Misfit stars: half planet, half star, newly discovered brown dwarfs are only as warm as a summer's day on Earth, defying definition even as they tempt astronomers with new insights.
For whatever reason, brown dwarfs form with too little mass to sustain nuclear fusion. Stars with 75 or more Jupiter masses have enough matter to continually fuse hydrogen into helium, releasing energy for millions or billions of years at steady temperatures. Brown dwarfs don't reach this mark: like regular stars, they can fuse the heavy form of hydrogen called deuterium, but they quickly consume that fuel and ultimately glow faintly with the heat left over from the gravitational collapse that birthed them. With no other lasting internal energy source of their own, brown dwarfs simply cool down as they age, radiating dimly at infrared wavelengths difficult for astronomers to detect amidst the background heat from other sources, such as Earth's atmosphere and the telescope we're using to observe them.
Researchers first theorized the existence of brown dwarfs in the 1960s and later thought they might account for a significant fraction of dark matter. We now know this is probably not the case, though the number of brown dwarfs likely approaches the number of regular stars. After the first brown dwarfs were confirmed in 1995, more than 1,000 have been spotted, with the coldest ones having surface temperatures as cool as 300 kelvins (80 [degrees]F). Thanks to the pressure exerted by electrons and atomic nuclei, most brown dwarfs have approximately the same diameter as Jupiter. Adding mass to a brown dwarf would make it denser, not larger.
To categorize brown dwarfs, astronomers extended downward the stellar classification system of O, B, A, F, G, K, and M, which groups stars based on their observed spectra and temperatures. Beneath the M class (which consists of mainly stars, although this group includes the youngest, hottest brown dwarfs) are the L dwarfs, slightly older and cooler brown dwarfs between 2400 K (3860 [degrees]F) and 1400 K (2060[degrees]F). L dwarfs eventually cool down to the T class, with atmospheres containing traces of steam, methane, and ammonia and at temperatures ranging from 1400 K down to 500 K (440[degrees]F). Whatever their temperature, many brown dwarfs' atmospheres have lithium (normally burned off during nuclear fusion), which is one way astronomers are able to pinpoint these stellar wannabes.
All brown dwarfs gradually fade at rates dependent on their mass, and end up in the coolest category, the Y class. These dwarfs show far more methane and ammonia in their atmospheres and potentially have water ice and salt clouds. They eventually reach Jupiter's temperature of around 130 K (-225?F). But the faint, long-infrared wavelengths of these older, ultra-cool Y dwarfs have been notoriously difficult to detect observationally. It wasn't until last year that the first eight Y dwarfs were discovered, six of which were found through the WISE mission.
The Search for Cold, Dark Beacons
WISE, launched into Earth orbit in December 2009 and retired in February 2011, snapped images every 11 seconds at four different mid-infrared wavelengths to map the entire sky. Scientists processed the data with automated software, pinpointed brown dwarf candidates, and used facilities such as NASA's Spitzer Space Telescope and the Keck Observatory in Hawaii to obtain photometry and spectra.
Last August, two papers announced that WISE researchers had found more than 100 new brown dwarfs, including the mission's first six confirmed Y dwarfs. These six objects are all relatively close, from about 10 to 30 light-years away. The coldest of these Y dwarfs, named WISEP J1828+2650, is only as warm as Earth on a summer's day: 300 K.
"That's just a glimpse of things to come," says J. Davy Kirkpatrick (IPACCaltech), lead author of one of the WISE papers. These ultra-cold brown dwarfs would appear 100 million times fainter than our Sun at the infrared wavelength of 1.2 microns if placed next to it.
One limitation of the new WISE data, however, is that all six Y dwarfs are isolated. Finding more in groups or near other objects would provide clues about their ages, masses, and other details.
"Right now we have 1,600 candidates in our list and we're not done searching the WISE data," says Michael Cushing (University of Toledo), lead author of the other WISE paper. While the bulk of the team's work will be completed in the next couple of years, scientists will be mining this dataset for decades.
Although WISE has been the most fruitful effort so far in finding ultra-cold brown dwarfs, two other groups spotted Y-dwarf candidates first, by looking for companions to particular stars. Last year, American and European researchers using the Keck II Telescope in Hawaii announced they had found a binary system containing an object, CFBDSIR J1458+1013B, that was only 370 K (210 [degrees]F)--the temperature of boiling water (S&T: June 2011, page 12).
