論文 / 著書情報
Article / Book Information
題目(和文)
Title(English)

Observational Studies of Stellar Magnetic Activity Variability using
Chromospheric Lines

著者(和文)

李尚姫

Author(English)

Sanghee Lee

出典(和文)

学位:博士(理学),学位授与機関:東京工業大学,
報告番号:甲第12488号,
授与年月日:2023年6月30日,
学位の種別:課程博士,
審査員:佐藤 文衛,中本 泰史,奥住 聡,井田 茂,中島 淳一

Citation(English)

Degree:Doctor (Science),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第12488号,
Conferred date:2023/6/30,
Degree Type:Course doctor,Examiner:,,,,

学位種別(和文)

博士論文

Category(English)

Doctoral Thesis

種別(和文)

論文要旨

Type(English)

Summary

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（博士課程）Doctoral Program論 文 要 旨ＴＨＥＳＩＳ ＳＵＭＭＡＲＹ

系・コース：

地球惑星科学

Department of, Graduate major in

学生氏名：
Student’s Name

李 尚姫系コース

申請学位（専攻分野）：

博士

Academic Degree Requested

Doctor of

指導教員（主）：Academic Supervisor(main)（ 理学 ）
佐藤 文衛

指導教員（副）：Academic Supervisor(sub)要旨（英文 800 語程度）
Thesis Summary （approx.800 English Words ）Sun-like stars are important targets for exploring exoplanet and various features of stars likethe Sun. In Sun-like stars, the atmosphere exhibits various phenomena, generally induced bythe magnetic fields and the active regions in the stellar convective outer atmosphere. Thestellar magnetic activity is observed on the outer atmosphere, the chromosphere. The
chromospheric emission is observed in various spectral lines, especially the Ca II H&K line
cores (3933, 3968 Å) are most well-studied in the diagnosis of the stellar magnetic activity.
The Ca II H&K line monitoring on a long time-scale has revealed stellar magnetic activity
cycles for the Sun-like stars. The magnetic activity cycle is important to understand the stellar
magnetic activity behaviors. The magnetic activity of stars having a convective zone is related
to the stellar dynamo and therefore the study of the magnetic activity cycle leads us tounderstand the stellar magnetic field and the dynamo. It has been reported that most G- andK-type stars show long (from 2.5 yr to more than 20 yr) and pronounced activity cycles, andthat some G- and K-type stars might have two activity cycles simultaneously by the previous
Ca II H&K observations. However, the solar and the stellar dynamo are still poorly
understood despite the continuous study of the solar cycle discovered many decades ago.
Meanwhile, for F-type stars, although there have not been so many cases to suggest theiractivity cycle, recent research suggested that they have a relatively short activity cycle within
1-2 years. This short activity cycle could provide an ideal time-frame to study the stellar
dynamo. It is important to increase observations of F-type stars.
The monitoring of the Ca II H&K line focusing on the planetary perspective has also been
performed. The existence of a hot-Jupiter can influence the stellar magnetic activity, and theeffect could be represented by enhancements of the activity variability (Star-Planet MagneticInteraction, SPMI). In addition, the stellar activity variability can affect the detectability ofexoplanets. Investigating the magnetic activity variability is therefore important for exoplanet
researches as well.
The Hα line (6563 Å) can also be an accessible indicator like the Ca II H&K line.
However, as observations in the Hα line have not been performed enough, the dependence on
stellar parameters has not been established observationally for the Hα line. Clarifying theactivity variability in the Hα line is helpful for future observational studies of the stellar
magnetic activity and exoplanet searches.

We report the results of intensive monitoring of the activity variability in the Hα line forseveral F-, G- and K-type stars over the last four years 2019-2022, in order to investigate theirlatest magnetic activity from various aspects, including activity cycles, amplitudes ofvariability, and search for exoplanets. The 4-year Hα line intensity data taken with 1.88-m
reflector at Okayama Branch Office, Subaru Telescope, shows their intrinsic magnetic activity
variability for each target star. We found the correlation between the variability in the Hα line
and the activity level derived from the Ca II H&K line. Thus, we conclude that HIDES Hαobservation is sufficient to present the magnetic activity variability of the target stars, and theHα line can be used as an activity indicator. In addition, we present the possible dependenceof the Hα variability on other stellar parameters, like stellar age or Rossby number. Our Hαvariability could help to estimate the amplitude range of activity variability that affects future
exoplanets or activity cycle searches for other stars.
We found the existence of the possible short-term activity cycle in three of the four F-typetarget stars, including τ Boo, whose Hα result is consistent with the previous results of Ca II
H&K line. It is suggested that the Hα results could support the short-term activity cycle trend
in F-type star.
For some G- and K-type target stars which are expected to show a shorter cycle ofco-existing cycles, we confirmed the shorter cycle of a K-type star, ε Eri in the Hα line.
Although we could not detect the expected shorter cycle of two G-type stars, β Com and κ1Cet, we estimate the non-detection limit of a shorter activity cycle from the amplitude of theHα variability. For the other G- and K-type target stars, as we could not find any periodicvariations, we also set the non-detection limit. Our Hα results provide supportive information
to further investigate a short-term period component of these stars for subsequent
observations.
In respect of the SPMI detection, we could not detect signature of the SPMI for some target
stars with a hot-Jupiter. We set an upper limit of non-detection of the SPMI. It suggests thatthe magnetic activity variability detected in this Hα observation is related to the stellar
intrinsic activity rather than the existence of exoplanets.
We suggest that the impact of the activity variability in the Hα line on the RV measurement
would be different by spectral type. Also, it is speculated that there is no significant
correlation between the activity variability detected in this observation and the previous RVmeasurements. We suggest that the magnetic activity variability with the detected amplitudein the Hα observation might not affect exoplanet searches for most individual target stars withthe precision of few m/s. There is still the possibility that the magnetic activity variability
may induce a RV variation for few stars, and we assume the amplitude range of the Hαvariability that could affect the RV for the individual stars.
備考 : 論文要旨は、和文 2000 字と英文 300 語を 1 部ずつ提出するか、もしくは英文 800 語を 1 部提出してください。
Note : Thesis Summary should be submitted in either a copy of2000 Japanese Characters and 300 Words (Engｌish) or 1copy of 800

Words (English).
注意：論文要旨は、東工大リサーチリポジトリ（T2R2）にてインターネット公表されますので、公表可能な範囲の内容で作成してください。
Attention: Thesis Summary will be published on Tokyo Tech Research Repository

Website (T2R2).