京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第18790号 / 理博第4048号 / 新制||理||1582(附属図書館) / 31741 / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 中村 卓史, 教授 鶴 剛, 教授 田中 貴浩 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

A major goal of the field of Galactic archeology is to understand the formation and evolution of the Milky Way disk. Stars migrate to different Galactic radii throughout their lifetimes, often leaving little dynamical signature of their initial orbits. Therefore, we need to look at the archaeological record preserved in stellar chemical compositions, which is indicative of their birth environment. In this thesis, we use the measurable properties of stars (chemical compositions and ages) to reconstruct the Milky Way disk's past. First, using hydrodynamical simulations, we find that a star's birth radius and age are linked to its chemical abundances. Subsequently, we learn that even with current-day measurement uncertainty and sample sizes, chemical abundances of Milky Way stars provide a route to reconstructing its formation over time. Extending the insights from hydrodynamical simulations to 30,000 stars observed across the Milky Way disk in the APOGEE survey reveals the importance of using the high-dimensional chemical abundance space. Specifically, we determine that we can use groups of chemically similar stars with 19 measured abundances to trace different underlying formation conditions. Using the high-dimensional abundance data for 10,000 stars from two spectroscopic surveys, APOGEE and GALAH, we empirically describe the chemical abundance trends across a vast radial extent of the Milky Way disk. To do this, we employ a novel approach of quantifying radial variations for individual abundances conditioned on supernovae enrichment history. This enables us to assess the information content in each of the 15 abundances examined and capture the fine-grained signatures in the disk's chemical evolution history. This thesis outlines the potential of using stellar chemistry to trace different evolutionary events of the Milky Way disk, particularly in a time where survey data sample size and precision are growing rapidly.

Upper Triassic strata that lie between the Chinle Formation and Nugget Sandstone along the south flank of the Uinta Mountains in northeastern Utah are distinctive. In the past, these rocks have been lumped together with the overlying or underlying units. These strata are equivalent to the Bell Springs Member of the Nugget Sandstone as defined in Wyoming and perhaps to the Rock Point Formation of the Chinle Formation near the Four Corners region. In this study, these rocks will be called the Bell Springs Formation following the usage of Lucas (1993) in Wyoming. The unit is regionally mappable in northeastern Utah, and is the sedimentologic transition from the fluvial-lacustrine environment of the Chinle Formation to the eolian depositional environment of the Nugget Sandstone. The Bell Springs Formation is comprised of interbedded fine- to medium-grained sandstone and siltstone, as well as planar laminated mudstone. The unit varies from planar laminated sandstone with abundant ripple marks, to cross-bedded sandstone that contains scoured channels filled with mudstone or sandstone. The mudstone beds are commonly mottled and contain desiccation cracks while both the mudstone and sandstone beds have rip-up clasts, occasional bioturbation, and small salt crystal casts. The thinly bedded mudstone and siltstone beds are purple to red to brown, and the sandstone beds vary in color from red to brown to orange or tan with green and gray mottling. The ripple structures with mud drapes indicate fluctuating deposition in low energy water. The presence of desiccation cracks, plant root traces, small eolian sand dunes, gypsum casts, crinkly algal mat beds, and bioturbation indicate intermittent subaerial exposure. Fluvial deposits by meandering streams, including point bar, levee, and splay deposits comprise a large part of this formation. Rocks of the Bell Springs Formation have previously been interpreted as either tidal flat or fluvial/lacustrine deposits. A tidal flat environment certainly may produce some of the features found in these deposits, such as, alternating erosion and deposition of interfingering channels and scours with rip-up clasts, ripples, flaser bedding, desiccation cracks, and bioturbation; however, these rocks lack some of the most important characteristics of tidal flat deposits such as herringbone-cross-stratification, general fining upward successions, and regionally associated sediments that would typically be found in shallow marine environments. We conclude that the sedimentary characteristics and regional setting of these rocks fit best with a fluvial environment interpreted as a meandering system being deposited on a broad floodplain in an arid to semi-arid climate. This depositional environment existed between the expanding Nugget Sandstone erg and the shrinking Chinle Formation as desertification increased during the Late Triassic and Early Jurassic in what is now the western United States. This study not only helps solidify the understanding of the depositional history of these strata, it also clarifies the nomenclature of these formations for future mapping and research.

No description available.

Development of Middle Triassic red-bed plays is commonly hampered by a lack of understanding of the stratigraphic relationships between reservoir sandstones. This inadequacy reflects poor palynological recoveries and a general deficiency in understanding the controls on palynological preservation. The sedimentology and palynology of Triassic red-bed facies, from the UK North Sea and onshore analogues, are studied to determine the sedimentary controls on palynological preservation and to investigate whether palynology is useful in these facies, where other stratigraphic techniques do not always provide unique solutions. The Skagerrak Formation (Quadrants 22, 29 and 30) typically comprises ephemeral channel and sheet-flood deposits in the north, but includes sediments deposited in perennially wet, alluvial plain and lacustrine settings in the south. Further north (Quadrant 210), the Cormorant Formation comprises dry alluvial deposits. Onshore, the Bromsgrove Sandstone Formation is characterised by ephemeral channel deposits that pass progressively upwards into tidally-influenced, fluvial and estuarine deposits; these are partly comparable with sediments in the Tarporley Siltstone Formation. Palynological analysis reveals that, in the absence of palynomorphs, palynodebris and absolute organic concentration can distinguish between preservational regimes, and thus environment. Palynological preservation demonstrates a correlation with facies deposited in perennially wet, alluvial plain, lacustrine and tidally influenced settings. Organic assemblages distinguish between members in the Bromsgrove Sandstone Formation, and can subdivide members on palaeoenvironmental criteria, which is of local value in correlation. Palynological assemblages are mostly lacking where ephemeral depositional processes were dominant. The assemblages demonstrate a close relationship with sedimentary facies, their associations, and sediment colour, but the oxidation potential of pore fluids, during and soon after deposition, is an overriding control on organic preservation. These relationships are all beneficial for targeting sediments for further palynological analysis.

