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An international team of astronomers, based in the US, Poland, Germany, Australia and France has made the first clear detection of a radioactive molecule in interstellar space: the isotopologue of aluminum monofluoride, ²⁶AlF. Recent data – obtained with NOEMA, ALMA, IRAM 30m and APEX – suggest that this radioactive molecule was ejected into space by the collision of two stars, an extremely rare cosmic event that was witnessed on Earth as a nova, in the year 1670.

When two Sun-like stars collide, the result can be a spectacular explosion and the formation of an entirely new star. It appeared to observers as a bright, red “new star”, known as CK Vulpeculae (CK Vul). Though initially visible with the naked eye, this burst of cosmic light quickly faded and now requires powerful telescopes to see the remains of this merger: a dim central star surrounded by a halo of glowing material flowing away from it.

²⁶AlF is the first molecule bearing an unstable radioisotope definitively detected outside of the solar system. Unstable isotopes have an excess of nuclear energy and eventually decay into a stable, less-radioactive form. In this case, the 26-aluminum (²⁶Al) decays to 26-magnesium (²⁶Mg).

“The first solid detection of this kind of radioactive molecule is an important milestone in our exploration of the cool molecular universe,” says Tomasz Kamiński, an astronomer with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and lead author on a paper appearing in Nature Astronomy.

The researchers detected the unique spectral signature of this molecule in the debris surrounding CK Vul, which is approximately 2,000 light-years from Earth. As this molecule spins and tumbles through space, it emits a distinctive fingerprint of millimeter-wavelength light.

Artist impression of the collision of two stars, like the ones that formed CK Vul. The inset illustrates the inner structure of one red giant before the merger. A thin layer of 26-aluminum (brown) surrounds a helium core. An extended convective envelope (not to scale), which forms the outermost layer of the star, can mix material from inside the star to the surface, but it never reaches deep enough to dredge 26-aluminum up to the surface. Only a collision with another star can disperse 26-aluminum. Credit: NRAO/AUI/NSF; S. Dagnello

The observation of this particular isotopologue provides fresh insights into the merger process that created CK Vul. It also demonstrates that the deep, dense inner layers of a star, where heavy elements and radioactive isotopes are forged, can be churned up and cast into space by stellar collisions. “We are observing the guts of a star torn apart three centuries ago by a collision,” observed Kamiński.
The astronomers also determined that the two stars that merged were relatively low-mass, with one being a red giant star with a mass somewhere between 0.8 and 2.5 times that of our Sun.

“This first direct observation of this isotope in a stellar-like object is also important in the broader context of the galactic chemical evolution,” notes Kamiński. “This is the first time an active producer of the radioactive nuclide ²⁶Al has been directly observationally identified.”

It has been known for decades that there is about three entire Suns’ worth of ²⁶Al spread across the Milky Way. But these observations, made at gamma-ray wavelengths, could only identify that the signal was there; they couldn’t pinpoint individual sources and it was unclear how the isotopes got there.

"With current estimates on the mass of ²⁶Al in CK Vul - about a quarter the mass of Pluto - and the rare occurrence of mergers such as this, it seems rather unlikely that mergers are solely responsible for this galactic radioactive material", Jan Martin Winters, co-author of the Nature Astronomy paper concludes.

However, NOEMA and ALMA can only detect the amount of ²⁶Al bound with fluorine. The actual mass of ²⁶Al in CK Vul (in atomic form) may be much greater. It is also possible that other merger remnants may have far greater amounts. Astronomers may also have underestimated the current merger rates in the Milky Way. “So this is not a closed issue and the role of mergers may be non-negligible,” speculates Kamiński.

More information:

This work is presented in “Astronomical detection of a radioactive molecule ²⁶AlF in a remnant of an ancient explosion,” by T. Kamiński et al., published in Nature Astronomy, a pdf version of the article can be found here.

The IRAM researchers involved are: Jan Martin Winters and Ka Tat Wong

Press contact:

Karin Zacher, IRAM press officer, Tel. 0033-476822103