Astronomical detection of a radioactive molecule 26AlF in a remnant of an ancient explosion

Decades ago, γ-ray observatories identified diffuse Galactic emission at 1.809 MeV originating from β^+ decays of an isotope of aluminium, ^26Al, that has a mean-life time of 1.04 Myr. Objects responsible for the production of this radioactive isotope have never been directly identified, owing to insufficient angular resolutions and sensitivities of the γ-ray observatories. Here, we report observations of millimetre-wave rotational lines of the isotopologue of aluminium monofluoride that contains the radioactive isotope (^26AlF). The emission is observed toward CK Vul which is thought to be a remnant of a stellar merger. Our constraints on the production of ^26Al combined with the estimates on the merger rate make it unlikely that objects similar to CK Vul are major producers of Galactic ^26Al. However, the observation may be a stepping stone for unambiguous identification of other Galactic sources of ^26Al. Moreover, a high content of ^26Al in the remnant indicates that prior to the merger, the CK Vul system contained at least one solar-mass star that evolved to the red giant branch.