2,022 research outputs found
Probabilistic computer model of optimal runway turnoffs
Landing delays are currently a problem at major air carrier airports and many forecasters agree that airport congestion will get worse by the end of the century. It is anticipated that some types of delays can be reduced by an efficient optimal runway exist system allowing increased approach volumes necessary at congested airports. A computerized Probabilistic Runway Turnoff Model which locates exits and defines path geometry for a selected maximum occupancy time appropriate for each TERPS aircraft category is defined. The model includes an algorithm for lateral ride comfort limits
Aircraft and avionic related research required to develop an effective high-speed runway exit system
Research was conducted to increase airport capacity by studying the feasibility of the longitudinal separation between aircraft sequences on final approach. The multidisciplinary factors which include the utility of high speed exits for efficient runway operations were described along with recommendations and highlights of these studies
Ferromagnetic redshift of the optical gap in GdN
We report measurements of the optical gap in a GdN film at temperatures from
300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is
1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split
bands below the Curie temperature. The paramagnetic gap is larger than was
suggested by very early experiments, and has permitted us to refine a
(LSDA+U)-computed band structure. The band structure was computed in the full
translation symmetry of the ferromagnetic ground state, assigning the
paramagnetic-state gap as the average of the majority- and minority-spin gaps
in the ferromagnetic state. That procedure has been further tested by a band
structure in a 32-atom supercell with randomly-oriented spins. After fitting
only the paramagnetic gap the refined band structure then reproduces our
measured gaps in both phases by direct transitions at the X point.Comment: 5 pages, 4 figure
Tracer particles for core-collapse supernova nucleosynthesis: The advantages of moving backward
After decades, the theoretical study of core-collapse supernova explosions is
moving from parameterized, spherically symmetric models to increasingly
realistic multi-dimensional simulations. Obtaining nucleosynthesis yields based
on such multi-dimensional core-collapse supernova (CCSN) simulations, however,
is not straightforward and frequently tracer particles are employed. Tracer
particles may be tracked in situ during the simulation, but often they are
reconstructed in a post-processing step based on the information saved during
the hydrodynamics simulation. Reconstruction can be done in a number of ways
and here we compare the approaches of backward and forward integration of the
equations of motion to the results based on inline particle trajectories. We
find that both methods agree reasonably well with the inline results for
isotopes for which a large number of particles contribute. However, for rarer
isotopes that are produced only by a small number of particle trajectories,
deviations can be large. For our setup, we find that backward integration leads
to a better agreement with the inline particles by more accurately reproducing
the conditions following freeze-out from nuclear statistical equilibrium,
because the establishment of nuclear statistical equilibrium erases the need
for detailed trajectories at earlier times. Based on our results, if inline
tracers are unavailable, we recommend backward reconstruction, to the point
when nuclear statistical equilibrium last applied, with an interval between
simulation snapshots of at most 1 ms for nucleosynthesis post-processing.Comment: 23 pages, 18 figures, sumitted to Ap
Field Blue Stragglers and Related Mass Transfer Issues
This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts.Comment: Chapter 4, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Harmonic generation in ring-shaped molecules
We study numerically the interaction between an intense circularly polarized
laser field and an electron moving in a potential which has a discrete
cylindrical symmetry with respect to the laser pulse propagation direction.
