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)

要約

Type(English)

OutlinePowered by T2R2 (Tokyo Institute Research Repository)

We investigated the magnetic activity variability for F-, G- and K-type stars (τ Boo, υ And, β Vir,α CMi, β Com, κ1 Cet, π1 UMa, β Aql, 61 Vir, ε Eri, η Cep, HD 102195) by monitoring HIDES-F Hα line
over the last four years, in terms of activity cycles, amplitudes of variability, and exoplanet searches. We
described HIDES-F Hα observations, the echelle data reduction, and analyses to evaluate the Hαvariability for each target star. As a result, the target stars have shown their intrinsic magnetic activityvariability, respectively.
We presented the possible dependence of the Hα variability on stellar parameters, especially on
the activity level of the target stars. Between the Hα variability and stellar age or Rossby number, there
might be a weak correlation which indicates that stars with the similar age or Rossby number could show
different activity variability. Although it is not easy to present a clear dependence of the activity
variability on stellar age and Rossby number in this Hα observation, it implies that the difference ofactivity variability is caused by a complex of stellar parameters and needs to be established through morecases. As the Hα variability is in a good agreement with log R'HK, the activity level calculated from the Ca
II H&K line, we suggest that the Hα variability can be properly used as an activity indicator. Our Hα flux
time series covered a new observing period, and its flux is strong enough to trace the activity variabilityof the F-, G- and K-type stars.
We suggested that the magnetic activity variability represents different behaviors of the activity
cycle depending on stellar spectral type. For the F-type target stars (τ Boo, υ And, β Vir, α CMi), theexistence of a possible activity variation has been presented. The clear cycle of 123-day for τ Boo has
been detected, and it is precisely consistent with the previous Ca II H&K results. Also, the possibility of ashort cycle within 1-2 years for β Vir and α CMi have been suggested by the Hα variability. For α CMi,
the possibility of temporal absence of its short cycle is speculated. We also suggested the range ofamplitudes for the Hα variability that could be detected as the short-term period for each star. Though aclear activity cycle period for υ And was not detected in this observation, our Hα results could be
supportive examples for the short-term activity cycle trend in F-type stars. The short time-frame of theactivity cycle in F-type stars would provide a better constraint to future dynamo studies for the Sun-likestars.
For the G-type (β Com, κ1 Cet, π1 UMa, β Aql, 61 Vir) and the K-type target stars (ε Eri, η Cep,HD 102195), they also presented the existence of the activity variation. The clear cycle of 2.9-year for εEri has been detected, and it is precisely consistent with its shorter cycle of co-existing cycles reported by
the previous Ca II H&K results. The Hα results of ε Eri could support the idea that the Sun-like stars have
two different cycles simultaneously. It also supports the suggestion of the two different dynamos
operation. For some G-type stars, we suggested their long-term activity variation possibly associated with
their longer cycle of co-existing cycles. The expected shorter cycle period was not detected in thisobservation, nevertheless, we set an upper limit of non-detection for the short-term period component ofactivity cycles for β Com and κ1 Cet. The Hα results of β Com and κ1 Cet are able to provide supportive
information of amplitudes range of activity variability for shorter cycles, to subsequent observations. For
the rest of the G- and K-type stars, we suggested an upper limit of non-detection for a periodic variation,
which also could be supportive conditions for following observations.
Additionally, we investigated the activity variations between the Hα and the Hβ line. Although we
could not find an identical correlation of long-term variations for the target stars, we suggested
observational evidence of the different relationship of each star. It is speculated that there might be a
relatively strong correlation for the stars which showed a long-term variation dominantly. It could be
helpful for future studies of relationships between the chromospheric lines.
With respect to the SPMI detection, for target stars with a hot-Jupiter, τ Boo, υ And, and HD
102195, any periods possibly induced by their hot-Jupiter have not been detected. We set an upper limit ofnon-detection of the SPMI. It is suggested that the magnetic activity variability detected in this Hαobservation is related to the stellar intrinsic activity rather than the existence of exoplanets. This also
could support that the short-term activity cycles in the F-type stars are induced by the stellar intrinsicdynamo.

We also investigated the magnetic activity variability depending on stellar spectral type andfeatures of individual stars with a focus on exoplanet searches. It is suggested that the magnetic activity
variability could affect the RV differently by spectral type. It would be supportive information for target
selection of exoplanet searches. We also speculated that the magnetic activity variability with the detected
amplitude in the Hα observation is not correlated with the RV variations for most individual target stars,
though there is still the possibility that the magnetic activity variability may induce RV variations for fewstars. In addition, we suggested the range of the amplitudes of the Hα variability that may or may not
induce a stellar noise to the RV. It could provide useful conditions to future exoplanet detection for theindividual stars.
We presented the efficiency of the Hα line with the difference of the variability amplitudes
between the Hα and the Ca II H&K line observationally. Though the Hα variability tends to show smaller
amplitudes, it would provide a strength in exploring activity variability in finer range of amplitudes. Our
Hα variability is expected to estimate adequately the amplitude range of activity variability that could
affect future exoplanet and activity cycles searches for other stars.
For the relationships between the stellar activity cycles and the rotational periods, our Hαobservations help to understand the scatters and possible tendency of the branches. With intensive andhigh cadence monitoring, we supported the existence of extremely short periods of F-type stars in theinactive branch and the possibility of a sub-sequence in the active branch, which might have been thereason of the scatters in the current diagram of the relationships. Additionally, we suggested that a distinct
short-term activity variation could be more dominant than a long-term variation, using the Hα results ofthe F-, G- and K-type stars together. The dominance or temporal absence of the short cycle suggested byour observations could explain few cases of the stars with detected co-existing cycles. Although ourresults of some G- and K-type target stars support the suggested conditions of co-existing cycles, further
observations might allow us to establish the possible condition as we speculated it could be more
complicated. It would also be helpful to select targets possibly have co-existing cycles and lead to better
understanding of the relationships between the stellar activity cycles and the rotational periods.