5,530 research outputs found
Constraint Damping in First-Order Evolution Systems for Numerical Relativity
A new constraint suppressing formulation of the Einstein evolution equations
is presented, generalizing the five-parameter first-order system due to Kidder,
Scheel and Teukolsky (KST). The auxiliary fields, introduced to make the KST
system first-order, are given modified evolution equations designed to drive
constraint violations toward zero. The algebraic structure of the new system is
investigated, showing that the modifications preserve the hyperbolicity of the
fundamental and constraint evolution equations. The evolution of the
constraints for pertubations of flat spacetime is completely analyzed, and all
finite-wavelength constraint modes are shown to decay exponentially when
certain adjustable parameters satisfy appropriate inequalities. Numerical
simulations of a single Schwarzschild black hole are presented, demonstrating
the effectiveness of the new constraint-damping modifications.Comment: 11 pages, 5 figure
Rare earth element deposition in pelagic sediment at the Cenomanian‐Turonian Boundary, Exmouth Plateau
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95314/1/grl8050.pd
The role of genes, stress, and dopamine in the development of schizophrenia
The dopamine hypothesis is the longest standing pathoetiologic theory of schizophrenia. Because it was initially based on indirect evidence and findings in patients with established schizophrenia, it was unclear what role dopamine played in the onset of the disorder. However, recent studies in people at risk of schizophrenia have found elevated striatal dopamine synthesis capacity and increased dopamine release to stress. Furthermore, striatal dopamine changes have been linked to altered cortical function during cognitive tasks, in line with preclinical evidence that a circuit involving cortical projections to the striatum and midbrain may underlie the striatal dopamine changes. Other studies have shown that a number of environmental risk factors for schizophrenia, such as social isolation and childhood trauma, also affect presynaptic dopaminergic function. Advances in preclinical work and genetics have begun to unravel the molecular architecture linking dopamine, psychosis, and psychosocial stress. Included among the many genes associated with risk of schizophrenia are the gene encoding the dopamine D2 receptor and those involved in the upstream regulation of dopaminergic synthesis, through glutamatergic and gamma-aminobutyric acidergic pathways. A number of these pathways are also linked to the stress response. We review these new lines of evidence and present a model of how genes and environmental factors may sensitize the dopamine system so that it is vulnerable to acute stress, leading to progressive dysregulation and the onset of psychosis. Finally, we consider the implications for rational drug development, in particular regionally selective dopaminergic modulation, and the potential of genetic factors to stratify patients
Late Tertiary history of hydrothermal deposition at the East Pacific Rise, 19°S: Correlation to volcano‐tectonic events
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95132/1/grl3614.pd
Geochemistry of rare earth elements in pacific hydrothermal sediments
The concentrations and mass accumulation rates of rare earth elements (REE) in hydrothermal sediments from two Pacific sites support earlier predictions that hydrothermal REE inputs are significant to the overall marine REE mass balance. Although REE are enriched in hydrothermal vent fluids, they are rapidly and extensively scavenged from the water column by Fe oxyhydroxides and undergo some degree of fractionation prior to removal. Evidence for significant light REE (LREE) hydrothermal inputs tends to be masked by the fact that the LREE patterns and inter-element ratios of both proximal and distal hydrothermal precipitates are similar to those of seawater. However, the heavy REE (HREE) patterns clearly reflect a basaltic source and the HREE ratios from both sites are similar to each other but distinct from their seawater analogs. This suggests HREE ratios potentialy could be used as proxy indicators in our attempts to reconstruct the history of seafloor hydrothermal activity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27113/1/0000105.pd
REE/Fe variations in hydrothermal sediments: Implications for the REE content of seawater
Seafloor hydrothermal vent solutions exhibit rare earth element (REE) enrichments ranging between one to three orders of magnitude greater than average seawater. To assess the impact of these hydrothermal inputs on ocean chemistry, we have examined the behavior of REEs for hydrothermal sediments collected adjacent to two Pacific spreading ridge sites: the East Pacific Rise at 19[deg]S, and the Southern Juan de Fuca Ridge at 4S[deg]N. In general, the REE/Fe ratios for both proximal and distal hydrothermal sediments are greater than vent solutions by a factor of 2 to 500, and these ratios increase with increasing distance away from the ridge axis. An evaluation of these results in the context of previous models of REE behavior indicates that, in fact, seawater experiences a net depletion in REEs as a result of hydrothermal activity. This is due primarily to the large scavenging capacity of iron oxyhydroxides which precipitate from these solutions. Such an interpretation explains why the REE content of seawater collected in the vicinity of hydrothermal vents is anomalously lower than "normal" seawater sampled from a comparable depth.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28043/1/0000482.pd
Metalliferous sediments and the scavenging residence time of Nd near hydrothermal vents
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94674/1/grl5993.pd
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