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2021.08.26
News

Face up and look at stars

Nishi-Harima Astronomical Observatory (NHAO) is a research facility and a public observatory located on the top of Mount Onade in Sayo Town. Sayo is an area with a dark and starry sky. We undertake important roles in astronomical research, education, and outreach. We also allow public visitors to gaze celestial objects and become familiar with them.

The NHAO hosts the 2-m Nayuta telescope and the 60-cm telescope. Nayuta is the second largest optical telescope in Japan and the largest-class public telescope in the world.

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The 2-m Nayuta telescope

Highlights of our recent researches are summarized below.

1. Collaborative observations of light accompanying gravitational waves from a neutron star merger (Tanaka et al. 2017, Utsumi et al. 2017)

On August 17, 2017, gravitational waves from a neutron star merger were detected for the first time. As a member of the Japanese follow-up team (J-GEM), Nayuta participated in the observations of its counterpart. The J-GEM observations revealed that the optical and near-infrared light was emitted by the radioactive decays of newly synthesized heavy nuclei. This finding provides a very important clue to solve the origin of heavy elements. One of the papers (Tanaka et al. 2017) received 2019 PASJ Excellent Paper Award.

2. Polarimetry of asteroid Ryugu, the target of Hayabusa2 mission (Kuroda et al. 2021)

Nayuta continuously supports Hayabusa2, the JAXA’s sample return mission, by ground-based observations. In 2020, Nayuta and other three telescopes conducted intensive polarimetry of Ryugu, which revealed that its polarization degree reaches as high as 53%. This is the highest value among asteroids and comets. The high polarization degree of Ryugu indicates that its surface is covered with large (submillimeter order) grains.

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Ryugu's surface

3. First detection of global polarization by Earth’s oceans (Takahashi et al. 2021)

How can we find an exoplanet with an ocean? Polarimetry is one of possible techniques to detect an ocean because glint on a smooth liquid surface is highly polarized. However, significance of the ocean polarization in Earth’s global polarization was not yet confirmed. Astronomers measured Earth’s polarization by observing lunar Earthshine with Nayuta. Earthshine is Earth’s light seen as a faint glow on the night side of the Moon. They found for the first time that Earth’s polarization degree is indeed higher when a larger area of the oceans faces to the observer. Therefore, polarimetry may be a promising technique in the search for exoplanets with an ocean. This work received the Best Presentation Award in the IAU Symposium 360.

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Lunar Earthshine