"These things are in our backyard, within 50 light-years of Earth," says Michael Liu (University of Hawaii), lead author of the J1458 paper. "Before this year, the coolest known object was estimated to be around 520 K," he says. "We're trying to map the sequence of changes along the way of low-mass stars to colder brown dwarfs to form a complete picture of these objects. "
The eighth potential Y dwarf comes from another 2011 announcement. WD 0806-661B is 63 light-years away and, at roughly 325 K (125 [degrees] F), it's a contender with WISEP J1828 for the coldest brown dwarf. Kevin Luhman (Penn State University) and colleagues were observing a white dwarf with Spitzer when they noticed the faint object orbiting the white dwarf at a large distance.
WD 0806B is a mystery. With only about seven times the mass of Jupiter, it's below the technical mass limit of about a dozen Jupiters that allows a brown dwarf to fuse deuterium. On the other hand, astronomers think that gas giants form in disks around stars no farther out than 100 a.u., yet this cold object orbits at a staggering 2,500 a.u. from its white dwarf companion. "You don't expect a gas giant to be out this far from a star unless something weird happened and its orbit was radically changed," says Luhman.
Codiscoverer Adam Burgasser (University of California, San Diego) says it doesn't really matter what it's called. "I'm less concerned about labeling something a planet or a brown dwarf--the important result is that this is a very cold object and its atmosphere is unique."
Despite the lack of a spectrum, WD 0806B's infrared luminosity suggests that it has the right temperature for a Y dwarf. It's hard to estimate the ages and distances of the free-floating WISE brown dwarfs, but because this object is paired with a white dwarf whose age and distance are well known, Luhman says the team has been able to determine characteristics such as mass more easily.
These new discoveries are blurring the observational line between planets and brown dwarfs. If a cool object is found alone and giving off faint light of its own, most astronomers would call it a brown dwarf. But there are exceptions, such as free-floating, Jupiter-mass "orphan planets" thought to have been ejected from planetary systems (June issue, page 16). Astronomers may be forced to give up on mass and location clues alone and try to differentiate brown dwarfs from planets by inferring how these objects formed.
Most theories suggest brown dwarfs originate not as planets do, but as stars do, from clouds of gas and dust that gravitationally collapse to form a dense core. But this process favors the formation of objects massive enough to sustain hydrogen fusion. With brown dwarfs, something must interrupt that formation process. The disturbance could be a larger, passing object that either steals material from the growing stellar embryo or flings the embryo out of the system before it can gather enough matter to sustain hydrogen fusion. Alternatively, the brown dwarf's formation may be triggered by turbulence in a nebula--such as a shock wave passing through--where there's too little mass to condense into a full star without a kick of this kind.
But others speculate that brown dwarfs could form similarly to large gas-giant planets, coalescing within a disk around a star before being ejected from the system to float freely in interstellar space. "Maybe it all happens at once, even in the same starformation region," says Kirkpatrick. "How exactly you get to brown dwarfs may have many different routes."
"The WISE objects are fascinating, but they're old and don't give many clues about their formation mechanism," says Cathie Clarke (University of Cambridge, England). She adds that looking into starforming clouds will help us understand how such cold, dense, hybrid objects form.
Clues to brown dwarf formation are likely to come with the international Atacama Large Millimeter/submillimeter Array (ALMA), a network of dishes currently being assembled in Chile that's touted as the world's largest astronomical project. The telescope, projected to be completed by 2013, will have 66 antennas in all, giving it a resolution sharper than the Hubble Space Telescope.
"ALMA is designed to look in the environments in which brown dwarfs are forming," says John Bally (University of Colorado, Boulder). There is a lot of interest in using ALMA to focus in on the formation of brown dwarfs, and astronomers hope that the telescope will play a major role in sorting out questions about Y dwarfs, he says. "It's very well suited for the study of low-luminosity objects."
