First Determination of the Actinide Thorium Abundance for a Red Giant of the Ursa Minor Dwarf Galaxy

• Published in: Publications of the Astronomical Society of Japan Vol. 59; no. 3; pp. L15 - L19
• Main Authors: Aoki, Wako, Honda, Satoshi, Sadakane, Kozo, Arimoto, Nobuo
• Format: Journal Article
• Language: English
• Published: Oxford, UK Oxford Universtiy Press 25.06.2007
• Subjects: galaxies: dwarf; nuclear reactions, nucleosynthesis, abundances; stars: abundances; stars: individual (Ursa Minor COS 82)
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/59.3.L15

The thorium abundance of the red giant COS 82 in the Ursa Minor dwarf spheroidal galaxy was determined based on the high-resolution spectrum. This is the first detection of actinides in an extragalactic object. A detailed abundance pattern was determined for 12 other neutron-capture elements from atomic number 39 to 68. These elements are significantly over-abundant with respect to other metals, like Fe ( $\gt$ 1 dex), and their abundance pattern agrees well with those of the r-process-enhanced, very metal-poor stars known in the galactic halo, while the metallicity of this object ([Fe $/$ H] $\sim -$ 1.5) is much higher than these field stars ([Fe $/$ H] $\sim-3.0$ ). The results indicate that the mechanism and the astrophysical site that are responsible for neutron-capture elements in COS 82 are similar to that for field r-process-enhanced stars, while the condition of low-mass star formation is quite different. An estimate of the age of this object based on the Th abundance ratio is discussed.