* Astronomy

First Detection of Thorium in an Extragalactic Star
Astronomers from the National Astronomical Observatory of Japan and Osaka Kyoiku University in Japan, have detected the element thorium in a red giant star called COS82, which lies in the Ursa Minor dwarf galaxy. Their discovery marks the first abundancedetermination of the actinides element thorium in a star beyond the Milky Way, and follows the detection of thorium in more than ten stars within the Milky Way. The derived thorium abundance indicates that the explosive synthesis of heavy elements occurs in similar-type environments in both the Milky Way and the Ursa Minor dwarf galaxy, which is a satellite of the Milky Way.
Thorium (Th) (along with uranium (U)) belongs to the actinide group of chemical elements. These are the heaviest elements in nature. Actinides are synthesized very quickly in such environments as supernova explosions. Although the mechanisms at work in the synthesis of such elements are still unclear, theoretical models predict that actinide production in an explosive environment is dependent on several factors, including the time-scale of the explosion.
The astronomers at National Astronomical Observatory and Osaka Kyoiku University used the Subaru Telescope High Dispersion Spectrograph (HDS) to determine the abundance of thorium in the Ursa Major dwarf target star. This star is known to have high abundance ratios of elements heavier than iron, and is thought to be severely contaminated by some explosive synthesis of heavy elements. The team's measurement of thorium revealed the overall abundance pattern of heavy elements for this object. Surprisingly for astronomers and nuclear physicists, the abundance ratios of thorium to other heavy elements are very similar to ratios found for Milky Way stars. This implies that the explosive synthesis of heavy elements occurs in very similar types of environments. This observational result provides a strong constraint on the theoretical models of nuclear reaction processes that produce heavy elements in the universe.
This result is published in the issue of June 25, 2007 of Publication of Astronomical Society of Japan.


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An optical image of the Ursa Minor dwarf galaxy from the Digitised Sky Surveys (DSS)* (14 arcmin by 14 arcmin) and the target star COS82. The morphology of the galaxy is not clear due to the low stellar density and the relatively short distance to the galaxy (220,000 light years).
Credit NAOJ