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Recent studies of solar-type superflare stars have suggested that even oldslowly rotating stars similar to the Sun can have large starspots andsuperflares. We conducted high dispersion spectroscopy of 49 nearbysolar-analog stars (G-type main sequence stars with$T_{\rm{eff}}\approx5,600\sim6,000$ K) identified as ROSAT soft X-ray sources,which are not binary stars from the previous studies. We expected that thesestars can be used as a proxy of bright solar-analog superflare stars, sincesuperflare stars are expected to show strong X-ray luminosity. More than half(37) of the 49 target stars show no evidence of binarity, and their atmosphericparameters ($T_{\rm{eff}}$, $\log g$, and [Fe/H]) are within the range ofordinary solar-analog stars. We measured Ca II 8542 and H$\alpha$ lines, whichare good indicators of the chromospheric activity. The intensity of these linesindicates that all the target stars have large starspots. We also measured$v\sin i$ (projected rotational velocity) and Lithium abundance for the targetstars. Li abundance is a key to understanding the evolution of the stellarconvection zone, which reflects the stellar age, mass and rotational history.We confirmed that many of the target stars rapidly rotate and have high Liabundance, compared with the Sun, as suggested by many previous studies. Thereare, however, also some target stars that rotate slowly ($v\sin i$=2$\sim$3 kms$^{-1}$) and have low Li abundance like the Sun. These results support thatold and slowly rotating stars similar to the Sun could have high activity leveland large starspots. This is consistent with the results of our previousstudies of solar-type superflare stars. In the future, it is important toconduct long-term monitoring observations of these active solar-analog stars inorder to investigate detailed properties of large starspots from the viewpointof stellar dynamo theory.

Spectroscopic observations of active solar-analog stars having high X-ray luminosity, as a proxy of superflare stars