Accurately measure the temperature of red supergiant stars

Red supergiants are a class of stars that end their lives in supernova explosions.

Their life cycles are not fully understood, in part because of the difficulties in measuring their temperatures. For the first time, astronomers are developing a precise method to determine the surface temperatures of red supergiant.

Stars are available in a wide range of sizes, masses and compositions. Our sun is considered a relatively small specimen, especially compared to something like Betelgeuse which is known as a red supergiant. Red supergiants are stars more than nine times the mass of our sun, and all of that mass means that when they die they do so with extreme ferocity in a huge explosion known as a supernova, especially what l ‘a type II supernova is called.

Type II supernovae seed the cosmos with elements essential to life; therefore, researchers want to know more about them. At this time, there is no way to accurately predict supernova explosions. Part of this puzzle is understanding the nature of the red supergiant that precede supernovae.

Although red supergiants are extremely bright and visible from great distances, it is difficult to determine their important properties, including their temperatures. This is due to the complex structures of their upper atmospheres which lead to inconsistencies in temperature measurements that might work with other types of stars.

“In order to measure the temperature of red supergiants, we had to find a visible, or spectral, property that was not affected by their complex upper atmospheres,” said graduate student Daisuke Taniguchi of the University’s Department of Astronomy. from Tokyo. “The chemical signatures known as absorption lines were the ideal candidates, but no line revealed temperature on its own. However, looking at the ratio of two different but related lines – those of iron – we found that the ratio itself was related to temperature. And it did so in a consistent and predictable way. “

Taniguchi and his team observed candidate stars with an instrument called WINERED that attaches to telescopes in order to measure the spectral properties of distant objects. They measured the iron absorption lines and calculated the ratios to estimate the respective temperatures of the stars. By combining these temperatures with precise distance measurements obtained by the European Space Agency’s Gaia Space Observatory, the researchers calculated the luminosity or power of stars and found their results consistent with theory.

“We still have a lot to learn about supernovae and related objects and phenomena, but I think this research will help astronomers fill in some of the blanks,” Taniguchi said. “The giant star Betelgeuse (on Orion’s shoulder) could become a supernova in our lifetime; in 2019 and 2020 it has faded unexpectedly. It would be fascinating if we could predict if and when it might become a supernova . I hope our new technique will help this effort and more. “


Journal article

Daisuke Taniguchi, Noriyuki Matsunaga, Mingjie Jian, Naoto Kobayashi, Kei Fukue, Satoshi Hamano, Yuji Ikeda, Hideyo Kawakita, Sohei Kondo, Shogo Otsubo, Hiroaki Sameshima, Keiichi Takenaka and Chikako Yasui. Effective temperatures of red supergiants estimated from line depth ratios of iron lines in YJ bands, 0.97-1.32 μm. Monthly notices from the Royal Astronomical Society.
DOI: 10.1093 / mnras / staa3855.

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