Washington, A team of astronomers has used the High Dispersion Spectrograph on the Subaru Telescope to conduct spectroscopic observations of Sun-like “superflare” stars first observed and cataloged by the Kepler Space Telescope.
The investigations focused on the detailed properties of these stars and confirmed that Sun-like stars with large starspots can experience superflares.
The team, made up of astronomers from Kyoto University, University of Hyogo, the National Astronomical Observatory of Japan (NAOJ), and Nagoya University, targeted a set of solar-type stars emitting very large flares that release total energies 10-10000 times greater than the biggest solar flares.
Solar flares are energetic explosions in the solar atmosphere and are thought to occur by intense releases of magnetic energy around the sunspots. Large flares often cause massive bursts of high-speed plasma called coronal mass ejections (CMEs), can lead to geomagnetic storms on Earth. Such storms can have severe impacts on our daily life by affecting such systems as communications and power grids.
The team found that more than half the observed 50 stars show no evidence of binarity (that is, they are not binary stars) and confirmed the characteristics of the target stars as similar to those of the Sun.
They also found that 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.
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.