Shallow ionized calcium absorption lines suggest that superflare stars have large starspots

 

Source: Subaru Telescope

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.

In the above figure, the bottom two images on the left show the Sun in visible light (left) and the Ca II line (right). The upper two images are imaginary drawings of a superflare star in visible light (left) and the Ca II line (right) where the areas around the starspots are bright. 

Superflare stars (the upper spectrum, shown in red) have a shallow (bright) core depth of the absorption line of Ca II 854.2 [nm] (ionized calcium) compared to the Sun (the bottom spectrum, in black). This suggests that superflare stars have large starspots.