An international team led by researchers from the Astrobiology Center has utilized data from the exoplanet exploration project using the Subaru Telescope’s near-infrared high-dispersion spectrograph “IRD” (IRD-SSP) and revealed the chemical compositions of 13 cool stars. The IRD-SSP searches for planets around lower-mass and cooler stars than the Sun (cool stars), also known as “M-type dwarfs”. This study elucidates the characteristics of M-type dwarfs themselves before planet discovery and represents the first achievement utilizing IRD-SSP data.
The chemical composition of a star is the ratio of each element, such as iron, sodium, and magnesium, among the components that make up the star. This information is necessary to determine the characteristics of exoplanets when they are discovered in the future, as it is also used to determine the formation materials of planets (exoplanets) that may exist around the star. It is also an important indicator of when a star was born in the evolution of the galaxy. For this reason, there is a long history of spectroscopic studies of the chemical composition of stars with temperatures similar to those of the Sun (F-, G-, and K-type stars) in visible light. On the other hand, it has been difficult to measure the chemical composition of M-type dwarfs using conventional methods because they are very faint in visible light and their low temperature makes spectroscopic data complicated.
The team developed a unique method using the near-infrared spectra collected by the IRD-SSP to measure the chemical composition of an initial sample of 13 M dwarfs (specifically, the ratios of the abundances of sodium, magnesium, potassium, calcium, titanium, chromium, manganese, iron, and strontium to hydrogen). IRD is optimized for observing M dwarfs, which are brighter in the near-infrared than in visible light. In addition, the large aperture of the Subaru Telescope, one of the largest in the world, makes it possible to study particularly faint M dwarfs. The Subaru Telescope’s large aperture, the largest in the world, also made it possible to study particularly faint M dwarfs, and the combined use of the data from multiple observations of the same M dwarf at different times was an advantage over a one-time observation.
The measurements revealed that the thirteen M-type dwarfs in this study have chemical compositions similar to those of F-, G-, and K-type stars near the Sun. Combining the data from the European Space Agency’s Gaia satellite, we also examined their motion in the Galaxy, suggesting that M-type dwarfs, which are particularly metal-poor, tend to move differently from the Sun (Figure 2). This trend is also known for F-, G-, and K-type stars and may reflect the chemical evolution of the Galaxy. Among the targets in this study is a well-known M-type dwarf star called Barnard’s star, which is a relatively new type of star in the Galaxy. There are multiple reports of evidence that this star is a relatively old type of star in the Galaxy, and the detailed chemical composition measurements obtained for the first time by this observation are consistent with this.
This achievement is significant not only because it reveals the detailed chemical compositions of 13 M-type dwarfs, but also because it indicates that the chemical compositions of approximately 100 M-type dwarfs for which IRD is being used to search for planets can be measured in the near future. It is expected to reveal for the first time what kind of stars are M-type dwarfs in the vicinity of our solar system. In addition, when IRD discovers a planet in the future, the presentation of the chemical composition of the planetary material will also provide clues to the characteristics of that planet.
This research was published in the “Astronomical Journal” on January 18, 2022 (Ishikawa et al. “Elemental Abundances of nearby M Dwarfs Based on High-resolution Near-infrared Spectra Obtained by the Subaru/IRD Survey: Proof of Concept“).
About Subaru Telescope
Subaru Telescope is a large optical-infrared telescope operated by the National Astronomical Observatory of Japan (NAOJ) and supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under the Large-Scale Scientific Frontier Initiative. Mauna Kea, where Subaru Telescope is located, is a precious natural environment and an important place in Hawaiian culture and history, and we are deeply grateful for the opportunity to explore the universe from Mauna Kea.
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Subaru Telescope March 28, 2022 Press Release
