Study investigates very metal-poor star HE 2315−4240

Based on the data from the Magellan-Clay telescope in Chile, astronomers have performed a chemo-dynamical study of a very metal-poor star known as HE 2315−4240. Results of the study, published on the preprint server arXiv, yield important insights into the nature of this star.

Metal-poor stars are rare objects, as only a few thousand stars with iron abundances [Fe/H] below -2.0 have been discovered to date. Expanding the still-short list of metal-poor stars is of high importance for astronomers, as such objects have the potential to improve our knowledge of the chemical evolution of the universe.

With a metallicity of approximately -2.89 dex, HE 2315−4240 is a very metal-poor star at an estimated distance of some 9,300 light years from Earth. Given that the star is poorly studied and very little is known about its properties, a team of astronomers led by Xinuo Wang of Cornell University in Ithaca, New York, decided to investigate it with the Magellan Inamori Kyocera Echelle (MIKE) spectrograph mounted on the Magellan-Clay telescope.

“In this study, we present a spectrum of a very metal-poor star, HE 2315−4240, with [Fe/H] = −2.89 based on a Magellan/MIKE high-resolution visual light spectrum,” states the study.

All in all, Wang’s team managed to derive abundances of 19 elements. It turned out that the alpha and iron-peak elements agree well with the abundance trend of other known metal-poor stars. This, according to the astronomers, confirms that at least one supernova enhanced these elements in the gas cloud that formed HE 2315−4240.

Furthermore, the observations found that HE 2315−4240 has low strontium-to-barium and carbon-to-iron abundance ratios. These results indicate that the star is accreted and formed in a dwarf galaxy. The effective temperature of HE 2315−4240 was estimated to be 5,181 K, which makes it a warm giant.

According to the study, the metallicity of HE 2315−4240, together with its abundances of magnesium and silicon suggest that the star formed from the gas enriched by a Type II supernova explosion. The astronomers suppose that the progenitor was most likely a massive (about 10 solar masses) Population III star. The hypothetical Population III stars, composed almost entirely of primordial gas, are theorized to be the first stars to form after the Big Bang.

Based on the conducted kinematics analysis, the researchers assume that HE 2315−4240 formed outside the Galactic disk, likely in a small dwarf galaxy, and was later absorbed by the growing Milky Way.

“The progenitor system was likely accreted before other systems, placing the star in the inner halo as we know it today,” the paper concluded.

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