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Geochemical and Sr-Nd isotopic constraints from the Kontum massif, central Vietnam on the crustal evolution of the Indochina block
No full text[[abstract]]The Kontum massif, central Vietnam, consists mainly of high-grade (amphibolite to granulite facies) metamorphic rocks and represents the largest basement exposure (core complex) of the Indochina block. To explore the crustal evolution of Indochina, Sr and Nd isotopic and geochemical data for various rock types from the massif are reported. The basement rocks show a wide range of present day epsilon(Nd) values from -22 (gneiss) to +15 (amphibolite), yielding depleted-mantle model ages (T-Dm) from 1.2 to 2.4 Ga along with an "exceptionally" old T-Dm of 2.7 Ga for a granulite. These data indicate that crustal formation in the Indochina block took place principally during the Paleoproterozoic and Mesoproterozoic, and do not support the conventional notion that the Kontum core complex is composed of Archean rocks. Geochemical data indicate that the gneisses and schists have heterogeneous compositions characterized by a calc-alkaline nature, whereas most of the amphibolites are tholeiitic basalts with intraplate magmatic signatures. Therefore, the former may be interpreted as products from pre-existing Proterozoic crustal materials and the latter as resulting from the Paleozoic rifting event that disintegrated the Indochina block from Gondwanaland. During its accretion with other SE Asian continental blocks in Permo-Triassic time, the Indochina core complex was subjected to the Indosinian orogeny, characterized by a high-temperature, granulite facies metamorphism in the lower crust with associated charnockite magmatism and subsequent regional exhumation.[[fileno]]2010101010033[[department]]物理
Laser post-ionization secondary neutral TOF-MS for isotopic analysis of pre-solar grains
No full text[[fileno]]2010101010045[[department]]物理
MAGNETIZED DISKS AROUND YOUNG STARS
No full text[[abstract]]We discuss the structure and evolution of a magnetized accretion disks around young stars that have dragged their magnetic field in the process of gravitational collapse. The disk evolves due to two diffusive processes: viscous stresses that redistribute mass and angular momentum, and the resistive diffusion of mass across magnetic field lines due to imperfect conduction. In steady-state there is an analytic model of the structure of these magnetized disks. We discuss the application of this model to disks around low and high mass young stars and recent results of time dependent models.[[fileno]]2010118030003[[department]]物理
Protostellar winds and chondritic meteorites - Formation of solids: The first step
No full text[[abstract]]We discuss the interaction between the magnetosphere of a young star and its surrounding accretion disk. We consider how an X-wind can be driven magnetocentrifugally from the inner edge of the disk where accreting gas is diverted onto stellar field lines either to flow onto the Sun or to be flung outwards with the wind. The X-wind satisfies many observational tests concerning optical jets, Herbig-Haro objects, and molecular outflows. Connections may exist between primitive solar system materials and X-winds. Chondrules and calcium-aluminum-rich inclusions (CAIs) experienced short melting events uncharacteristic of the asteroid belt where meteorites originate. The inner edge of the solar nebula has the shortest orbital timescale available to the system, a few days. Protosolar flares introduce another timescale, tens of minutes to hours. CAIs may form when solids are lifted from shaded portions of the disk close to the Sun and are exposed to its intense light for a day or so before they are flung by the X-wind to much larger distances. Chondrules were melted, perhaps many times, by flares at larger distances from the Sun before being launched and annealed, but not remelted, in the X-wind. Aerodynamic sorting explains the narrow range of sizes with which CAIs and chondrules are found in chondritic meteorites. Flare-generated cosmic-rays may induce spallation reactions that produce some of the short-lived radioactivities associated with primitive solar system rocks.[[fileno]]2010118030012[[department]]物理
Ambipolar diffusion in molecular cloud cores and the gravomagneto catastrophe
Get PDF[[abstract]]This paper reexamines the problem of ambipolar diffusion as a mechanism for the production and runaway evolution of centrally condensed molecular cloud cores, a process that has been termed the gravomagneto catastrophe. Our calculation applies in the geometric limit of a highly flattened core and allows for a semianalytic treatment of the full problem, although physical fixes are required to resolve a poor representation of the central region. A noteworthy feature of the overall formulation is that the solutions for the ambipolar diffusion portion of the evolution for negative times (t 0). The treatment shows that the resulting cores display nonzero, but submagnetosonic, inward velocities at the end of the diffusion epoch, in agreement with current observations. Another important result is the derivation of an analytic relationship between the dimensionless mass-to-flux ratio lambda(0) equivalent to f(0)(-1) of the central regions produced by runaway core condensation and the dimensionless measure of the rate of ambipolar diffusion is an element of. In conjunction with previous work showing that ambipolar diffusion takes place more quickly in the presence of turbulent fluctuations, i.e., that the effective value of is an element of can be enhanced by turbulence, the resultant theory provides a viable working hypothesis for the formation of isolated molecular cloud cores and their subsequent collapse to form stars and planetary systems.[[fileno]]2010118010009[[department]]物理
Gravitational collapse of magnetized clouds. I. Ideal magnetohydrodynamic accretion flow
Get PDF[[abstract]]We study the self-similar collapse of an isothermal magnetized rotating cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance from the accreting protostar, we find an analytic solution that corresponds to free fall onto a central mass point. The density distribution is not spherically symmetric but depends on the mass loading of magnetic field lines, which can be obtained by matching our inner solution to an outer collapse solution previously computed by Allen et al. The concentration of magnetic field trapped by the central mass point under field freezing, independent on the details of the starting state, creates a split-monopole configuration in which the magnetic field strength increases as the inverse square of the distance from the center. Under such conditions, the inflow eventually becomes sub-Alfvenic and the outward transfer of angular momentum by magnetic braking very efficient, thus preventing the formation of a centrifugally supported disk. Instead, the azimuthal velocity of the infalling gas decreases to zero at the center, and the gas spirals into the star. Therefore, the dissipation of dynamically important levels of magnetic field is a fundamental requisite for the formation of protoplanetary disks around young stars.[[fileno]]2010118010012[[department]]物理
Does magnetic levitation or suspension define the masses of forming stars?
