20,995 research outputs found
Risk Assessments for Chemical Stockpile Incinerators: Is the Supporting Guidance Adequate
The author identifies potential deficiencies in guidance used in the United States for risk assessments of chemical weapons incinerators
Identifying Dark Matter Burners in the Galactic center
If the supermassive black hole (SMBH) at the center of our Galaxy grew
adiabatically, then a dense "spike" of dark matter is expected to have formed
around it. Assuming that dark matter is composed primarily of weakly
interacting massive particles (WIMPs), a star orbiting close enough to the SMBH
can capture WIMPs at an extremely high rate. The stellar luminosity due to
annihilation of captured WIMPs in the stellar core may be comparable to or even
exceed the luminosity of the star due to thermonuclear burning. The model thus
predicts the existence of unusual stars, i.e. "WIMP burners", in the vicinity
of an adiabatically grown SMBH. We find that the most efficient WIMP burners
are stars with degenerate electron cores, e.g. white dwarfs (WD) or degenerate
cores with envelopes. If found, such stars would provide evidence for the
existence of particle dark matter and could possibly be used to establish its
density profile. In our previous paper we computed the luminosity from WIMP
burning for a range of dark matter spike density profiles, degenerate core
masses, and distances from the SMBH. Here we compare our results with the
observed stars closest to the Galactic center and find that they could be
consistent with WIMP burners in the form of degenerate cores with envelopes. We
also cross-check the WIMP burner hypothesis with the EGRET observed flux of
gamma-rays from the Galactic center, which imposes a constraint on the dark
matter spike density profile and annihilation cross-section. We find that the
EGRET data is consistent with the WIMP burner hypothesis. New high precision
measurements by GLAST will confirm or set stringent limits on a dark matter
spike at the Galactic center, which will in turn support or set stringent
limits on the existence of WIMP burners at the Galactic center.Comment: 2 pages, 2 figures; to appear in the Proc. of the First Int. GLAST
Symp. (Stanford, Feb. 5-8, 2007), eds. S.Ritz, P.F.Michelson, and C.Meegan,
AIP Conf. Pro
How severe is the time-inconsistency problem in monetary policy?
This study analyzes two monetary economies, a cash-credit good model and a limited-participation model. In these models, monetary policy is made by a benevolent policymaker who cannot commit to future policies. The study defines and analyzes Markov equilibrium in these economies and shows that there is no time-inconsistency problem for a wide range of parameter values.Monetary policy ; Inflation (Finance) ; Money supply
Expectation Traps and Monetary Policy
We describe a class of monetary economies that generate persistent episodes of high and low inflation. In this class of economies, variations in expectations can lead private agents to take actions which then make it optimal for the monetary authority to validate those expectations. We think these models deserve attention because they display several good empirical properties.Inflation Time-Consistency
Expectation traps and monetary policy
Why is inflation persistently high in some periods and low in others? The reason may be absence of commitment in monetary policy. In a standard model, absence of commitment leads to multiple equilibria, or expectation traps, even without trigger strategies. In these traps, expectations of high or low inflation lead the public to take defensive actions, which then make accommodating those expectations the optimal monetary policy. Under commitment, the equilibrium is unique and the inflation rate is low on average. This analysis suggests that institutions which promote commitment can prevent high inflation episodes from recurring.Monetary policy ; Inflation (Finance) ; Consumer behavior
Depression in small-vessel disease relates to white matter ultrastructural damage, not disability.
OBJECTIVE: To determine whether cerebral small-vessel disease (SVD) is a specific risk factor for depression, whether any association is mediated via white matter damage, and to study the role of depressive symptoms and disability on quality of life (QoL) in this patient group.
METHODS: Using path analyses in cross-sectional data, we modeled the relationships among depression, disability, and QoL in patients with SVD presenting with radiologically confirmed lacunar stroke (n = 100), and replicated results in a second SVD cohort (n = 100). We then compared the same model in a non-SVD stroke cohort (n = 50) and healthy older adults (n = 203). In a further study, to determine the role of white matter damage in mediating the association with depression, a subgroup of patients with SVD (n = 101) underwent diffusion tensor imaging (DTI).
RESULTS: Reduced QoL was associated with depression in patients with SVD, but this association was not mediated by disability or cognition; very similar results were found in the replication SVD cohort. In contrast, the non-SVD stroke group and the healthy older adult group showed a direct relationship between disability and depression. The DTI study showed that fractional anisotropy, a marker of white matter damage, was related to depressive symptoms in patients with SVD.
CONCLUSION: These results suggest that in stroke patients without SVD, disability is an important causal factor for depression, whereas in SVD stroke, other factors specific to this stroke subtype have a causal role. White matter damage detected on DTI is one factor that mediates the association between SVD and depression
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