An article published in the journal “Nature Astronomy” describes the discovery of aluminum monofluoride (AlF) molecules containing aluminum-26, a radioactive isotope of this element, in interstellar space. A team of astronomers used the ALMA and NOEMA radio telescopes to trace its origins to the variable star CK Vulpeculae (CK Vul), the remnant of the merger between two stars observed from the Earth between 1670 and 1672 and called Nova Vulpecola 1670. It’s the first observation of aluminum-26 that leads to the identification of its origin, which occurred in a very rare event.
In 1670 a new star, defined for that reason “nova”, became visible in the sky with the naked eye and during the following two years it passed through a series of changes in brightness. When it started fading for good, it became necessary to use a telescope to see it, a type of instrument then state-of-the-art while the ones available today allow us to still study the remains, called CK Vul. Over time various hypotheses tried to explain that nova and perhaps the correct one was reported in an article published in March 2015 in the journal “Nature”, which attributed that phenomenon to the merger between two stars.
Today, astronomical studies are conducted not only at visible light frequencies but also at many other frequencies of the electromagnetic spectrum, depending on the need. In the case of CK Vul, two radio telescopes were used: ALMA (Atacama Large Millimeter/submillimeter Array), inaugurated in March 2013, and NOEMA (NOrthern Extended Millimeter Array). They allowed a team led by Tomasz Kaminski of the Harvard-Smithsonian Center for Astrophysics to detect the chemical “signature” of aluminum-26, a radioactive and unstable isotope that will decay in magnesium-26.
On the Earth there’s aluminum-27, a stable isotope of this element. Instead, aluminum-26 doesn’t exist on Earth so it was necessary to use the aluminum-27 isotope’s signature to derive accurate data for aluminum-26. This radioactive isotope was already detected in space in the past thanks to its gamma ray emissions but it’s the first time that it was directly identified so it was possible to trace its origin to CK Vul. Aluminum-26 was found to be bound to fluorine to form aluminum monofluoride.
The top image (ALMA (ESO/NAOJ/NRAO), T. Kaminski, Gemini, NOAO/AURA/NSF, NRAO/AUI/NSF, B. Saxton) shows CK Vul with radioactive molecules of aluminum monofluoride emitted in space visible in the orange double-lobe structure at the center. The image in red is ALMA’s view, the one in blue is the Gemini Observatory’s view.
The bottom image (NRAO/AUI/NSF; S. Dagnello) shows an artistic impression of the collision between two stars like the one that formed CK Vul. The inset illustrates the internal structure of a red giant star before the merger. A thin layer of aluminum-26, seen in brown, surrounds a helium core. A convective envelope (not in scale) forms the star’s outermost layer and can mix materials from the inside to the surface but never reaches aluminum-26. To disperse that radioactive element, a merger with another star was needed.
The amount of aluminum-26 estimated in CK Vul suggests that the amount ejected as a result of mergers between stars is not enough to explain the estimate of the total amount existing in the Milky Way. However, ALMA and NOEMA could only detect aluminum-26 bound to fluorine and there could be more in its atomic form ejection of a greater amount of this isotope.
In essence, this research represents a step forward in the explanation of the presence of aluminum-26 but the problem of its amount is far from solved. It’s also possible that the estimate of the number of star mergers is wrong. Last generation instruments are offering many new data on many cosmic phenomena that bring many answers but also new questions, in this case about radioactive elements in space.