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京都大学 / 新制・課程博士 / 博士(理学) / 甲第23706号 / 理博第4796号 / 新制||理||1686(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)講師 LEE Shiu Hang, 教授 嶺重 慎, 准教授 前田 啓一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

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Stellar structure is investigated within the framework of scalar-tensor gravity. Novel perturbative analytical results are obtained for constant-density stars and for Newtonian polytropes in the quadratic model with coupling function A(Φ) = exp(αΦ+1/2βΦ^2). They are compared to full numerical calculations, and possible applications to main-sequence stars, white dwarfs, and the Chandrasekhar mass are indicated. It is found that Buchdahl's theorem is violated in Brans-Dicke theory for stars with exponentially-decaying density profiles. However, the mass-to-radius ratio M/R tends to the constant-density value in a certain limit. It is observed that for β < 0, there exists a maximum value of η = P0/ρo for constant-density stars, where P0 and ρ0 are the central pressure and density, respectively. It is conjectured that if such a maximum value also exists for other equations of state, and is less than the constant-density maximum value, then knowledge of P/ρ in the centre of a star can be used to constrain β. / Thesis / Master of Science (MSc)

Ein präzises Verständnis über die Entstehung der Elemente im Universum stellt ein hoch- relevantes Kernthema der nuklearen Astrophysik dar. Vor diesem Hintergrund wurde die 12C(p,γ)13N-Reaktion untersucht, die als Startreaktion des CNO-Zyklus Einfluss auf das Verhältnis von 12C zu 13C im Universum nimmt. Die analysierten Messdaten wurden in in- verser Kinematik am Tandetron-Beschleuniger des Helmholtz-Zentrum Dresden-Rossendorf aufgenommen. Der resultierende S-Faktor, vermessen im Bereich der 421keV-Resonanz, liegt im Mittel 23% unterhalb etablierter Literaturdaten, deckt sich aber mit den Ergeb- nissen anderer kürzlich veröffentlichter Messdaten. Die in dieser Analyse ebenfalls erschwerte präzise Untersuchung niedriger, aber astrophy- sikalisch relevanter Energien kann durch Untertagelabore und der damit einhergehenden Abschirmung vor kosmischer Strahlung erreicht werden. In der vorliegenden Arbeit werden in diesem Bestreben erste mit dem 5 MV-Tandem-Beschleuniger untersuchte Kernreaktio- nen am Felsenkeller-Untertagelabor in Dresden vorgestellt. Aus Untersuchungen der 14N(α,γ)18F-, der 13C(p,γ)14N- und der 27Al(p,γ)28Si-Reaktion wurden dabei präzise Werte für die Energiekalibration des Ionenbeschleunigers ermittelt. Es wird ein Vergleich mit weiteren Möglichkeiten zur Bestimmung dieses Kalibrationsfaktors präsentiert und aus diesem Vergleich ein Wert von k = 0,9572 ± 0,0004 zur Kalibration der Hochspannung des Beschleunigers abgeleitet. Die vorgestellte Herangehensweise zur Bestimmung dieses Faktors und die dokumentier- ten Erkenntnisse und Analysen zu optimalen Betriebsparametern von Beschleuniger und der zugehörigen Radiofrequenz-Ionenquelle werden auch für zukünftige protonen- und he- liumstrahlinduzierte Untersuchungen im Felsenkeller-Untertagelabor von Relevanz sein.

