High Dispersion Spectroscopy of the Superflare Star KIC6934317

TL;DR

This study uses high-resolution spectroscopy to analyze the superflare star KIC6934317, revealing high chromospheric and coronal activity, large starspots, and a nearly pole-on orientation, despite its old age.

Contribution

It provides detailed stellar parameters and activity analysis of KIC6934317, a superflare star, highlighting its high activity levels and geometric orientation, which are novel insights.

Findings

KIC6934317 exhibits superflares with energies ~10^35 erg.

The star shows high chromospheric and coronal activity despite low lithium.

It has a small inclination angle, explaining the observed brightness variations.

Abstract

We conducted the high-resolution spectroscopic observation with Subaru/HDS for a G-type star (KIC6934317). We selected this star from the data of the Kepler spacecraft. This star produces a lot of superflares, and the total energy of the largest superflare on this star is ~10^3 times larger (~2.2*10^35 erg) than that of the most energetic flare on the Sun (~10^32 erg). The core depth and emission flux of Ca II infrared triplet lines and Ha line show high chromospheric activity in this star, in spite of its low lithium abundance and the small amplitude of the rotational modulation. Using the empirical relations between emission flux of chromospheric lines and X-ray flux, this star is considered to show much higher coronal activity than that of the Sun. It probably has large starspots which can store a large amount of magnetic energy enough to give rise to superflares. We also estimated the stellar parameters, such as effective temperature, surface gravity, metallicity, projected rotational velocity (v sin_i), and radial velocity. KIC6934317 is then confirmed to be an early G-type main sequence star. The value of v sin_i is estimated to be ~1.91 km s^-1. In contrast, the rotational velocity is calculated to be ~20 km s^-1 by using the period of the brightness variation as the rotation period. This difference can be explained by its small inclination angle (nearly pole-on). The small inclination angle is also supported by the contrast between the large superflare amplitude and the small stellar brightness variation amplitude. The lithium abundance and isochrones implies that the age of this star is more than about a few Gyr, though a problem why this star with such an age has a strong activity remains unsolved.