ベイズ推論による恒星磁気活動の探究

京都大学新制・課程博士博士(理学)甲第23006号理博第4683号新制||理||1672(附属図書館)京都大学大学院理学研究科物理学・宇宙物理学専攻(主査)准教授 野上 大作, 教授 一本 潔, 教授 太田 耕司学位規則第4条第1項該当Doctor of ScienceKyoto UniversityDFA

Do Kepler superflare stars really include slowly-rotating Sun-like stars ? - Results using APO 3.5m telescope spectroscopic observations and Gaia-DR2 data -

We report the latest view of Kepler solar-type (G-type main-sequence) superflare stars, including recent updates with Apache Point Observatory (APO) 3.5m telescope spectroscopic observations and Gaia-DR2 data. First, we newly conducted APO3.5m spectroscopic observations of 18 superflare stars found from Kepler 1-min time cadence data. More than half (43 stars) are confirmed to be "single" stars, among 64 superflare stars in total that have been spectroscopically investigated so far in this APO3.5m and our previous Subaru/HDS observations. The measurements of $v\sin i$ (projected rotational velocity) and chromospheric lines (Ca II H\&K and Ca II 8542\AA) support the brightness variation of superflare stars is caused by the rotation of a star with large starspots. We then investigated the statistical properties of Kepler solar-type superflare stars by incorporating Gaia-DR2 stellar radius estimates. As a result, the maximum superflare energy continuously decreases as the rotation period $P_{\mathrm{rot}}$ increases. Superflares with energies $\lesssim 5\times10^{34}$ erg occur on old, slowly-rotating Sun-like stars ($P_{\mathrm{rot}}\sim$25 days) approximately once every 2000--3000 years, while young rapidly-rotating stars with $P_{\mathrm{rot}}\sim$ a few days have superflares up to $10^{36}$ erg. The maximum starspot area does not depend on the rotation period when the star is young, but as the rotation slows down, it starts to steeply decrease at $P_{\mathrm{rot}}\gtrsim$12 days for Sun-like stars. These two decreasing trends are consistent since the magnetic energy stored around starspots explains the flare energy, but other factors like spot magnetic structure should also be considered.Comment: 71 pages, 31 figures, 10 tables. Accepted for publication in The Astrophysical Journal (on March 29, 2019

Starspot Mapping with Adaptive Parallel Tempering. II. Application to TESS Data for M-dwarf Flare Stars AU Microscopii, YZ Canis Minoris, and EV Lacertae

Starspots and stellar flares are indicators of stellar magnetic activity. The magnetic energy stored around spots is thought to be the origin of flares, but the connection is not completely understood. To investigate the relation between spot locations deduced from light curves and the occurrence of flares therein, we perform starspot modeling for the TESS light curves of three M-dwarf flare stars, AU Mic, YZ CMi, and EV Lac, using the code implemented in Paper I. The code enables us to deduce multiple stellar/spot parameters by the adaptive parallel tempering algorithm efficiently. We find that flare occurrence frequency is not necessarily correlated with the rotation phases of the light curve for each star. The result of starspot modeling shows that any spot is always visible to the line of sight in all phases, and we suggest that this can be one of the reasons why there is no or low correlation between rotation phases and flare frequency. In addition, the amplitude and shape of the light curve for AU Mic and YZ CMi have varied in two years between different TESS cycles. The result of starspot modeling suggests that this can be explained by the variations of spot size and latitude

Starspot mapping with adaptive parallel tempering I: Implementation of computational code

Starspots are thought to be regions of locally strong magnetic fields, similar to sunspots, and they can generate photometric brightness modulations. To deduce stellar and spot properties, such as spot emergence and decay rates, we implement computational code for starspot modeling. It is implemented with an adaptive parallel tempering algorithm and an importance sampling algorithm for parameter estimation and model selection in the Bayesian framework. For evaluating the performance of the code, we apply it to synthetic light curves produced with 3 spots. The light curves are specified in the spot parameters, such as the radii, intensities, latitudes, longitudes, and emergence/decay durations. The spots are circular with specified radii and intensities relative to the photosphere, and the stellar differential rotation coefficient is also included in the light curves. As a result, stellar and spot parameters are uniquely deduced. The number of spots is correctly determined: the 3-spot model is preferable because the model evidence is much greater than that of 2-spot models by orders of magnitude and more than that of 4-spot model by a more modest factor, whereas the light curves are produced to have 2 or 1 local minimum during one equatorial rotation period by adjusting the values of longitude. The spot emergence and decay rates can be estimated with error less than an order of magnitude, considering the difference of the number of spots.Comment: 27 pages, 14 figures, 2 tables, accepted for publication in Ap

Surgical Outcomes of Minimally Invasive Stabilization for Spinal Fractures in Patients with Ankylosing Spinal Disorders

Study Design A retrospective study. Purpose To evaluate the clinical and radiological outcomes of ankylosing spinal disorder (ASD) patients with spinal fractures treated by minimally invasive stabilization (MISt) using percutaneous pedicle screws (PPSs). Overview of Literature ASDs, such as ankylosing spondylitis (AS) and diffuse idiopathic skeletal hyperostosis (DISH), increase susceptibility to spinal fractures because of extremely decreased spinal flexibility. Such fractures tend to be unstable and, consequently, should be treated with multiple-segmental internal fixation. However, conventional internal fixation procedures can severely damage the soft tissue, resulting in severe hemorrhage. Therefore, MISt is the preferred approach to treat spinal fractures in ASD patients. Methods Nine ASD patients (four males and five females; three AS and six DISH patients) with spinal fractures who were treated by MISt using PPSs, were reviewed from April 2009 to August 2016. One patient died of aspiration pneumonia during follow-up (FU), and the remaining eight patients underwent clinical and radiological evaluation. Results The mean age at surgery was 79.6 years (range, 68–95 years). The mean duration of postoperative FU was 14.2 months (range, 3–30 months). All treated fractures were anterior and posterior element injuries with distraction. Three patients presented delayed onset preoperative neurological deficit following trauma. The mean operation time was 179.6 minutes (range, 92–340 minutes). The mean hemorrhage was 103.6 mL (range, unquantifiable to 480 mL). Radiological evaluations at FU showed preservation of the acceptable postoperative correction of the fractured vertebra, as there were no re-collapses of the fractured vertebrae during FU. Conclusions ASD patients must be acknowledged as highly susceptible to unstable spinal fractures, even after relatively mild trauma. MISt using PPSs may be an effective treatment for spinal fractures in such patients