Although the older Y dwarfs will provide a more complete picture of a brown dwarf's life cycle, they may also be helpful in another area of study. Because their temperatures drop to almost as cold as Jupiter's 130 K, Y dwarfs may turn out to be proxies for exoplanet atmospheres. The coldest known brown dwarf is 300 K, the same temperature Jupiter would have if it was 1 a.u. from the Sun, but researchers think they'll find colder examples with atmospheric conditions similar to the solar system's gaseous planets.
Some young exoplanets have been directly imaged, such as the Beta Pictoris planet and four planets around HR 8799 (page 18), but the majority of exoplanets are detected indirectly when they pass in front of their stars or cause a slight wobble in the star's position as they orbit. And the reflected glow from gas giants is usually washed out by the bright light from their nearby stars, making it difficult to study the planets' atmospheres. Because Y dwarfs are often alone rather than near other stars, astronomers can observe them directly and without an interfering glare, making their atmospheres easier to study than those of similar-sized exoplanets.
"Brown dwarfs are the simplest case," says Kirkpatrick. "If we get the models right, we can better compare those to the exoplanets. We're just beginning to see hints of how diverse these cold atmospheres can get."
"These are basically stars that have ice in their atmospheres," adds Burgasser. The Y dwarfs likely host some combination of methane, water, and ammonia gases, as well as both solid and liquid forms of water and salts. Some models suggest that these molecules could form clouds or belts like those seen on Jupiter and Saturn, which could show up as brightness variations as the dwarf rotates. Burgasser is part of a collaboration to monitor several sources for clues about brown dwarf atmospheres and weather.
There may be millions of ultra-cold brown dwarfs with ice-friendly temperatures and possibly even water cycles, according to Burgasser. But water cycles on Y dwarfs wouldn't look like Earth's because of the lack of solid surfaces. Instead of forming bodies of liquid water, molecules in Y dwarfs might circulate in the atmosphere while changing from gas to liquid to ice as they rise, then back to gas as they sink, he says. He adds that this scenario is highly speculative, since "we basically have never had those kinds of atmospheres to study before."
"There could be some atmospheres where floating life could actually form," Kirkpatrick muses, echoing an idea popularized by writers such as Arthur C. Clarke and Carl Sagan. "It's not out of the realm of possibility."
In addition to their fascinating atmospheres, Y dwarfs may hold hints of their past surrounding conditions. "One of the consequences of not fusing hydrogen is that brown dwarfs retain a record of the chemical abundance of the environment in which they formed," says Burgasser. Brown dwarfs, unlike the Sun, are fully convective, meaning material all the way down to the core is dredged up to the cloud tops, revealing the entire object's composition. This information would not only provide a glimpse into dwarfs' long-gone nurseries but it would also be useful to astronomers curious about environments in the early universe when the first stars formed. "These brown dwarfs would be relics of this period," he says.
As astronomers uncover more of these dim star-planet hybrids around us, it may turn out that Y dwarfs are among our closest neighbors, useful atmospheric laboratories, and ancient record-keepers waiting to tell us more about the universe.
RELATED ARTICLE: All in the Family
Although brown dwarfs have a wide range of masses, they are always about the size as Jupiter. Shown here (left to right) are the Sun (facing page), a low-mass M dwarf star, an L dwarf, a T dwarf, a Y dwarf, and Jupiter, plotted to scale. The visible-light colors of the brown dwarfs are chosen for an age of 1 billion years, except for the Y dwarf: since Y dwarfs have only been detected in infrared, the purple is an artistic choice.
L dwarfs: The youngest brown dwarfs. Bright and warm, they glow at around 1700 K (2600[degrees]F). Compared with M dwarfs, L dwarfs have less titanium oxide and vanadium oxide in their atmospheres but more sodium, potassium, and water.
T dwarfs: Middle-aged and cooler, these failed stars come in around 1200 K (1700[degrees]F) and have methane in their atmospheres.
S&T: GREGG DINDERMAN
Y dwarfs The coolest of the cool. Water, ammonia, and methane characterize their atmospheres. Effective temperatures are less than 500 K (440[degrees]F).
Kristina Grifantini is a science journalist based in Cambridge, Massachusetts. She won the 2010 American Astronomical Society's Solar Physics Division Popular Writing Award for her March 2009 S&T cover story "Solar Impact."
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|Title Annotation:||Stellar Halflings|
|Publication:||Sky & Telescope|
|Date:||Jul 1, 2012|
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