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A SYSTOLIC ALGORITHM FOR EXTRACTING REGIONS FROM A PLANAR GRAPH
No full text[[abstract]]In this paper, we describe a systolic algorithm for extracting all of the fundamental regions in a planar graph. It takes O(n) computation time and uses O(n) processing elements, where n is the number of edges of the input planar graph.[[fileno]]2030256010084[[department]]資訊工程學
Nd-Sr isotopic composition and geochemistry of sediments from Taiwan and their implications
No full text[[fileno]]2010101010030[[department]]物理
Stability and dissolution of heavy minerals in the Neogene-Pleistocene sandstones from western foothills, Taiwan
No full text[[abstract]]Stability and dissolution of heavy minerals were studied in the sandstones from the Western Foothills, Taiwan, where the continuous sequence from Miocene to Late Pleistocene develops. On the basis of modal proportions of heavy minerals in sandstones, it is concluded that many heavy minerals have been dissolved even in Late Pliocene sandstones. Stable minerals throughout the sequence are zircon, tourmaline, rutile, monazite, apatite and spinel. Ilmenite was dissolved in the Early Miocene sandstone. Dissolutions of aluminous silicates such as chloritoid, staurolite and Al_2SiO_5 polymorphs are observed in the Middle Miocene sandstones. Epidote, titanite and grossular-andradite garnet are more unstable minerals. Chemical analyses of detrital almandine garnets revealed that garnet with grossular component more than 10% was selectively dissolved in the Early Miocene sandstone. It is important for provenance study at first to recognize the selective dissolution of heavy minerals in the sandstones.[[fileno]]2010101010039[[department]]物理
The Role of Magnetic Fields in the Protostellar Accretion Phase
No full text[[abstract]]We summarize recent work addressing the role of magnetic fields in the process of star formation and disk accretion. After a short review of the basic observational results, we concentrate on the efficiency of magnetic braking during cloud collapse and its consequences on the formation of centrifugally supported disks around young stars. Then, we relate this issue to the well-known magnetic flux problem of star formation, and we show that the introduction of non-ideal MHD effects is a necessary step toward the development of self-consistent models for the collapse of molecular clouds and the formation of disks. Finally, we discuss the structure and evolution of magnetized accretion disks around young stars that have dragged their magnetic field in the phase of gravitational collapse and we focus on two main diffusive processes: viscous stresses that redistribute mass and angular momentum, and the resistive diffusion of mass across magnetic field lines.[[fileno]]2010118030001[[department]]物理
The challenge of sub-Keplerian rotation for disk winds
Get PDF[[abstract]]Strong magnetization makes the disks surrounding young stellar objects rotate at rates that are too sub-Keplerian to enable the thermal launching of disk winds from their surfaces unless the rate of gas diffusion across field lines is dynamically fast. This underappreciated implication of disk magnetization poses a considerable challenge for disk-wind theory.[[fileno]]2010118010007[[department]]物理
Nonaxisymmetric neutral modes in relativistic disks
Get PDF[[abstract]]We perform a linear stability analysis of the axisymmetric, relativistic, self-similar, isothermal disk against nonaxisymmetric perturbations. Two sets of neutral modes are discovered. The first set corresponds to marginally unstable perturbations driven by gravitational radiation, and the other signals the onset of bifurcation to nonaxisymmetric equilibrium solutions to the Einstein equations.[[fileno]]2010118010017[[department]]物理
Singular isothermal disks. I. Linear stability analysis
Get PDF[[abstract]]As part of st larger effort to understand how binary and single stars form from the collapse of magnetized molecular cloud cores, we perform a global stability analysis of isopedically magnetized, singular isothermal disks (SIDs). The work described here has precedents in earlier studies of disturbances in power-law disks by Zang in 1976, Toomre in 1977, Lynden-Bell & Lemos in 1993, Syer & Tremaine in 1996, and Goodman & Evans in 1999. We find the analytic criteria for the bifurcation of axisymmetric disks into nonaxisymmetric forms with azimuthal periodicities m = I (eccentric displacements), 2 (oval distortions), 3 (triangular distortions), etc. These bifurcations, which occur at zero frequency, are the compressible and differentially rotating analogs of how the classical sequence of incompressible and uniformly rotating Maclaurin spheroids bifurcate (secularly, under dissipative forces) to become Dedekind ellipsoids with figure axes that remain fixed in space. Like Syer & Tremaine and Lynden-Bell & Lemos, Ne also find that zero-frequency logarithmic spirals are possible scale-free disturbances, but our interpretation of the existence of such steadily propagating wavetrains is different. We give a dynamical instability interpretation based on the onset of swing amplification by overreflection at the corotation circle of prograde spiral density waves the pattern speeds of which have nonzero and positive values. Our analysis yields identical instability criteria as the global normal-modes treatment of Goodman & Evans, and we tentatively also identify dynamical barred-spiral instabilities as the "breathing mode" limit of two-armed ordinary-spiral instabilities. We prove a general "reciprocity theorem," which states that the overreflection factors are identical for spiral density waves launched from cavities interior or exterior to Q-barriers that straddle the corotation circle. This globally valid result supports a unifying interpretation, advocated for many years by C. C. Lin and his colleagues (see, e.g., work by Bertin gr Lin): the coexistence of spiral structure in galaxies arising from the instability of internal normal modes in the combined star/gas disk or from driving by external tidal influences associated with the chance passages of companion bodies.[[fileno]]2010118010041[[department]]物理
BENDING WAVES IN SATURNS RINGS
No full text[[abstract]]We investigate certain brightness variations seen in Saturn's A ring and find them to be due to vertical corrugations of the local ring plane caused by a spiral bending wave. This wave is resonantly excited by Mimas and propagates inward via the collective gravity of the ring particles. B. A. Smith et al. [Science212, 163–191 (1981)] had previously associated vertical relief with this feature due to its observed azimuthal variations and its proximity to an inclination resonance with Mimas. We develop the theory of forced bending waves, some aspects of which have been treated in the galactic context by C. Hunter and A. Toomre [Astrophys. J.155, 747–776 (1969)] and by G. Bertin and J.W.-K. Mark [Astron. Astrophys.88, 289–297 (1980)]. Our theory is in good agreement with the observations. In particular, the presence of these bending waves may resolve the conflict between ground-based estimates of 1–2 km for the global ring thickness [e.g., A. Brahic and B. Sicardy, Nature289, 447–450 (1981)] and Voyager stellar occultation measurements of <200 m for the local ring thickness [A. L. Lane et al., Science215, 537–543 (1982); E. A. Marouf and G. L. Tyler, Science217, 243–245 (1982)].[[fileno]]2010118010101[[department]]物理
THE EVOLUTION OF PROTOSTARS .1. GLOBAL FORMULATION AND RESULTS
No full text[[abstract]]A new formulation for the problem of protostar evolution is developed on the basis of a method that involves the division of the global problem into a set of more manageable subproblems. Jump conditions of the radiative accretion shock which joins the hydrostatic mass-gaining core to the dynamic inner cloud envelope are derived. Standard integration schemes yield the high accuracy needed to resolve complex spatial structures which span many orders of magnitude in density and temperature. It is shown that a 1-solar-mass protostar ends its main accretion phase moderately high up in the H-R diagram; this star begins its pre-main-sequence phase of quasi-static contraction on a convective Hayashi track.
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