Free Astronomy Magazine September-October 2018

19 SEPTEMBER-OCTOBER 2018 SPACE CHRONICLES powerful telescopes to see the remains of this merger, a dim central star surrounded by a halo of glowing mate- rial flowing away from it. 348 years after the initial event was ob- served, the remains of this explosive stellar merger have led to the clear and convinc- ing signature of a ra- dioactive version of aluminum, known as aluminium-26. This is the first unstable ra- dioactive molecule de- finitively detected outside of the Solar System. Unstable iso- topes have an excess of nuclear energy and eventually decay into a stable form. “This first observation of this isotope in a star-like object is also important in the broader context of galactic chemical evolution,” notes Kami ń ski. “This is the first time an active producer of the ra- dioactive nuclide alu- minum-26 has been directly identified.” Kami ń ski and his team detected the unique spec- tral signature of molecules made up of aluminum-26 and fluorine ( 26 AlF) in the debris surrounding CK Vulpeculae, which is about 2000 light-years from Earth. As these molecules spin and tumble through space, they emit a distinctive fin- gerprint of millimetre-wavelength light, a process known as rotational transition. Astronomers consider this the “gold standard” for detec- tions of molecules. The observation of this particular isotope provides where between 0.8 and 2.5 times that of our Sun. Being radioactive, alu- minium-26 will decay to become more sta- ble and in this process one of the protons in the nucleus decays into a neutron. During this process, the ex- cited nucleus emits a photon with very high energy, which we ob- serve as a gamma ray. Previously, detections of gamma ray emis- sion have shown that around two solar masses of aluminium- 26 are present across the Milky Way, but the process that created the radioactive atoms was unknown. Furthermore, owing to the way that gamma rays are detected, their precise origin was also largely unknown. With these new meas- urements, astrono- mers have definitively detected for the first time an unstable ra- dioisotope in a mole- cule outside of our Solar System. At the same time, however, the team have concluded that the production of aluminium- 26 by objects similar to CK Vulpecu- lae is unlikely to be the major source of aluminium-26 in the Milky Way. The mass of aluminium-26 in CK Vulpeculae is roughly a quarter of the mass of Pluto, and given that these events are so rare, it is highly unlikely that they are the sole pro- ducers of the isotope in the Milky Way galaxy. This leaves the door open for further studies into these radioactive molecules. C omposite image of CK Vulpeculae, the remains of a double-star collision. This impact launched radioactive molecules into space, as seen in the orange double-lobe structure at the centre. This is an ALMA image of 27-aluminum monofluoride, but the rare isotropic version of AIF resides in the same region. The red, diffuse image is an ALMA image of the broader dust in the region. The blue is opti- cal data from the Gemini observatory. [ALMA (ESO/NAOJ/NRAO), T. Kami ń ski; Gemini, NOAO/AURA/NSF; NRAO/AUI/NSF, B. Saxton] fresh insights into the merger pro- cess that created CK Vulpeculae. It also demonstrates that the deep, dense, inner layers of a star, where heavy elements and radioactive iso- topes 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,” remarked Kami ń ski. The astronomers also determined that the two stars that merged were of relatively low mass, one being a red giant star with a mass some- !