No description available.

No description available.

How are stars formed? This is one of the most fundamental questions in astronomy. It is therefore ironic that to date, no object has been unambiguously identified as a true protostar; an object which derives the bulk of its luminosity from accretion. While this may be ironic, it is not surprising. Stars are believed to form as a result of the gravitational collapse of a portion of a molecular cloud. Theory predicts that the cloud core in which the star is formed will be cold, dense and possess hundreds of magnitudes of extinction, rendering it opaque at visible and near-infrared wavelengths. Continuum observations at far-infrared, submillimeter, and millimeter wavelengths can be used to identify candidate protostars, but spectroscopic observations are needed to detect infall. The difficulties arise when there are systematic velocity fields present in the cloud core which are not the result of infall, such as would be produced by either a molecular outflow or rotation. In this dissertation we use both observations and theoretical models to sort through these problems and develop a strategy which could be used to identify and study protostars.

We have conducted a CS survey of the 10 outflows and 30 IRAS sources identified by Margulis (1987) in the Mon OB1 dark cloud to study the relationship between outflows, YSOs, and dense cores in this cloud. We have found that the CS J = 2 → 1 transition traces a large portion of the dense, low-velocity components of the outflows in Mon OB1. We find the mass of this component to be nearly an order of magnitude greater than previous estimates of the outflow "core" component. We detected little CS gas around the quiescent sources in this cloud. CS 2 → 1 temperatures and integrated intensities are 2 to 7 and 2 to 14 times higher, respectively, in the vicinities of IRAS sources associated with outflow activity than about the quiescent sources. This implies CS abundances, temperatures and/or densities are enhanced in regions where outflows impact the ambient cloud. The CS 2 → 1 emission is concentrated in two regions encompassing 6 of the 10 previously identified outflows in this cloud. Four of these six outflows are identifiable in CS. Two, previously identified as monopolar outflows, exhibit bipolar structure in CS. We have detected the CS J = 5 → 4 transition in the vicinity of 4 of the 10 outflows in this cloud, and around none of the quiescent IRAS sources. The CS 5 → 4 emission is extended around two of the outflow sources and has been mapped in these regions. CS J = 7 → 6 emission has been mapped about the brightest outflow source in this cloud. The morphology of the 7 → 6 region suggests it may have been part of the collimating structure for the outflow associated with this sources. The velocity structure and binding energies of the 5 → 4 and 7 → 6 cores suggest the outflows are disrupting these cores. The addition of the low-velocity CS outflow component to previous estimates of outflow energetics implies multiple generations of outflows need not be required to support this cloud against collapse. Our results neither support nor rule out the existence of fossil outflows in this cloud. A full-cloud, unbiased survey is required to search for such outflows.

New observations of the star formation region NGC 7538 are presented. Energetic outflows are commonly associated with regions of active star formation, despite the fact that the star formation process itself must be predominantly one of infall. This work shows how multi-wavelength observations can be used to study such phenomena on a variety of scales, in an attempt to infer their connection with star formation processes. Included are near-infrared spectroscopy of IRS 2; carbon monoxide J = 1-0 emission line mapping of the central regions of the NGC 7538 molecular cloud; and high spatial resolution maps of IRS 1 at 5 and 15 GHz. Other recent observational data are also considered, including far-infrared continua mapping, maser sources, and spectroscopy of numerous atomic and molecular species. A very large (r ≥ 1.5 pc) and massive (m ≥ 100 solar masses) distribution of high velocity (ΔV(FWHM) ≃ 35 - 40 km s⁻¹) molecular gas is identified in NGC 7538. The correspondence of far-infrared emission with the extent of the high velocity gas, along with the near-equality between the observed momentum flux of the gas and that which is available for radiation pressure L(*) /c (IRS 1-3) leads to the proposal of an in situ mechanism for radiative acceleration of the gas. This mechanism for radiative acceleration of the gas. This mechanism operates in cases where the optical depth of dust is insufficient to permit the "snowplow" outflow effect from strong radiation pressure. On the 1/2 - 1 pc minimum scale of these molecular observations, no obvious bipolarity or collimation is detected, consistent with the in situ mechanism. The highly luminous infrared source IRS 1 is identified as the probable source of this high velocity phenomenon. It is the most luminous source of the three (IRS 1-3) upon which the high velocity gas distribution is centered. The presence on a scale of 100 - 100 AU of a high density ridge perpendicular to a collimated distribution of ionized gas is strongly indicated in both mid-infrared and radio emission, and by anomalous optical and infrared extinctions. There is thus a startling discontinuity between collimation of gas on this scale and the lack of it observed at the arcminute scale of the molecular observations.