Another Origin for Cosmic Rays

Some superfast particles arriving at Earth may originate from shock waves in turbulent stellar clusters, a gamma-ray study published in the November 25th issue of Science suggests. The observations are the first firm direct evidence of a longstanding theory for the origin of these particles, called cosmic rays, but they don’t do anything for another, even longer-standing theory in favor of supernova remnants.

Cosmic rays were first discovered in 1912 by Victor Hess, who won a Nobel Prize for his detection of this strange source of radiation entering the atmosphere from space. Until the 1930s scientists thought cosmic rays were some sort of electromagnetic wave — hence their name. But the deceptively dubbed “rays” are actually speedy charged particles whizzing through the universe. They’re mostly protons from hydrogen atoms stripped of their electrons, but they can also be heavier atomic nuclei, electrons, and other subatomic particles.

Cyg X in gamma and infrared
Gamma rays detected by the Fermi LAT (top image) are emitted by freshly accelerated cosmic rays traveling through the stormy Cygnus X region (in infrared, bottom image). The cosmic ray "cocoon" fills the cavities carved out around and between two star clusters, Cyg OB2 and NGC 6910.
NASA / DOE / Fermi LAT / I. Grenier / L. Tibaldo
Yet even after 99 years, astronomers still don’t know for sure where cosmic rays receive their energy boost. The problem with figuring out where cosmic rays come from is that they appear to come from everywhere. Because they’re charged particles, cosmic rays react to whatever magnetic fields they encounter, and there are a lot of magnetic fields in galaxies, whether from stars or planets or even the galaxy itself. By the time the particles reach Earth, they’re hitting us from all sides.

Gamma rays don’t have this problem. The most energetic photons in the universe, gamma rays basically travel in straight lines from their sources to us. And because cosmic rays are stupendously energetic, they produce gamma rays when they run into stuff.

Astronomers have used gamma rays to probe likely sites of cosmic ray acceleration. For several decades researchers have suspected that our galaxy’s rays come from supernova remnants, and X-ray and gamma-ray observations do indicate that electrons are being accelerated to high energies at remnants’ shock fronts as they slam into surrounding gas and dust, sending the electrons surfing in and out of the blast wave. But there’s no conclusive evidence of proton and nuclei acceleration, and these heavier particles make up 99% of cosmic rays, says Isabelle Grenier (Paris Diderot University and CEA Saclay), a coauthor on the new study. “We have no smoking gun,” she says. “We have very strong hints, but no proof.”

To hunt for cosmic rays’ origin, Grenier and her colleagues turned the Fermi Gamma-ray Space Telescope’s Large Area Telescope to point at the star-forming region Cygnus X, a tumultuous section of space about 4,500 light-years away. The team detected a diffuse gamma-ray glow from inside a superbubble, a giant cavity in the gas and dust that's been blown out by thousand-mile-per-second stellar winds from the young, massive stars of two of the region’s star clusters, Cyg OB2 and NGC 6910. What’s more, the radiation looks like it’s coming from protons, not electrons.

The average energies Grenier’s team observed are much higher than the energies of cosmic rays near Earth. Add that higher energy to the emission’s localization inside the superbubble (meaning, the particles haven’t had a chance to move very far from their energizing source), and the fact that the gamma rays come from protons, and it looks like the team’s caught, as they put it, “freshly accelerated cosmic rays” that haven’t slowed down to near-Earth energy levels yet.

To find the source, the team focused at first on a strong gamma-ray-emitting supernova remnant that appears in the same part of the sky. The remnant’s distance isn’t pinned down, so it’s not clear if it’s actually associated with Cygnus X. But that it might be there, in the same place as cosmic rays, sparked the researchers’ interest. “We were so excited,” says Grenier. “And I must say that, several months after, I’m not convinced that it’s the best scenario anymore.” The diffuse gamma-ray emission showed no sign of any connection with the remnant.

But the astronomers discovered something else intriguing: the diffuse gamma-rays are completely confined to the superbubble created by the stars’ strong winds, even edged by an infrared-emitting shell of dust grains heated by the intense starlight.

That made the researchers turn to a second theory for cosmic ray production, one involving exactly this kind of environment. Astronomers have suspected since the 1980s or so that cosmic rays may also come from clusters of massive, young stars called OB associations, where the O and B stand for the two hottest, most massive types of the family of stars that fuse hydrogen in their cores. The suspicion stems from the cosmic rays’ composition. Many of the common heavier elements, such as carbon and silicon, are about as abundant among the particles as they are in the solar system, but there are some elements that are overrepresented. Particularly, a heavy isotope of neon, neon-22, is about five times as abundant in cosmic rays as it is in the solar system. But Ne-22 is seen in the outer layers thrown off by really massive, young, windy stars called Wolf-Rayet stars. Overall, the cosmic rays’ chemical makeup suggests that about 20% are created by WR stars, while the rest are other particles found in the interstellar medium, the stuff between the stars.

A sizable fraction of cosmic rays may be born in WR stars’ massive outflows, but that’s not necessarily where they gain their energy. In 1999 Richard Mewaldt (Caltech) and his colleagues reported the presence in cosmic rays of the cobalt isotope cobalt-59. Co-59 is a daughter isotope, an atom formed by the radioactive decay of nickel-59 when that atom captures an electron and shoves it together with one of its proton to make a neutron. Such a snatch can’t happen when the nickel atom’s nucleus is accelerated to high energies and stripped of its electrons, as cosmic ray particles are. That means that the nuclei that make up cosmic rays aren’t born with their high energies: they hang around a while — about 100,000 years, the team concluded — before being sped up and out into interstellar space.

“This rules out a supernova accelerating its own ejecta,” Mewaldt says, although some of the heavier cosmic ray nuclei probably first formed in supernova explosions. “But [it] is consistent with accelerating cosmic rays from a region where massive stars are born, a region that will be enriched in WR material because of the high-velocity winds of these stars.”

Grenier’s team didn’t measure specific chemical composition, so they don’t know what the cosmic rays are made of. Whatever the ingredients — and they’re probably a combination of interstellar medium, old supernova ejecta, and outflows from an earlier batch of Wolf-Rayet stars — it looks like they’re now being accelerated by the current stellar clusters’ winds.

“This is a very important paper,” says Mewaldt of Grenier’s study, “because it provides the first direct evidence for the distributed acceleration of cosmic rays in OB associations.”

The cosmic rays are still confined in a “cocoon” because they can’t spread out fast in the torrid environment inside the superbubble, Grenier says. The massive stars are only a few million years old, and their powerful winds and ultraviolet radiation create a maelstrom inside the cavity, twisting magnetic fields into tangles that trap the cosmic rays. Over time the particles will escape into quieter regions, but what happens to their energies while inside the cocoon remains a mystery.

It’s a mystery that’s particularly intriguing to Grenier. Low-energy cosmic rays (at least, lower energy than the ones the team observed) “are very, very important for the structure of the clouds of the gas from which we form stars,” she explains. Dense clumps of clouds eventually collapse under their own gravity to make stars. While the clouds are pretty opaque to light, cosmic rays can sneak inside, bringing with them heat and catalyzing the formation of molecules. How that heat and chemistry influence star formation isn’t known, and Grenier is pursing the question with her colleagues. What is clear is that “if you radiate those clouds with more cosmic rays or [fewer] cosmic rays, you change the game.”

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