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Title:
High dispersion spectroscopy of solar-type superflare stars. II. Stellar rotation, starspots, and chromospheric activities
Authors:
Notsu, Yuta; Honda, Satoshi; Maehara, Hiroyuki; Notsu, Shota; Shibayama, Takuya; Nogami, Daisaku; Shibata, Kazunari
Affiliation:
AA(Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan ), AB(Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo-cho, Sayo, Hyogo 679-5313, Japan), AC(Kiso Observatory, Institute of Astronomy, School of Science, The University of Tokyo, 10762-30, Mitake, Kiso-machi, Kiso-gun, Nagano 397-0101, Japan; Okayama Astrophysical Observatory, National Astronomical Observatory of Japan, 3037-5 Honjo, Kamogata, Asakuchi, Okayama 719-0232, Japan), AD(Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan), AE(Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan), AF(Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan; Kwasan and Hida Observatories, Kyoto University, Yamashina-ku, Kyoto, Kyoto 607-8471, Japan), AG(Kwasan and Hida Observatories, Kyoto University, Yamashina-ku, Kyoto, Kyoto 607-8471, Japan)
Publication:
Publications of the Astronomical Society of Japan, Volume 67, Issue 3, id.3314 pp. (OUP Homepage)
Publication Date:
06/2015
Origin:
OUP
Astronomy Keywords:
stars: activity, stars: flare, stars: rotation, stars: solar-type, starspots
Abstract Copyright:
The Author 2015. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved.
DOI:
10.1093/pasj/psv002
Bibliographic Code:
2015PASJ...67...33N

Abstract

We conducted high dispersion spectroscopic observations of 50 superflare stars with Subaru/HDS. These 50 stars were selected from the solar-type superflare stars that we had discovered from the Kepler data. More than half (34 stars) of these 50 target superflare stars show no evidence of binarity, and we estimated stellar parameters of these 34 stars in our previous study (Notsu et al. 2015, PASJ, 67, 32). According to our previous studies using Kepler data, superflare stars show quasi-periodic brightness variations whose amplitude (0.1%-10%) is much larger than that of the solar brightness variations (0.01%-0.1%) caused by the existence of sunspots on the rotating solar surface. In this study, we investigated whether these quasi-periodic brightness variations of superflare stars are explained by the rotation of a star with fairly large starspots, by using stellar parameters derived in Paper I. First, we confirmed that the value of the projected rotational velocity, v sin i, is consistent with the rotational velocity estimated from the period of the brightness variation. Next, we measured the intensity of Ca II infrared triplet lines and Halpha line, good indicators of the stellar chromospheric activity, and compared them with other stellar properties. The intensity of Ca II infrared triplet lines indicates that the mean magnetic field strength (<fB>) of the target superflare stars can be higher than that of the Sun. A correlation between the amplitude of the brightness variation and the intensity of Ca II triplet line was found. All the targets expected to have large starspots because of their large amplitude of the brightness variation show high chromospheric activities compared to the Sun. These results support the idea that the brightness variation of superflare stars is due to the rotation with large starspots.
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