AsianScientist (Feb. 25, 2015) – Astronomers have confirmed that supernova explosions are a major source of lithium (Li) in the galaxy. Their results, published in Nature, were based on observations of the Nova Delphini 2013 star that was first discovered by an amateur.
Immediately after the Big Bang, the universe consisted primarily of light elements such as hydrogen (H) and helium (He). Astronomers want to understand how the heavier elements—such as carbon, oxygen and iron—came to be.
Li is the third lightest element following H and He, and is familiar to us as the base material for the Li-ion batteries used in PCs, smart phones, eco-cars, etc. Big Bang nucleosynthesis only produced a very small amount of Li. Collisions between atomic nuclei in the interstellar medium are also assumed to produce Li by breaking the nuclei of heavy elements.
Because many sites and events can produce Li, Li is used as an indicator to probe the complete chemical evolution of the universe. Many scientists have studied this element by measuring the amount of Li found in various stars in our galaxy. This allowed them to estimate the amount produced through each process. Today, as a result of these indirect approaches, low-mass stars or nova explosions are thought to be the most important candidates for Li production in the current galaxy epoch. However, there have been no direct observations of the processes up till now.
On August 14th, 2013, the well-known Japanese amateur astronomer Mr. Koichi Itagaki found a bright new star in the constellation Delphinus. This star, which was named Nova Delphini 2013 (=V339 Del), was at magnitude 6.8 at discovery and peaked at 4.3 mag within two days. It was the first naked-eye nova since 2007, when V1280 Sco was found.
About 40 days later, in September 2013, a team of astronomers from National Astronomical Observatory of Japan (NAOJ), Osaka Kyoiku University, Nagoya University and Kyoto Sangyo University observed the nova to investigate the materials expelled by the explosion.
Using the Subaru Telescope, they identified absorption lines originating from many elements such as H, He, and Fe. Among them were sets of strong absorption lines in the ultraviolet (UV) range (wavelength ~313 nanometers) of the spectrum. Comparing these lines with other lines originating from hydrogen, calcium and other elements, the researchers identified an isotope of beryllium (Be), 7Be, the fourth-lightest element in the universe.
In a classical nova, the isotopes of He (3He) and plentiful 4He transferring from the companion star are fused together to form radioactive 7Be in the high-temperature environment on the surface of a white dwarf. This radioactive isotope decays to form an isotope of lithium (7Li) within a short time (half-life of 53.22 days).
This discovery of 7Be within 50 days after the nova explosion means that this explosion is actually producing a large amount of 7Li formed from 7Be. Because 7Be is found in the gas blobs blown away from the central region of the nova at high velocities (~1000 km/s), 7Li formed from this 7Be was not destroyed in the high-temperature environment close to the surface of the white dwarf.
The observations made using the Subaru Telescope provide the first strong evidence that novae produce significant amounts of Li in the universe. This discovery confirms the chemical evolution model from the Big Bang to the present universe, as predicted by scientists.
Furthermore, the observed amount of Li produced in this nova explosion is proven to be higher than predicted by theoretical estimates. Nova Delphini 2013 shows rather typical characteristics of classical novae. If other novae also produce a large amount of Li as Nova Delphini 2013 did, nova explosions must be recognized as very major Li factories in the universe. In near future, more observations of other nova explosions will provide much clearer model of Li evolution.
The article can be found at: Tajitsu et al. (2015) Explosive Lithium Production in the Classical Nova V339 Del (Nova Delphini 2013).
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Source: National Astronomical Observatory of Japan.
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