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A pair of colliding stars spills radioactive molecules into space



Artistic representation of the collision of two stars, as formed by CK Vul. The insert 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

When two sun-like stars collide, the result can be a spectacular explosion and the formation of a whole new star. Such an event was seen in 1670 from Earth. It appeared to the observers as a bright, red "new star". Although initially visible to the naked eye, this cosmic burst of light quickly faded and now requires powerful telescopes to see the remnants of this blending: a dark central star surrounded by a halo of luminous material flowing away from it.

Approximately 348 years after this event, an international team of astronomers using the Atacama Large Millimeter / Submillimeter Array (ALMA) and the NOEMA (Northern Extended Millimeter Array) radio telescope investigated the remnants of this explosive stellar fusion known as CK Vulpeculae (CK Vul ) – and discovered the clear and convincing signature of a radioactive version of aluminum ( 26 Al, an atom with 13 protons and 13 neutrons) bound to fluorine atoms to form 26-aluminum monofluoride ( 26 AlF).

This is the first molecule with an unstable radioisotope that has definitely been detected outside our solar system. Unstable isotopes have a surplus of nuclear energy and eventually disintegrate into a stable, less radioactive form. In this case, the 26-aluminum (26Al) decomposes into 26-magnesium ( 26 Mg).

"The first evidence of this kind of radioactive molecule is an important milestone in our exploration of the cool molecular universe," said Tomasz Kamiński, astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and lead author of an article in Nature Astronomy .

The researchers discovered the unique spectral signature of these molecules in the wreckage around CK Vul, which is about 2,000 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 "rotation transition." Astronomers consider this a "gold standard" for molecular detection.

These characteristic molecular fingerprints are usually taken from laboratory experiments and then used to identify molecules in space. 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 fingerprint data from stable and abundant 27 AlF molecules to obtain accurate data for the rare 26 AlF molecule. "This extrapolation method is based on the so-called Dunham approach," explains Alexander Breier from the Kasseler team. "It allows researchers to calculate the rotation transitions of 26 AlF to an accuracy far beyond the needs of astronomical observers."

Composite image of CK Vul, the remains of a binary 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 isotopic version of AlF is in the same region. The red, diffused image is an ALMA image of the broader dust in the region. The blue is optical hydrogen emission as seen by the Gemini Observatory. Credit: ALMA (ESO / NAOJ / NRAO), T. Kami? Ski & M. Hajduk; Twins, NOAO / AURA / NSF; NRAO / AUI / NSF, B.Saxton

The observation of this particular isotopologue provides new insights into the fusion process that produced CK Vul. 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 that was ripped apart three centuries ago by a collision," noted Kamiński. "How cool is that?"

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

"This first direct observation of this isotope in a stellar-like object is also important in the broader context of galactic chemical evolution," noted Kamiński. "This is the first time that an active producer of radioactive nuclide 26 Al has been directly observable identified."

It has been known for decades that there are about three complete solar values ​​ 26 Al spread across the Milky Way. But these observations, made at gamma wavelengths, could only determine that the signal was there; they could not locate individual sources, and it was unclear how the isotopes got there.

With current estimates of the mass of 26 Al in CK Vul (about a quarter of the mass of Pluto) and the Rare. English: bio-pro.de/en/region/stern/magazine/. In mergers like these, it seems unlikely that mergers alone are responsible for this galactic radioactive material, astronomers conclude.

However, ALMA and NOEMA can only compare the amount of [21] with […]. The actual mass of 26 in CK Vul (in atomic form) can be much larger. It is also possible that other merge residues may have much larger amounts. Astronomers may also have underestimated the current merger rates in the Milky Way. "So this is not a closed topic and the role of mergers can not be insignificant," speculated Kamiński.


Further research:
Researchers see that a ray of light from the first confirmed neutron star fusion appears behind the sun

Further information:
Tomasz Kamiński et al, Astronomical Detection of the Radioactive Molecule 26AlF in the Remnant of an Ancient Explosion, Nature Astronomy (2018). DOI: 10.1038 / s41550-018-0541-x

Sources in Journal:
Nature Astronomy

Provided by:
National Radio Astronomy Observatory


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