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Stellar corpse reveals the origin of radioactive molecules



Observations with ALMA find radioactive isotope aluminum-26 from the rest of CK vulpeculae.

Astronomers using ALMA and NOEMA have made the first definitive proof of a radioactive molecule in interstellar space. The radioactive part of the molecule is an aluminum isotope. The observations show that the isotope was scattered into space after the collision of two stars leaving behind what is known as the CK vulpeculae. This is the first time that this element has been directly observed from a known source. Earlier identifications of this isotope are due to the detection of gamma rays, but their exact origin was unknown.

  Composite image of CK Vulpeculae, the remnants of a double star collision. This impact brought radioactive molecules into space, as seen in the orange double lobe structure in the middle. This is an ALMA image of 27-aluminum monofluoride, but the rare isotropic version of AIF is in the same region. The red, diffused image is an ALMA image of the wider dust in the region. The blue is optical data from Gemini Observatory. Credit: ALMA (ESO / NAOJ / NRAO), T. Kamiński; Twins, NOAO / AURA / NSF; NRAO / AUI / NSF, B. Saxton

Composite image of CK Vulpeculae, the remains of a double star collision. This impact brought radioactive molecules into space, as seen in the orange double lobe structure in the middle. This is an ALMA image of 27-aluminum monofluoride, but the rare isotropic version of AIF is in the same region. The red, diffused image is an ALMA image of the wider dust in the region. The blue is optical data from Gemini Observatory. Credit: ALMA (ESO / NAOJ / NRAO), T. Kamiński; Twins, NOAO / AURA / NSF; NRAO / AUI / NSF, B. Saxton

The team headed by Tomasz Kamiński (Harvard-Smithsonian Center for Astrophysics, Cambridge, USA) used the Atacama Large Millimeter / Submillimeter Array (ALMA) and the NO Extended Millimeter Array ( NOEMA) for the detection of a source of the radioactive isotope aluminum-26. The source, known as CK Vulpeculae, was first seen in 1670 and at the time it appeared to observers as a bright, red "new star". Although initially visible to the naked eye, it quickly faded and now requires powerful telescopes to see the remnants of this blending, a dark central star surrounded by a halo of glowing material flowing away from it. The remains of this explosive stellar fusion have become the clear and convincing signature of a radioactive version of Aluminum, which is referred to as aluminum – 26. This is the first unstable radioactive molecule that has definitely been detected outside the solar system. Unstable isotopes have an excess of nuclear energy and eventually disintegrate 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 Kamiński. "This is the first time that an active producer of the aluminum-26 radioactive nuclide has been directly identified."

Kamiński and his team found the unique spectral signature of molecules that consist of aluminum-26 and fluorine (26AlF) surrounding CK Vulpeculae, which is about 2000 light-years from Earth. As these molecules spin and rotate through space, they emit a characteristic fingerprint of millimeter-wavelength light, a process known as rotational transition. Astronomers consider this a "gold standard" for the recognition of molecules [1].

  Artistic representation of the collision of two stars, as they formed CK Vulpeculae. The inset shows the internal structure of a red giant before the merger. A thin layer of 26-aluminum (brown) surrounds a helium core. An extended convective envelope (not to scale) that forms the outermost layer of the star can mix material from the interior of the star to the surface, but it never reaches deep enough to dredge 26-aluminum to the surface. Only a collision with another star can disperse 26-aluminum. Credit: NRAO / AUI / NSF; S. Dagnello

Artistic representation of the collision of two stars, as formed by CK Vulpeculae. The inset shows the internal structure of a red giant before the merger. A thin layer of 26-aluminum (brown) surrounds a helium core. An extended convective envelope (not to scale) that forms the outermost layer of the star can mix material from the interior of the star to the surface, but it never reaches deep enough to dredge 26-aluminum to the surface. Only a collision with another star can disperse 26-aluminum. Credit: NRAO / AUI / NSF; S. Dagnello

The observation of this particular isotope provides new insights into the fusion process that produced CK vulpeculae. It also shows that the deep, dense inner layers of a star, in which heavy elements and radioactive isotopes are forged, can be whirled up by star collisions and thrown into space.

"We are watching the entrails of a star torn apart three hundred years ago by a collision," noted Kamiński.

The astronomers also found that the two stars growing together had a relatively low mass. One of them was a red giant star with a mass between 0.8 and 2.5 times our Sun.

Because aluminum is radioactive, it decays to become more stable, and in this process one of the protons in the nucleus breaks down into a neutron. During this process, the excited nucleus emits a very high energy photon, which we observe as a gamma ray [2].

So far, measurements of gamma radiation have shown that around two solar masses of aluminum-26 are present in the Milky Way, but the process that generated the radioactive atoms was unknown. Because of the way gamma rays are detected, their exact origins were also largely unknown. With these new measurements, astronomers have for the first time detected an unstable radioisotope in a molecule outside our solar system.

At the same time, however, the team has come to the conclusion that the production of aluminum 26 by similar objects such as CK-Vulpeculae are unlikely to be the main source of aluminum 26 in the Milky Way. The mass of aluminum-26 in CK Vulpeculae is about a quarter of the mass of Pluto, and given that these events are so rare, they are very unlikely to be the only producers of the isotope in the Milky Way. This opens the door for further studies on these radioactive molecules.

Notes

[1] The characteristic molecular fingerprints are usually taken from laboratory tests. In the case of 26AlF, this method is not applicable because 26-aluminum is not present on Earth. Laboratory astrophysicists at the University of Kassel therefore used the fingerprint data from stable and abundant 27AlF molecules to obtain accurate data for the rare 26AlF molecule.

[2] Aluminum-26 contains 13 protons and 13 neutrons in its nucleus (one less than the stable isotope, 27 aluminum). When it decays, aluminum 26 becomes 26-magnesium, a completely different element.


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