- More than half the observed 50 stars show no evidence of binarity (that is, they are not binary stars). The team confirmed the characteristics of the target stars as similar to those of the Sun.
- On the basis of the Kepler data, superflare stars show somewhat regular, periodic changes in their brightnesses. The typical periods range from one day to a few tens of days. Such variations are explained by the rotation of the star and its starspots. As shown in Figure 1, the stars seem to become dimmer when their starspots are on their visible sides. Moreover, the timescales of the brightness variations should correspond to the stars' rotation speeds. Spectroscopic observations allow observers to estimate the rotation velocity from the broadening of absorption lines (Figure 2), and confirm that a velocity derived from spectroscopic data matches the brightness variation timescale as the star rotates. In addition, the measured rotation velocity of some target superflare stars is as slow as that of the Sun.
- Based on solar observations, astronomers know that if there are large dark star spots on a stellar surface, the "core depth" (the depth and width of a spectral line) of the Ca II 854.2[nm] (ionized Calcium) absorption line becomes shallow. Using this, they investigated the core depth of Ca II 854.2 [nm] line, and found that superflare stars have large starspots compared to sunspots (Figure 3).
- Yuta Notsu (Department of Astronomy, Kyoto University)
- Satoshi Honda (Center for Astronomy, University of Hyogo)
- Hiroyuki Maehara (Okayama Astrophysical Observatory, NAOJ)
- Shota Notsu (Department of Astronomy, Kyoto University)
- Takuya Shibayama (Solar-Terrestrial Environment Laboratory, Nagoya University)
- Daisaku Nogami (Department of Astronomy, Kyoto University)
- Kazunari Shibata (Kwasan and Hida Observatories, Kyoto University)
- Paper 1: "High Dispersion Spectroscopy of Solar-type Superflare Stars. I. Temperature, Surface Gravity, Metallicity, and v sin i"
- Paper 2: "High Dispersion Spectroscopy of Solar-type Superflare Stars. II. Stellar Rotation, Starspots, and Chromospheric Activities"
Authors of both papers:
Notsu, Y., Honda, S., Maehara, H., Notsu, S., Shibayama, T., Nogami, D., and Shibata, K.
This work was supported by the Grant-in-Aids from the Ministry of Education, Culture, Sports, Science and Technology of Japan (No.25287039, 26400231, and 26800096).