[P-AE16] Exoplanets

[PAE16-P01] The research on the existence of helium in the atmosphere of small planets around M-type stars with Subaru/IRD

*Kiyoe Kawauchi¹, Norio Narita¹, Masayuki Kuzuhara^2,3, Teruyuki Hirano^2,3, Hiroki Harakawa⁴, Sebastien Vievard^2,4, Aoi Takahashi^2,3, Takayuki Kotani^2,3, Motohide Tamura^1,2,3 (1.The University of Tokyo, 2.Natural Institute of Natural Sciences Astrobiology Center, 3.National Astronomical Observatory of Japan, NAOJ, 4.Subaru Telescope)

Keywords:exoplanet atmosphere, transmission spectroscopy, small planets

Many small planets, whose radius is between 1 and 4 Earth radius, were found by Kepler space telescope. It was found that these radii show a distribution of the bimodal (Fulton et al. 2017). From the observation with the ground-based telescopes, these masses have been estimated. The mean densities calculated by these estimated radii and masses can constrain the bulk composition and internal structure. However, we cannot constrain that the planet has a rocky core enveloped in the H-He gas (primary gas) or H2O dominated ices/fluids because they show the same mean density (Zeng et al. 2018). To resolve this degeneracy, we should research the atmosphere directly. The atmospheres of some small planets were already observed but it has been difficult for resolving them because the signals of the molecule lines become weak due to aerosols (e.g. Tsiaras et al. 2019). Helium is suitable for identifying whether the planets have a primary gas or not since it is abundant in the upper atmosphere above the aerosols layer if it presents and it is created in the protoplanetary disk only.

Recently, by the development of the high-resolution spectrograph in the near-infrared region, He triplet lines have been detected in the atmosphere of dozens hot Jupiters (e.g. Nortmann et al. 2018). He triplet lines in three small planets were investigated with high-resolution transmission spectroscopy on the ground by Kasper et al. 2020, but there are only a few small planets whose atmospheres are possible to be investigated as almost all discovered small planets are far from our solar system. However, more than 80 candidates of the small planets located near our solar system have been discovered by Transiting Exoplanet Survey Satellite (TESS) launched in 2018. It will be possible to search for the He triplet lines in many small planets.

We performed the high-resolution transmission spectroscopy of He triplet lines on three small planets discovered by TESS projects with Subaru/IRD. In this presentation, we will report on these results.