Two recent studies suggest that brown dwarfs, or so-called “failed stars,” are nevertheless more like stars than planets.

brown dwarf's atmosphere
Artist's conception of a brown dwarf.
NASA / JPL-Caltech

Brown dwarfs are the exceedingly common runts of the stellar litter. But even though they’re everywhere, their faint glow makes them difficult to observe and understand. Two recent studies shed light on the formation of these once-exotic objects.

First proposed as an idea in the 1960s and finally discovered in the 1990s, brown dwarfs bridge the gap between the smallest stars and the largest planets, never igniting hydrogen fusion in their cores. They cool off over time, slowly shedding the nascent heat leftover from their formation as a dim glow.

In the last two decades, astronomers have scrutinized hundreds of these objects, studying their properties and pondering their formation. Did brown dwarfs form like stars, condensing out of gigantic clouds of dust and gas? Or did they come together in the same way as planets do, within the disk around another star? The debate rang out in meetings all over the world, but gradually evidence pointed to a scaled-down version of star formation.

Now, two exciting new studies suggest that brown dwarfs (including one with a mass of just 12 times that of Jupiter) are mimicking their bigger stellar siblings in other ways.

A Brown Dwarf’s Star-Like Jet

Brown dwarf launches jet
This image shows the jet launched by a brown dwarf in the outer periphery of the sigma Ori cluster. Traced by emission from singly ionized sulfur, which appears green in the image, the jet extends 0.7 light years northwest of the brown dwarf. Click image for larger version.

Basmah Riaz (Max Planck Institute for Extraterrestrial Physics, Germany) and colleagues used the Southern Astrophysical Research telescope (SOAR) to study a young brown dwarf, dubbed Mayrit 1701117, in the 3 million-year old sigma Ori star cluster. The observations, to be published in the Astrophysical Journal, showed the brown dwarf to be powering a jet of material that launches gas up to 0.7 light-years from the object.

Jets have often been observed coming from young stars as well as brown dwarfs, but brown dwarf jets have typically been much smaller than their stellar counterparts. This jet, though, is the largest ever observed from a brown dwarf. And just like jets coming from more massive stars, this one also varies with time, its gas clumping as it flows outward. That kind of knottiness indicates that the gas is probably powered by accreting material that falls irregularly onto the brown dwarf.

“The jet shows all the familiar hallmarks of outflows from stars . . . it checks all the boxes quite convincingly,” says coauthor Emma Whelan (National University of Ireland, Maynooth).

Star-like Disk Feeds Infant Brown Dwarf

Brown dwarfs form as stars do, fed by accretion disks
Artists' impression of the gas and dust disk around the planet-like object OTS44. First radio observations indicate that OTS44 has formed in the same way as a young star.
Johan Olofsson (Univ. Valparaiso & MPIA)

Young stars are often found surrounded by disks of gas and dust leftover from their formation. Some of this disk material falls onto the star itself, while other parts eventually form planets and other small objects. Some brown dwarfs have such disks too, but until now, such disks have only been discovered around much more massive brown dwarfs.

In another study, published in Astrophysical Journal Letters and led by Amelia Bayo (University of Valparaíso, Chile), focused on OST44, a planetary-mass object in the Chameleon star-forming region. It isn’t a brown dwarf exactly — the boundaries that divide stars, brown dwarfs and planets are fuzzy. Brown dwarfs traditionally contain between 13 and 75 times the mass of Jupiter, but the exact boundaries can shift depending on what the object’s made of. With a mass 12 times Jupiter’s, OST44 lies right on the planet-brown dwarf boundary. .

Bayo used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to examine the disk of cool gas around OST44, the least massive brown dwarf-ish object to host an accretion disk. At just 2 million years old, the object is an infant in astronomical terms and it’s still growing, as gas flows inward from its disk.

From the ALMA data Bayo and colleagues measured the amount of dust around OST44 and found that it matched what’s expected based on observations of disks around other stars and more massive brown dwarfs. So it seems the astronomers have caught this object in the act of forming like a star.

But these new observations present a new challenge: From what we know, star formation methods shouldn’t be able to produce planet-mass objects. Yet that seems to be exactly what happened here.

To understand how these runts can act like their bigger stellar siblings, producing large-scale jets and feeding from swirling disks of gas, astronomers will need more data. You can expect more exciting results from ALMA in this regime, as it studies more young, low-mass objects in the future.


Image of Donald W.-Bennett

Donald W.-Bennett

May 26, 2017 at 9:57 am

" . . . From what we know, star formation methods shouldn’t be able to produce planet-mass objects. Yet that seems to be exactly what happened here. "

Ha! The Universe will always get the final say . . . no matter what we expect to see.

Great article.

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