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Stars and solar physics

Stars and solar physics

Merging stars spewed radioactive molecules

06 Aug 2018
Aluminum monofluoride
Radioactive source: image of CK Vulpeculae taken by ALMA showing two orange nodes corresponding to aluminium monofluoride. (Courtesy: ALMA (ESO/NAOJ/NRAO), T. Kamiński & M. Hajduk; Gemini, NOAO/AURA/NSF; NRAO/AUI/NSF, B. Saxton)

Radioactive molecules have been observed directly in interstellar space for the first time. Astronomers led by Tomasz Kamiński at the Harvard-Smithsonian Center for Astrophysics concluded that aluminium monofluoride containing the unstable isotope aluminium-26 was ejected into space during the merger of binary star system CK Vulpeculae. Their work boosts our understanding of how radioactive aluminium has been dispersed across the Milky Way and could lead to the discovery of other sources of the isotope.

Between 1670 and 1672, astronomers in Europe thought they were witnessing the birth of a new star when a mysterious object called CK Vulpeculae appeared in the sky – emitting red light that was initially bright enough to be seen with the naked eye. We now know that CK Vulpeculae is in the Milky Way about 2300 light-years from Earth, but its precise nature had long puzzled astronomers.

In 2013, the mystery surrounding CK Vulpeculae was partially solved by researchers using the APEX radio telescope in Chile, which spotted a highly unusual molecular gas flowing out from its source. Further analysis suggested that the gas was, in fact, the partial remnant of a rare merger of a low-mass binary star system – which probably produced the light seen in the 1670s.

Gamma-ray sources

When aluminium-26 decays it emits a gamma ray and by detecting this radiation, astronomers have known for several decades that the Milky Way contains about two solar masses of the isotope. However, gamma-ray detectors have poor angular resolution and therefore the sources of aluminium-26 could not be located.

In this latest study, Kamiński’s team used the ALMA radio telescope in Chile, and the NOEMA radio telescope in France to observe the distinctive, millimetre-wavelength signature associated with aluminium monofluoride molecules that contain aluminium-26. This allowed them to conclude that CK Vulpeculae is indeed a source of aluminium-26.

The research provides new insights into how low-mass binary mergers take place – and evidence that heavy elements inside stars can be ejected into space during such mergers.

Other sources needed

The discovery could also give some idea of the origins of the Milky Way’s interstellar aluminium-26. However, the relatively small amount of the isotope emitted by CK Vulpeculae and the current belief that such mergers are exceptionally rare events suggests that there are probably other types of sources of aluminium-26.

The work of Kamiński and colleagues has also shown that modern millimetre-wave interferometers like ALMA could be much better at searching for aluminium-26 than gamma-ray observatories. This could help astronomers look for other sources of the radioactive isotope.

The study is described in Nature Astronomy.

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