Space shuttle OMS helium regulator design and development

Analysis, design, fabrication and design verification testing was conducted on the technological feasiblity of the helium pressurization regulator for the space shuttle orbital maneuvering system application. A prototype regulator was fabricated which was a single-stage design featuring the most reliable and lowest cost concept. A tradeoff study on regulator concepts indicated that a single-stage regulator with a lever arm between the valve and the actuator section would offer significant weight savings. Damping concepts were tested to determine the amount of damping required to restrict actuator travel during vibration. Component design parameters such as spring rates, effective area, contamination cutting, and damping were determined by test prior to regulator final assembly. The unit was subjected to performance testing at widely ranging flow rates, temperatures, inlet pressures, and random vibration levels. A test plan for propellant compatibility and extended life tests is included

Spectroscopic test of Bose-Einstein statistics for photons

Using Bose-Einstein-statistics-forbidden two-photon excitation in atomic barium, we have limited the rate of statistics-violating transitions, as a fraction $\nu$ of an equivalent statistics-allowed transition rate, to $\nu<4.0\times10^{-11}$ at the 90% confidence level. This is an improvement of more than three orders of magnitude over the best previous result. Additionally, hyperfine-interaction enabling of the forbidden transition has been observed, to our knowledge, for the first time

Analytical formula for the Uehling potential

The closed analytical expression for the Uehling potential is derived. The Uehling potential describes the lowest-order correction on vacuum polarisation in atomic and muon-atomic systems. We also derive the analytical formula for the interaction potential between two electrically charged point particles which includes correction to the vacuum polarisation, but has correct asymptotic behaviour at larger $r$. Our three-term analytical formula for the Uehling potential opens a new avenue in the study of the vacuum polarisation in light atomic systems.Comment: arXiv admin note: substantial text overlap with arXiv:1103.204

Flows, Fragmentation, and Star Formation. I. Low-mass Stars in Taurus

The remarkably filamentary spatial distribution of young stars in the Taurus molecular cloud has significant implications for understanding low-mass star formation in relatively quiescent conditions. The large scale and regular spacing of the filaments suggests that small-scale turbulence is of limited importance, which could be consistent with driving on large scales by flows which produced the cloud. The small spatial dispersion of stars from gaseous filaments indicates that the low-mass stars are generally born with small velocity dispersions relative to their natal gas, of order the sound speed or less. The spatial distribution of the stars exhibits a mean separation of about 0.25 pc, comparable to the estimated Jeans length in the densest gaseous filaments, and is consistent with roughly uniform density along the filaments. The efficiency of star formation in filaments is much higher than elsewhere, with an associated higher frequency of protostars and accreting T Tauri stars. The protostellar cores generally are aligned with the filaments, suggesting that they are produced by gravitational fragmentation, resulting in initially quasi-prolate cores. Given the absence of massive stars which could strongly dominate cloud dynamics, Taurus provides important tests of theories of dispersed low-mass star formation and numerical simulations of molecular cloud structure and evolution.Comment: 32 pages, 9 figures: to appear in Ap

On the interaction of a single-photon wave packet with an excited atom

The interaction of a single-photon wave packet with an initially excited two-level atom in free space is studied in semiclassical and quantum approaches. It is shown that the final state of the field does not contain doubly occupied modes. The process of the atom's transition to the ground state may be accelerated, decelerated or even reversed by the incoming photon, depending on parameters. The spectrum of emitted radiation is close to the sum of the spectrum of the incoming single-photon wave packet and the natural line shape, with small and complicated deviations.Comment: 17 pages, 5 figure

Posterior Cingulate and Lateral Parietal Gray Matter Volume in Older Adults with Depressive Symptoms

Depressive symptoms occurring late in life are an important risk factor for Alzheimer’s disease (AD). The latest research finds that onset of depressive symptoms in late life may herald the development of AD, not only for amnestic Mild Cognitive Impairment (aMCI) patients, but also for cognitively-normal older adults. Neuroimaging of brain structure, blood flow, and glucose metabolism indicates that depressive symptoms in late life are accompanied by structural and functional changes in limbic brain regions vulnerable to AD. The present cross-sectional study was guided by the hypothesis that compared to their non-depressed counterparts, older adults with mild to moderate depressive symptoms have less volume in limbic structures vulnerable to changes in AD—specifically, cortical midline structures such as anterior cingulate and posterior cingulate cortex as well as mesial temporal regions such as bilateral hippocampi and amygdalae. Consistent with our hypothesis, results of a voxel-based morphometry analysis revealed smaller retrosplenial, posterior cingulate, and precuneus gray matter volumes in depressed individuals relative to healthy controls. Right lateral parietal cortex—another region vulnerable to change in AD—was also smaller in the group with depressive symptoms. Contrary to our hypothesis, no volumetric differences were found in the anterior cingulate cortex or mesial temporal lobe. Results of this study show a relationship between geriatric depressive symptoms and brain volume in regions vulnerable to AD. Follow-up of participants over time will tell if brain changes detected here predict development of AD

Lung cancer in lifetime nonsmoking men – results of a case-control study in Germany

Epidemiological studies of lung cancer among nonsmoking men are few. This case–control study was conducted among lifetime nonsmoking men between 1990 and 1996 in Germany to examine lung cancer risk in relation to occupation, environmental tobacco smoke, residential radon, family history of cancer and previous lung disease. A total of 58 male cases with confirmed primary lung cancer and 803 male population controls who had never smoked more than 400 cigarettes in their lifetime were personally interviewed by a standardized questionnaire. In addition, 1-year radon measurements in the living and bedroom of the subjects' last dwelling were carried out. Unconditional logistic regression was used to calculate odds ratios (OR) and 95% confidence intervals (CI). Having ever worked in a job with known lung carcinogens was associated with a two-fold significantly increased lung cancer risk (OR = 2.2; Cl = 1.0–5.0), adjusted for age and region. The linear trend test for lung-cancer risk associated with radon exposure was close to statistical significance, demonstrating an excess relative risk for an increase in exposure of 100 Bq m−3 of 0.43 (P = 0.052). Nonsignificantly elevated effects of exposure to environmental tobacco smoke in public transportation and in social settings were observed. No associations with a family history of cancer or previous lung diseases were found. Our results indicate that occupational carcinogens and indoor radon may play a role in some lung cancers in nonsmoking men. © 2001 Cancer Research Campaign http://www.bjcancer.co

Turbulent Flow-Driven Molecular Cloud Formation: A Solution to the Post-T Tauri Problem?

We suggest that molecular clouds can be formed on short time scales by compressions from large scale streams in the interstellar medium (ISM). In particular, we argue that the Taurus-Auriga complex, with filaments of 10-20 pc $\times$ 2-5 pc, most have been formed by H I flows in $\lesssim 3$Myr, explaining the absence of post-T Tauri stars in the region with ages $\gtrsim 3$ Myr. Observations in the 21 cm line of the H I `halos' around the Taurus molecular gas show many features (broad asymmetric profiles, velocity shifts of H I relative to $^{12}$CO) predicted by our MHD numerical simulations, in which large-scale H I streams collide to produce dense filamentary structures. This rapid evolution is possible because the H I flows producing and disrupting the cloud have much higher velocities (5-10 kms) than present in the molecular gas resulting from the colliding flows. The simulations suggest that such flows can occur from the global ISM turbulence without requiring a single triggering event such as a SN explosion.Comment: 26 pages, 12 ps figures. Apj accepte