Science and Exploration

Star with Strange Chemistry Is from Out of Town

By Keith Cowing
Press Release
May 1, 2019
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Star with Strange Chemistry Is from Out of Town
(Top Left) The star J1124+4535 that shows chemical abundance ratios similar to some dwarf galaxy stars (image from SDSS). The color and luminosity of this star are typical as a usual red giant. (Right) Chemical abundance ratios of J1124+4535 measured by Subaru/HDS: (a. Mg/Fe, b. Eu/Fe, c: Eu/Mg). Asterisks indicate stars in the Ursa Minor dwarf galaxy, whereas others mean stars in the Milky Way. The values indicate ratios of number density of atoms of elements normalized to solar-system values. (Credit: NAOJ)

Astronomers have discovered a star in the Milky Way Galaxy with a chemical composition unlike any other star in our galaxy.
This chemical composition has been seen in a small number of stars in dwarf galaxies orbiting the Milky Way. This suggests that the star was part of a dwarf galaxy that merged into the Milky Way.

In the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) survey data, researchers noticed the star J1124+4535 for its unusual chemical composition. Initial observations showed that J1124+4535, located in the constellation Ursa Major (the Great Bear), had low abundances of certain elements, such as magnesium. Follow-up observations with the High Dispersion Spectrograph on the Subaru Telescope confirmed the low levels of magnesium but found comparatively high levels of europium. This is the first time an element ratio like this has been observed in a star in the Milky Way.

Stars form from clouds of interstellar gas. The element ratios of the parent cloud impart an observable chemical signature on stars formed in that cloud. So stars formed close together have similar element ratios. The composition of J1124+4535 doesn’t match any other stars in the Milky Way, indicating that it must have formed elsewhere.

Chemical signatures similar to J1124+4535 have been observed in some stars in dwarf galaxies orbiting the Milky Way. Galaxy evolution models and simulations suggest that galaxies like the Milky Way grow by absorbing neighboring dwarf galaxies. Thus it makes sense that J1124+4535 was born in a now vanished dwarf galaxy which merged into the Milky Way.

Reference: “Evidence for the Accretion Origin of Halo Stars with an Extreme r-Process Enhancement,” Qian-Fan Xing et al., 2019 April 29, Nature Astronomy [].

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