The electronic structure of atoms and diatomic hydrides

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A global picture of quantum de Sitter space

Perturbative gravity about a de Sitter background motivates a global picture of quantum dynamics in `eternal de Sitter space,' the theory of states which are asymptotically de Sitter to both future and past. Eternal de Sitter physics is described by a finite dimensional Hilbert space in which each state is precisely invariant under the full de Sitter group. This resolves a previously-noted tension between de Sitter symmetry and finite entropy. Observables, implications for Boltzmann brains, and Poincare recurrences are briefly discussed.Comment: 17 pages, 1 figure. v2: minor changes, references added. v3: minor changes to correspond to PRD versio

Magnetic resonance volume flow and jet velocity mapping in aortic coarctation

AbstractObjectives. Nuclear magnetic resonance (MRI) velocity mapping was used to characterize flow waveforms and to measure volume flow in the ascending and descending thoracic aorta in patients with aortic coarctation and in healthy volunteers. We present the method and discuss the relation between these measurements and aortic narrowing assessed by MRI. Finally, we compare coarctation jet velocity measured by MRI velocity mapping with that obtained from continuous wave Doppler echocardiography.Background. The development of a noninvasive imaging method for morphologic visuslization of aortic coarctation and for measurement of its impact on blood flow is highly desirable in the preoperative and postoperative management of patients.Methods. Magnetic resonance imaging phase-shift velocity mapping was used to measure ascending and descending aortic volume flow in 39 patients with aortic coarctation and in 12 healthy volunteers. Magnetic resonance imaging was also used for anatomic and peak jet velocity measurements. The latter were compared with those available from continuous wave Doppler study in 40% of the patients.Results. Whereas ascending aortic volume flow measurement did not show significant differences between the patient and healthy control groups, volume flow curves in the descending aorta did show significant differences between the two groups. Peak volume flow (mean ± SD) was 10.6 ± 5.3 liters/min in patients and 19.6 ± 4.7 liters/min in control subjects (p < 0.001). Time-averaged flow was 2.5 ± 0.9 liters/min in patients and 3.9 ±1.1 liters/min in control subjects (p < 0.05). The descending/ ascending aorta flow ratio was 0.47 ± 0.19 in patients and 0.64 ±0.08 in control subjects (p < 0.05). These variables correlate well with the degree of aortic narrowing. Peak coarctation jet velocity measured by MRI velocity mapping is comparable to that obtained from continuous wave Doppler study (r = 0.95).Conclusions. We established normal ranges for volume flow in the descending aorta and demonstrated abnormalities in patients with aortic coarctation. These abnormalities are likely to be related to resistance to flow imposed by the coarctation and could represent an additional index for monitoring patients before and after intervention

Selective aortic arch perfusion: a first-in-human observational cadaveric study.

BACKGROUND: Selective aortic arch perfusion (SAAP) is a novel endovascular technique that combines thoracic aortic occlusion with extracorporeal perfusion of the brain and heart. SAAP may have a role in both haemorrhagic shock and in cardiac arrest due to coronary ischaemia. Despite promising animal studies, no data is available that describes SAAP in humans. The primary aim of this study was to assess the feasibility of selective aortic arch perfusion in humans. The secondary aim of the study was to assess the feasibility of achieving direct coronary artery access via the SAAP catheter as a potential conduit for salvage percutaneous coronary intervention. METHODS: Using perfused human cadavers, a prototype SAAP catheter was inserted into the descending aorta under fluoroscopic guidance via a standard femoral percutaneous access device. The catheter balloon was inflated and the aortic arch perfused with radio-opaque contrast. The coronary arteries were cannulated through the SAAP catheter. RESULTS: The procedure was conducted four times. During the first two trials the SAAP catheter was passed rapidly and without incident to the intended descending aortic landing zone and aortic arch perfusion was successfully delivered via the device. The SAAP catheter balloon failed on the third trial. On the fourth trial the left coronary system was cannulated using a 5Fr coronary guiding catheter through the central SAAP catheter lumen. CONCLUSIONS: For the first time using a perfused cadaveric model we have demonstrated that a SAAP catheter can be easily and safely inserted and SAAP can be achieved using conventional endovascular techniques. The SAAP catheter allowed successful access to the proximal aorta and permitted retrograde perfusion of the coronary and cerebral circulation

The effect of an embedded solid block on the onset of convection in a porous cavity

Purpose – The purpose of this paper is to determine how the presence of an embedded, centrally placed, solid but heat-conducting block affects the onset and development of Darcy-Bénard convection. Design/methodology/approach – Steady solutions are obtained using finite difference methods with SOR as the smoother. A detailed presentation is given of how the interface conditions are modelled, and how a continuity of pressure argument is used to determine the value of the streamfunction on the solid block. Findings – The presence of the block affects strongly both the onset of convection and the nonlinear properties such as the mean Nusselt number and the strength of the fluid circulation. The smallest possible critical Darcy-Rayleigh is found to be 22.0152, which is smaller than 4π2, the value when the block is absent. Research limitations/implications – The Darcy-Rayleigh number is restricted to values at or below 200, which is five times the critical value without a solid block, but the size and conductivity of the block vary between all admissible values. Originality/value – This is the first investigation of the effect of internal obstacles on Darcy-Bénard convection. </jats:sec

Faint High Latitude Carbon Stars Discovered by the Sloan Digital Sky Survey: Methods and Initial Results

We report the discovery of 39 Faint High Latitude Carbon Stars (FHLCs) from Sloan Digital Sky Survey commissioning data. The objects, each selected photometrically and verified spectroscopically, range over 16.6 < r* < 20.0, and show a diversity of temperatures as judged by both colors and NaD line strengths. At the completion of the Sloan Survey, there will be many hundred homogeneously selected and observed FHLCs in this sample. We present proper motion measures for each object, indicating that the sample is a mixture of extremely distant (>100 kpc) halo giant stars, useful for constraining halo dynamics, plus members of the recently-recognized exotic class of very nearby dwarf carbon (dC) stars. Motions, and thus dC classification, are inferred for 40-50 percent of the sample, depending on the level of statistical significance invoked. The new list of dC stars presented here, although selected from only a small fraction of the final SDSS, doubles the number of such objects found by all previous methods. (Abstract abridged).Comment: Accepted for publication in The Astronomical Journal, Vol. 124, Sep. 2002, 40 pages, 7 figures, AASTeX v5.

Natural Flood Management Through Peatland Restoration: : Catchment-Scale Modeling of Past and Future Scenarios in Glossop, UK

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