Collider in the sky

Star system accelerates protons to energies rivaling the LHC

HEIDELBERG, Germany — Physicists toiling at the Large Hadron Collider now have a heavenly rival. Protons caught between two fierce stellar furnaces are being boosted to energies comparable to those that can be attained at the world’s most powerful atom smasher, a new study suggests.

Composite image shows portraits of the massive star system Eta Carinae as seen in low-energy gamma rays (left), visible light (middle) and high-energy gamma rays (right). The high-energy gamma rays are believed to be generated when protons revved up by the star system’s winds collide with charged atoms in the wind, in something like a scaled-up version of the Large Hadron Collider. R. Walter/U. of Geneva, NASA, Fermi, INTEGRAL

The two orbiting stars — among the most massive known in the galaxy — together form the Eta Carinae star system, some 7,500 light-years from Earth. Massive stars expel dense winds of material; Eta Carinae casts out the mass equivalent of about an Earth every week.

NASA’s Fermi Gamma-ray Space Telescope recently recorded gamma rays from Eta Carinae ranging from 20 billion to 50 billion electron volts, Roland Walter of the University of Geneva in Switzerland reported December 9 at the biannual Texas Symposium on Relativistic Astrophysics, held in Heidelberg, Germany.

The most plausible explanation for the energetic emissions is that protons, which are trapped by the system’s intense magnetic fields, bounce back and forth between the two stellar winds, gaining energy during each bounce. The protons attain energies of at least 10 trillion electron volts, higher than the maximum energy of the 7 trillion electron volt proton beams in the Large Hadron Collider, Walter and his colleagues estimate. “Eta Carinae is a scaled-up version of the Large Hadron Collider,” Walter said.

In addition to the 10 TeV protons, Eta Carinae’s strong winds carry flows of charged atoms. When the energized protons collide with these ions or each other, the protons produce particles called pions that decay into the energetic gamma rays observed by Fermi.

Felix Aharonian of the Dublin Institute for Advanced Studies and the Max Planck Institute for Nuclear Physics in Heidelberg says that it’s plausible that highly energetic protons are producing the energetic gamma rays observed from Eta Carinae. Although he stops short of calling it proof, Aharonian calls the recent recordings “the best observational evidence we have that stellar winds are accelerating protons to high energies.”

The study indicates that collisions between stellar winds are a probable source of the streams of high-speed protons and other energetic charged particles called cosmic rays in the Milky Way. That had long been predicted, but the Eta Carinae emissions provide some of the first tentative proof of that model, Walter said. Other massive star systems, also expected to have strong stellar winds, may also be prime candidates for generating cosmic rays, he added.

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