Figure 1: Left: The brightness variation of solar-type superflare stars (from Kepler data). In addition to the sudden brightenings caused by flares, quasi-periodic brightness variations with periods of about 15 days are seen. Right: An artificial image of a superflare star seen with visible light. This figure shows a large superflare (shown in white) occurring in the large starspot area. (Credit: Kyoto University)

.

Based on the initial discovery, the team carried out spectroscopic observations on 50 solar-type superflare stars selected from the Kepler Space Telescope's data. From the investigation of the detailed properties of spectral lines, the team obtained the following results:

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).

The results of these observations and analysis confirm that stars similar to the Sun can have superflares if they have large starspots. In the future, in addition to the continuing spectroscopic observations with Subaru Telescope, the team will conduct observations with the Kyoto University's Okayama 3.8m telescope, which is now under construction. This will allow them to investigate more detailed properties and changes in long-term activity of superflare stars.

Figure 2: Left: Four neutral iron (Fe I) absorption lines are shown. As mentioned above, spectral lines show some broadening because of the Doppler effect on the light from the rotating stellar surface. Slowly rotating stars like the Sun have a narrow line profile, while rapidly rotating stars have a wide line profile. Measuring these broadenings allows an estimate of the stellar rotation velocity. Right: The wavelength of light from the surface of a rotating star shifts because of the Doppler effect. For example, the wavelength of light from point A becomes a bit short (is blue-shifted) since this point is approaching us (the observer). By contrast, the wavelength of light from point C is a bit long (is red-shifted) since this point moves away from us. The wavelengths of light from point B have no shifts since this point moves perpendicular to the line of sight. Finally, this line-shift effect results in broadening of spectral lines. (Credit: Kyoto University)