<p dir="ltr">Advancements in astronomical imaging all-sky surveys are revolutionising the field of time domain astronomy. However, the immense volume of alert data presents a critical bottleneck in maximizing scientific returns from these surveys. Effectively analyzing alert streams to discover transients in unexplored physical parameter spaces is crucial for enhancing scientific output. Building robust infrastructure to identify, prioritize, and execute efficient follow-up strategies on alert streams from all-sky surveys becomes critical. My thesis confronts this challenge through a multidisciplinary approach, by integrating statistical methods, machine learning algorithms, and hydrodynamical simulations to constrain transient explosion properties and motivate effective follow-up initiatives. </p><p dir="ltr">I analyze a sample of 45 Type II supernovae from the Zwicky Transient Facility (ZTF) public survey using a grid of hydrodynamical models in order to assess whether theoretically driven forecasts can intelligently guide follow-up observations supporting all-sky survey alert streams. I estimate several progenitor properties and explosion physics parameters, including zero-age main-sequence (ZAMS) mass, mass-loss rate, kinetic energy, ⁵⁶Ni mass synthesized, host extinction, and the time of the explosion. This work involves simulations of real-time of evolving incomplete light curves of the sample (∆t < 25 days, ∆t < 50 days, all data) leading to the conclusion that certain physical parameters exhibit greater reliability as indicators of true values during early epochs. This study emphasises the vital role of real-time modeling of transients, supported by multi-band synthetic light curves tailored to survey passbands, for identifying interesting transients based on their progenitor and explosion properties and determining critical epochs for follow-up observations.</p><p dir="ltr">In my thesis, I report multi wavelength observations and characterization of the ultraluminous transient AT 2021lwx (ZTF20abrbeie; aka“Barbie”) identified in the ZTF alert stream, that was flagged as an anomaly by the Recommender Engine For Intelligent Transient Track-ing (REFITT). From a spectroscopically measured redshift of 0.9945, and slowly fading g and r light curves spanning over 1000 observer-frame days that peak with an absolute magnitude of Mr = −25.7 mag, AT 2021lwx has an extraordinary peak pseudo-bolometric luminosity of log (Lmax/[erg/s]) = 45.7. The total radiative energy is over 10⁵³ erg, and as of today, the transient continues to decline slowly following a t^−5/3 power-law. Modeling available photometry with MOSFiT suggests that AT 2021lwx is a tidal disruption event (TDE) candidate involving a ≈ 14 or 15 solar mass star accreting onto a supermassive black hole (SMBH) with mass M_BH ≈ 10⁸ solar mass. But, intriguingly, no host galaxy associated with the theorized SMBH is detected yet. The Pan-STARRS non-detections do not definitively exclude the existence of a galaxy hosting AT 2021lwx. Utilizing EzGal, upper limits on the stellar mass for different stellar population models were determined. These upper limits suggest that a 10¹⁰ solar mass host galaxy cannot be ruled out.</p><p dir="ltr">The enhanced sensitivity of upcoming large-scale all-sky surveys enables the early detection of transients, providing unique insights into their progenitor systems, an example being detection of shock cooling emission (SCE) in light curves of stripped-envelope supernovae(SESNe). Leveraging a statistically significant sample of these events with early detections from all-sky surveys presents an invaluable opportunity to constrain their environments, pro-genitors, and explosion properties. In my final study contributing to this thesis, I analyze a sample of 16 SESNe identified from the ZTF survey, characterised by prominent shock cooling emission features in their light curves. By modeling the SCE and the radioactive peak in these transients, I derive estimates of progenitor radius, mass of the extended envelope and explosion properties of SESNe. This analysis yields upper limits and ranges for the compactness and envelope structures of the SESN progenitors that exhibit SCE in their light curves. I conclude my thesis with a summary of the findings and their future applications.</p>

Measuring accurate stellar abundances of planet-forming elements is critical to our understanding of exoplanet compositions and their formation processes. While these values can be reliably derived from optical spectra for FGK-type stars, the recovery of accurate abundances for M dwarfs is complicated due to persistent discrepancies between models and observed spectra, such as blended absorption features and broad molecular bands that obscure the continuum. These lingering uncertainties in M dwarf chemical compositions inhibit our ability to accurately model the interiors and atmospheres of exoplanets around M dwarfs. To address this issue, we have built a custom framework to extract elemental abundances from the spectra of cool stars via the spectral synthesis method. We showcase our methodology as well as the derived elemental abundances for a pair of cool stars. SPIRou, with its high spectral resolution and broad near-IR wavelength range, is the ideal instrument to help mitigate the difficulties present in the recovery of M dwarf elemental abundances. By combining the capabilities of SPIRou with our framework, we are well equipped to ensure the accuracy of derived elemental abundances in M dwarfs. Our results will ultimately be applied to planet-hosting M dwarfs in order to place strong constraints on the planets’ refractory and volatile abundances, both of which are important diagnostics of planetary formation histories and interior compositions. / Thesis / Master of Science (MSc) / Understanding what planets are made of helps us learn how they form. Since planets and their stars are created from the same materials, we can study a star’s composition to learn more about the planets that orbit it. Measuring the abundances of planet-forming elements like magnesium, silicon, and iron is routinely performed for Sun-like stars, but the task proves to be much more difficult for smaller, cooler stars like M dwarfs. M dwarfs are very common and host most of the super-Earths within the Milky Way that could potentially support life, so studying them is crucial. My Master’s thesis focuses on developing a method to accurately measure the elemental abundances in M dwarfs using high-resolution spectra taken at infrared wavelengths where M dwarfs emit most of their light. My work is helping to improve our understanding of the composition and formation pathways of exoplanets around M dwarfs.

