UdeM Astronomer Finds that Aging Brown Dwarfs Grow Lonely

It takes two to tango, but in the case of brown dwarfs that were once paired as binary systems, that relationship doesn’t last for very long. A new study led by a UdeM astronomer shows that older brown dwarfs rarely have a companion.

Clémence Fontanive, the researcher who led the study. Credit : Alessandro della Bella.

Clémence Fontanive, a researcher at the Trottier Institute for Research on Exoplanets at Université de Montréal, used NASA’s Hubble Space Telescope to study some of the coldest and lowest-mass brown dwarfs of the Solar neighbourhood. 

Brown dwarfs are interstellar objects more massive than Jupiter but less massive than the smallest stars. Like stars, brown dwarfs can be born in pairs and orbit around each other, in what is known as a binary system. In a paper published in The Monthly Notices of the Royal Astronomical Society, Dr Fontanive’s team report that no companions were found around 33 brown dwarfs observed with Hubble.

The fact that these relatively old brown dwarfs have no companions suggests that if some of them formed in pairs, they must have parted ways over time.

“Our survey confirms that widely separated companions are extremely rare around the older, lower-mass brown dwarfs. Since binary brown dwarfs are observed at younger ages, this suggests that such systems do not survive over time,” said Clémence Fontanive.

Less massive? Less chance of having a companion! 

More than half the stars in our galaxy are found in binary systems, with the most massive stars usually accompanied by another star.

“There is a trend that the less massive an object is, the less likely it is to be found in a pair. The motivation for this study was to see if we continue to see this trend when we reach the very lowest-mass brown dwarfs,” said Dr. Fontanive. 

The very low frequency of pairs deduced from this study is indeed compatible with this trend extending all the way down to the least massive objects. It also supports the theory that brown dwarfs are born the same way as stars, through the gravitational collapse of a cloud of molecular hydrogen. The only difference: they do not have enough mass to sustain nuclear fusion of hydrogen for generating energy, whereas stars do. 

Young brown dwarf couples exist, but separations are frequent

Formation processes are known to produce low-mass binary brown dwarfs. Dr. Fontanive discovered just such a system in 2020 in the young star-forming region of Ophiuchus. The team’s recent results therefore provide evidence that these young binary systems are likely to get disrupted and will not survive to old ages. 

Why? Dr. Fontanive details the most plausible explanation: 

“When they’re young, these pairs of brown dwarfs are part of a molecular cloud that disperses as it evolves. As that happens, things start to move and stars pass relatively close to each other. Because brown dwarfs are so light, the gravitational hold tying wide binary pairs is fragile, and bypassing stars can easily tear these binaries apart.”

The Hubble telescope in search of companions

For their NASA’s Hubble Space Telescope survey, the team selected a sample of brown dwarfs previously identified by another NASA’s space telescope, the Wide-Field Infrared Survey Explorer. The sample consists of 33 of the nearest and coldest rogue brown dwarfs, that had never been explored for binarity before. 

For these nearby objects, Hubble can detect binaries as close together as 500 million kilometres, the approximate distance between our Sun and the asteroid belt.

These brown dwarfs are so cool (a few hundred degrees warmer than Jupiter in most cases) that their atmospheres contain water vapour that condensed out. To find the coolest companions, the astronomers used two different near-infrared filters, one in which cold brown dwarfs are bright, and another where they appear very faint due to the water absorption in their atmospheres. 

“We could not have done this kind of survey without Hubble’s sharpness and sensitivity. It’s interesting but not surprising that we did not find anything,” says Dr. Fontanive. This confirms earlier models by providing the best observational evidence to date.”

About this study

The article An HST survey of 33 T8 to Y1 brown dwarfs: NIR photometry and multiplicity of the coldest isolated objects was published on September 22, 2023 in Monthly Notices of the Royal Astronomical Society. In addition to Clémence Fontanive (iREx, UdeM, Canada), the team includes 7 co-authors from Italy, the USA and the UK. 

About the Hubble Space Telescope

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. Goddard also conducts mission operations with Lockheed Martin Space based in Denver, Colorado. The Space Telescope Science Institute in Baltimore, Maryland, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

Media Contact 

Marie-Eve Naud
Education and Public Outreach Coordinator,
Trottier Institute for Research on Exoplanets
Université de Montréal, Montréal, Canada
514-279-3222, marie-eve.naud@umontreal.ca  

Science Contacts 

Clémence Fontanive
Trottier Postdoctoral Fellow,
Trottier Institute for Research on Exoplanets at Université de Montréal
Université de Montréal, Montréal, Canada
clemence.fontanive@umontreal.ca 

Multimedia

Artistic rendition of a brown dwarf

Caption: This artist’s concept shows a brown dwarf, an object more massive than a planet but smaller than a star. We know that brown dwarfs can have binary companions. But as they age, some of these binary systems gravitationally fall apart, and each brown dwarf goes its separate way, according to a recent study led by UdeM astronomer. Credit: NASA, ESA, Joseph Olmsted (STScI)

Clémence Fontanive

Caption: Clémence Fontanive, the researcher who led the study. Credit: Alessandro della Bella.

Links

• Press release of STScI
• Press release of UdeM
• Article (MNRAS)