8.11.2024

Controversial observation of bizarre, paired brown dwarfs could upend star-formation models

In this JWST image the Trapezium star cluster, in the core of the Orion nebula, lights up gas and dust.ASA; ESA; CSA; M. MCCAUGHREAN; S. PEARSON; CC BY-SA 3.0 IGO

How small can a star be? Training its powerful eye on nearby star-forming regions in the Milky Way, NASA’s JWST space observatory may be seeing that lower limit for brown dwarfs, which form like stars but aren’t massive enough for hydrogen fusion to ignite in their cores. Three teams have identified a dozen or so brown dwarfs between three and eight Jupiter masses—and no smaller.

“For the first time, you can actually look for objects down to at least a couple of Jupiter masses,” says Ray Jayawardhana, an astrophysicist at Johns Hopkins University and member of one team. What they’re seeing matches the lower mass limit predicted by theories of star formation, adding to astronomers’ confidence that they might finally be seeing the smallest brown dwarfs that nature can build.

However, one controversial unpublished study from a fourth team claims the detection of paired brown dwarfs that are nearly as small as Jupiter, an unexpected finding that would upend star formation models. “If one of those turns out to be a true Jupiter-mass binary, it would be really great,” says Alexander Scholz of the University of St. Andrews. But Koraljka Mužic of the Institute of Astrophysics and Space Sciences in Lisbon, Portugal, thinks that’s unlikely. “I’m very skeptical about that finding,” she says.

Many astronomers define brown dwarfs based on what they burn. Objects above about 70 Jupiter masses have gravity strong enough to fuse hydrogen atoms and are classified as traditional stars. Brown dwarfs above 13 Jupiter masses can only fuse a hydrogen isotope called deuterium, providing a weak nuclear burn for a few million years.

But distinguishing between smaller brown dwarfs and planets requires looking at how they form. Even though they can end up about as massive as 10 Jupiters, planets always arise around a star, from its surrounding disk of gas and dust.

In contrast, stars, including brown dwarfs, form on their own within giant collapsing clouds of gas. Under the influence of gravity, a cloud contracts and fragments into globs of different sizes that undergo further collapse until they become stars. But the gas heats up as it gets denser, creating pressure that can keep it from further contraction and fragmentation. Theories have often pegged the smallest possible brown dwarf at about seven Jupiter masses—or 0.007 times the Sun’s mass. “For obvious reasons, that’s historically been called the James Bond mass,” says Mark McCaughrean of the Max Planck Institute for Astronomy.

Before JWST’s launch in 2021, telescopes had seen brown dwarfs as small as five Jupiter masses, but those observations were at the very limits of the instruments’ abilities. It was possible they were missing smaller objects hiding in the dusty stellar nurseries. JWST, with its keen vision and infrared detectors, could in principle see the faint glow of much smaller objects, so the three groups used it to scan nearby star-forming regions and infer the masses of objects they found based on their color.

In a study published late last year, the first team reported finding a handful of brown dwarfs ranging from three to eight Jupiter masses in the cluster IC 348, located about 1000 light-years away. In September, the group that included Jayawardhana said it had discovered six low-mass objects, the lightest five times as heavy as Jupiter, in the star-forming region NGC 1333. A third paper posted to the preprint server arXiv that same month reported several brown dwarfs as small as three Jupiter masses in the Flame nebula, roughly 1350 light-years away. In all these cases, JWST should theoretically have been able to spot smaller objects yet didn’t.

Not everyone believes the case is closed. “At the moment, I think the jury is still out,” says Elena Sabbi of the International Gemini Observatory. For one thing, the nearby stellar nurseries these studies examined might not be typical for the Milky Way. But the biggest challenge comes from McCaughrean and his colleague Samuel Pearson of the European Space Agency, who, in an October 2023 preprint posted to arXiv, claim to have spotted a bevy of bewildering brown dwarfs that shouldn’t exist.

In the Trapezium cluster, in the heart of the Orion nebula some 1400 light-years away, McCaughrean and Pearson noticed 42 brown dwarfs orbiting in pairs, a few of them nearly as light as Jupiter. They named these head scratchers Jupiter-Mass Binary Objects, or JuMBOs. In one spot, they found five such curiosities in a row, leading McCaughrean to dub the area “JuMBO alley.” “I think it’s fair to say we didn’t expect that,” Scholz says.

The JuMBOs seem to contradict both star-formation theory and the conventional understanding that pairs of stars get ever rarer at lower masses. One possibility is that they are not brown dwarfs at all, but planets that formed around other stars and were later ejected by gravitational interactions with different objects—but the prospect of duos being chaotically flung from a system and remaining bound together seems vanishingly slim. Another possibility is that the JuMBOs were once larger but have been whittled down in size by blasts of radiation from giant stars that are plentiful in Trapezium.

McCaughrean and Pearson say the objects likely contain methane and water, which are common in the atmospheres of cool, newborn brown dwarfs. They inferred the presence of these gases not from detailed spectra, but from the objects’ colors—which raises a third, mundane possibility about what they are, says Kevin Luhman, an astronomer at Pennsylvania State University. He reanalyzed the data and found that most of the objects lack the distinctive color patterns that would be expected for relatively cool brown dwarfs with large amounts of water and methane. Instead, he thinks the colors match those of stars and galaxies reddened by the dust of Orion. The JuMBOs are likely background objects peeking through the cluster rather than shining inside it, he says. McCaughrean counters that it’s statically implausible for random background alignments to mimic brown dwarf binaries, especially a row of them like in JuMBO alley.

McCaughrean and Pearson have since taken spectroscopic images of the cluster and hope to publish new results in the coming months, which McCaughrean believes will corroborate at least some of their initial objects.

He is also looking to push the telescope’s sensitivity further and has applied for time to hunt for Saturn-size brown dwarfs. “Never say never, right?” he says. “I’m an observer. Let’s go and see what’s there.”

Quelle: AAAS