Dans ce travail de thèse, nous analysons pour la première fois à l'aide de la technique d'imagerie doppler, les raies photosphériques de deux étoiles T Ttauri à faibles raies d'émission, pour cartographier leur brillance de surface et localiser ainsi les inhomogenéités en température, qui signent la présence de forts champs magnétiques. Pour ce faire, nous avons développé une méthode numerique fondée sur la paramétrisation de taches, dont les caractéristiques sont déterminées lors de la minimisation, de type moindres carrés, des différences entre les modèles de raie calculés et l'ensemble des raies (issues de divers éléments chimiques) observées à plusieurs phases rotationnelles. Nous étudions le type et l'amplitude de déformations engendrées par la présence de taches froides, ce qui nous permet d'en déduire les conditions optimales d'application de l'imagerie doppler. Nous etudions par ailleurs les limitations intrinséques de cette technique déterminées par la qualité des données (résolution instrumentale et rapport signal sur bruit). L'application à des étoiles T Tauri, jeunes étoiles en fin de formation, similaires au soleil dans sa jeunesse, nous permet de mettre en évidence la présence de larges taches froides, préférentiellement localisées aux latitudes élevées. La position de ces taches suggère une géometrie de type dipolaire pour le champ magnétique. Seule une étude exhaustive pourra déterminer si cette géometrie est commune à toute cette classe d'étoiles, ou si elle est liée à des caractéristiques particulières (fort taux de rotation 70km/s ; 80 km/s). cette réponse est cruciale pour déterminer l'origine du champ magnétique : fossile, engendré par un mecanisme particulier de type dynamo, peut-etre les deux à la fois. L'application future de cette technique aux etoiles T Tauri classiques, supposées etre entourées d'un disque d'accrétion, sera determinante pour comprendre l'interaction entre l'environnement circumstellaire et l'objet central.

M dwarfs have been the subject of renewed interest as potential habitable planet hosts and have increasingly become the targets of planet detection surveys. Currently, however, the number of detections of transiting M dwarf planets remain low. The characterisation of M dwarf activity is an important consideration for such surveys, and provides constraints on the modelling of magnetically active low mass stars. Currently the spottedness of M dwarfs is not well understood owing to their intrinsic faintness and the lack of diagnostics for assessing starspot morphologies and distributions. The WFCAM Transit Survey (WTS) contains long term observations of M dwarfs in the near infra-red and presents an opportunity to study the long term variability of M dwarfs. The M dwarfs in the WTS are identified by use of colour-spectral type relations, and the periodically variable M dwarfs in the sample are detected using a Lomb-Scargle periodogram analysis. A total of 72 periodically variable M dwarfs are found with periods ranging from 0.16 to 90.33 days. The relations between the spectral subtypes, amplitudes and periods are studied and comparisons to earlier works studying M dwarf rotation are made. A number of examples of significant spot evolution are found, which exhibit complex light curve morphologies that vary in form and amplitude over periods of months to years. This provides an indication as to the nature of the spottedness of these stars. Simulations are performed to probe the connection between spot coverage, temperature and light curve amplitude. Using the results from these simulations, the spot coverage fractions of the WTS M dwarfs are estimated and they are found to be heavily spotted. Dynamic models with spots evolving at various average rates are used to explore how spot evolution can drive increased dispersion in the light curves, and to what extent this affects the detectability of periodicity by the method used. It is found that spot evolution can invoke significant noise in an M dwarf light curve, and in combination with photon noise, can in some instances inhibit the detection of a period. In reflection of the results, the relation between the light curve dispersion and spot coverage of the WTS M dwarfs is considered and it is found that more heavily spotted M dwarfs have intrinsically noisier light curves. The morphologies of the light curves produced by the simulations, and the manner in which they evolve, are qualitatively similar to the real M dwarfs in the WTS sample and indicate how models extrapolated from sunspot distributions can explain behaviour seen in active M dwarf light curves.

We investigate binary hypotheses for the formation of bipolar nebulae using the smoothed particle hydrodynamics code Gadget-2. In the general case, we present a parameter study of mass loss from very simple common envelope models, which seems to show a strongly bipolar trend for sufficiently oblate envelopes and low enough spiral-in injection energy. The density profiles of the envelopes produce differing structure within the ejecta. We also investigate 3 specific bipolar mass loss events. In the case of the outer nebula of SN 1987A, we study the effects of fast polar jets interacting with a pre- existing progenitor wind and find that these are consistent with the observed light echoes from the nebula, in particular for the feature known as ’Napoleon’s Hat’. In the case of Cas A, we briefly study the effects of close binarity on supernova ejecta and suggest the influence of a close, compact companion may lead to at least one jet-like disturbance that may be observable in a supernova remnant. Finally, we study whether a fast bipolar wind, similar to that of Eta Car’s present wind, may inflate ejecta similar to that produced in the common envelope models to a shape consistent with that of Eta Car’s Homunculus Nebula.