Get PDF[[abstract]]We investigate whether magnetic tension can define the masses of forming stars by holding up the subcritical envelope of a molecular cloud that suffers gravitational collapse of its supercritical core. We perform an equilibrium analysis of the initial and final states assuming perfect field freezing, no rotation, isothermality, and a completely flattened configuration. The sheet geometry allows us to separate the magnetic tension into a levitation associated with the split monopole formed by the trapped flux of the central star and a suspension associated with curved field lines that thread the static pseudodisk and envelope of material external to the star. We find solutions where the eigenvalue for the stellar mass is a fixed multiple of the initial core mass of the cloud. We verify the analytically derived result by an explicit numerical simulation of a closely related three-dimensional axisymmetric system. However, with field freezing, the implied surface magnetic fields much exceed measured values for young stars. If the pinch by the central split monopole were to be eliminated by magnetic reconnection, then magnetic suspension alone cannot keep the subcritical envelope (i.e., the entire model cloud) from falling onto the star. We argue that this answer has general validity, even if the initial state lacked any kind of symmetry, possessed rotation, and had a substantial level of turbulence. These findings strongly support a picture for the halt of infall that invokes dynamic levitation by YSO winds and jets, but the breakdown of ideal magnetohydrodynamics is required to allow the appearance in the problem of a rapidly rotating, centrifugally supported disk. We use these results to calculate the initial mass function and star formation efficiency for the distributed and clustered modes of star formation.[[fileno]]2010118010018[[department]]物理
[[alternative]]Gods on Deeds---Engaged Religions and Multiculturalism among the Chinese in Contemporary Malacca, Malaysia
No full text[[abstract]]宗教在公領域中所扮演的角色近年來漸受重視。關注的焦點之一在於宗教如何淑世。現有理論文獻指出兩種可能的角色：宗教所提供社會連結然可以成為淑世運動的社會與理念基礎；宗教團體及機構可以直接提供慈善救濟以及其他文教服務、資源。然而，兩種宗教淑世角色如何在多元文化主義的脈絡中呈現仍是一個尚待實證研究來回答的課題。本計畫以當代馬來西亞多元文化主義中的華人宗教淑世活動為研究對象，聚焦馬六甲華人社會宗教淑世活動變遷，檢視當地社會中的佛教、民間宗教與基督教的淑世活動，並分析宗教傳統、族群認同與界線、現代化如何影響兩種宗教淑世角色。本計畫結合文獻分析及人類學民族誌田野研究方法，在馬來西亞麻六甲進行實地研究。本期中報告嘗試從社會資本理論角度分析淑世宗教。
This paper examines the relations among religion, ethnicity, and social capital in an Asian society. The subject matter is the new faces of the public contributions of Buddhism, Christianity, and popular temples among the Chinese in Malacca, Malaysia. Malacca is a particularly fruitful place for the examination of this topic due to its long and rich history of engaged religions among the Chinese, and since religious revitalization occurred in tandem with modernization and globalization. Parallel to the emergence of multiculturalism in the wake of the New Economic Policy, a change occurred in the scope and salience of Malacca’s engaged religions: old and established engaged religions such as popular temples and Christian churches began to embrace global and cosmopolitan approaches; and new global Buddhist philanthropic organizations emerged in the local religious landscape. Such changes parallel the modernization of religion in Malaysia as well as within the Chinese diaspora. The patterns of engaging the public good may provide a model for categorizing engaged religions in comparative studies. Based upon preliminary research in Malaysia, I propose two categories of engaged religions: provider and facilitator. The former directly provides goods and services whereas the later facilitates the act of contribution through networks and inspiration. I would try to bring in Putnam’s distinction of bridging and bonding, and discuss how the two types of social capital work with the two types of engaged religions.[[fileno]]204072607000
Thermodynamic Properties of the Exchange Interaction Model of Ferromagnetism
No full text[[abstract]]Terms through T-7 are obtained for the zero field-high temperature partition function series for general spin S and arbitrary lattice. For the f.c.c. lattice the fraction of the entropy change occurring above Tc is found to be (S∞-Sc)/S∞ = 0.494 - 0.353/(2S+1).[[fileno]]2010111010003[[department]]物理