Detection of Low-Level Activities in Solar-Analog Stars from Emission Strengths of the Ca II 3934 Line

Activity studies of solar-type stars, especially with reference to the status of our current Sun among them, have exposed the importance of (1) homogeneously selecting the sample stars and (2) reliably evaluating their activities down to a considerably low level. Motivated by these requirements, we conducted an extensive study on the activities of 118 solar-analog stars (of sufficiently similar properties to each other) by measuring the emission strength at the core of Ca II 3933.663 line (K line) on the high-dispersion spectrogram obtained by Subaru/HDS, where special attention was paid to correctly detecting the chromospheric emission by removing the wing-fitted photospheric profile calculated from the classical solar model atmosphere. This enabled us to detect low-level activities down to log R' similar to -5.4 (R' is the ratio of the chromospheric core emission flux to the total bolometric flux), by which we could detect subtle activity differences that were indiscernible in previous studies. Regarding the Sun, we found log R-circle dot' = -5.33 near to the low end of the distribution, which means that it belongs to the distinctly low-activity group among solar analogs. This excludes the once-suggested possibility for the high frequency of Maunder-minimum stars showing appreciably lower activities than the minimum-Sun.