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

TL;DR

This study measures low-level chromospheric activity in 118 solar-analog stars using Ca II 3934 line emission, revealing the Sun's activity level and challenging previous ideas about Maunder-minimum stars.

Contribution

It introduces a precise method for detecting subtle chromospheric activities in solar analogs, improving sensitivity down to log R' ~ -5.4, and provides new insights into the Sun's activity status.

Findings

The Sun's activity level is near the low end among solar analogs.

Low-level activities can be reliably detected with the new measurement technique.

The high frequency of Maunder-minimum stars with lower activity is unlikely.

Abstract

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' ~ -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 which were indiscernible in previous studies. Regarding the Sun, we found log R'sun = -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.