understanding both brown dwarfs and stars for a very long time,” said Liu.
Astronomers have been using binaries to measure masses of stars for more than a century. To determine the masses of a binary, one measures the size and speed of the stars’ orbits around an invisible point between them where the pull of gravity is equal (known as the “center of mass”).
However, binary brown dwarfs orbit much more slowly than binary stars, due to their lower masses. And because brown dwarfs are dimmer than stars, they can only be studied in detail with the world’s most powerful telescopes.
The information that Dupuy and his discovery team have assembled has allowed them to draw a number of conclusions about what distinguishes stars from brown dwarfs.
Objects heavier than 70 Jupiter masses are not cold enough to be brown dwarfs, implying that they are all stars powered by nuclear fusion. Below 70-Jupiter mass objects are fated to be brown dwarfs. This minimum mass is somewhat lower than theories had predicted but still consistent with the latest models of brown dwarf evolution.
In addition to the mass cutoff, they discovered a surface temperature cutoff. Any object cooler than 1,600 Kelvin (about 2,400 degrees Fahrenheit) is not a star, but a brown dwarf.
This simple division between stars and brown dwarfs has been used for a long time. In fact, astronomers have had theories about how massive the collapsing ball has to be in order to form a star for over 50 years. However, the dividing line in mass has never been confirmed by observation, until now.
This new work will help astronomers understand the conditions under which stars form and evolve — or sometimes fail. In turn, the success or failure of star formation has an impact on how, where, and why solar systems form.
This research will be published in the next issue of The Astrophysical Journal Supplement, and a preprint can be found online at arxiv.org/abs/1703.05775.