This setup serves as a simple model, e.g., for benzene and other aromatic
compounds. From general symmetry considerations, within a Floquet approach,
selection rules for the harmonic generation [O. Alon Phys. Rev. Lett. 80 3743
(1998)] have been derived recently. Instead, the results we present in this
paper have been obtained solving the time-dependent Schroedinger equation ab
initio for realistic pulse shapes. We find a rich structure which is not always
dominated by the laser harmonics.Comment: 15 pages including 7 figure
A One-Health environmental risk assessment of contaminants of emerging concern in London’s waterways throughout the SARS-CoV-2 pandemic
The SARS-CoV-2 pandemic had huge impacts on global urban populations, activity and health, yet little is known about attendant consequences for urban river ecosystems. We detected significant changes in occurrence and risks from contaminants of emerging concern (CECs) in waterways across Greater London (UK) during the pandemic. We were able to rapidly identify and monitor large numbers of CECs in n=390 samples across 2019–2021 using novel direct-injection liquid chromatography-mass spectrometry methods for scalable targeted analysis, suspect screening and prioritisation of CEC risks. At total of 10,029 measured environmental concentrations (MECs) were obtained for 66 unique CECs. Pharmaceutical MECs decreased during lockdown in 2020 in the R. Thames (p≤0.001), but then increased significantly in 2021 (p ≤0.01). For the tributary rivers, the R. Lee, Beverley Brook, R. Wandle and R. Hogsmill were the most impacted primarily via wastewater treatment plant effluent and combined sewer overflows. For the R. Hosgmill in particular, pharmaceutical MEC trends were generally correlated with NHS prescription statistics, likely reflecting limited wastewater dilution. Suspect screening of ∼1,200 compounds tentatively identified 25 additional CECs at the five impacted sites, including metabolites such as O-desmethylvenlafaxine, an EU Watch List compound. Lastly, risk quotients (RQs) ≥0.1 were calculated for 21 compounds across the whole Greater London freshwater catchment, of which 7 were of medium risk (RQ ≥1.0) and three were in the high-risk category (RQ ≥10), including imidacloprid (RQ=19.6), azithromycin (15.7) and diclofenac (10.5). This is the largest spatiotemporal dataset of its kind for any major capital city globally and the first for Greater London, representing ∼16 % of the population of England, and delivering a foundational One Health case study in the third largest city in Europe across a global pandemic
First Space-Based Microlens Parallax Measurement: Spitzer Observations of OGLE-2005-SMC-001
We combine Spitzer and ground-based observations to measure the microlens
parallax of OGLE-2005-SMC-001, the first such space-based determination since
S. Refsdal proposed the idea in 1966. The parallax measurement yields a
projected velocity \tilde v ~ 230 km/s, the typical value expected for halo
lenses, but an order of magnitude smaller than would be expected for lenses
lying in the Small Magellanic Cloud (SMC) itself. The lens is a weak (i.e.,
non-caustic-crossing) binary, which complicates the analysis considerably but
ultimately contributes additional constraints. Using a test proposed by Assef
et al. (2006), which makes use only of kinematic information about different
populations but does not make any assumptions about their respective mass
functions, we find that the likelihood ratio is L_halo/L_SMC = 20. Hence, halo
lenses are strongly favored but SMC lenses are not definitively ruled out.
Similar Spitzer observations of additional lenses toward the Magellanic Clouds
would clarify the nature of the lens population. The Space Interferometry
Mission could make even more constraining measurements.Comment: ApJ, in press. Text and figures are updated to match the journal
versio
On the decay of deformed actinide nuclei
decay through a deformed potential barrier produces significant
mixing of angular momenta when mapped from the nuclear interior to the outside.
Using experimental branching ratios and either semi-classical or
coupled-channels transmission matrices, we have found that there is a set of
internal amplitudes which are essentially constant for all even--even actinide
nuclei. These same amplitudes also give good results for the known anisotropic
particle emission of the favored decays of odd nuclei in the same mass
region.
PACS numbers: 23.60.+e, 24.10.Eq, 27.90.+bComment: 5 pages, latex (revtex style), 2 embedded postscript figures
uuencoded gz-compressed .tar file To appear in Physical Review Letter
Comprehensive study of the magnetic stars HD 5797 and HD 40711 with large chromium and iron overabundances
We present the results of a comprehensive study of the chemically peculiar
stars HD 5797 and HD 40711. The stars have the same effective temperature, Teff
= 8900 K, and a similar chemical composition with large iron (+1.5 dex) and
chromium (+3 dex) overabundances compared to the Sun. The overabundance of
rare-earth elements typically reaches +3 dex. We have measured the magnetic
field of HD 5797. The longitudinal field component Be has been found to vary
sinusoidally between -100 and +1000 G with a period of 69 days. Our estimate of
the evolutionary status of the stars suggests that HD 5797 and HD 40711, old
objects with an age t \approx 5 \times 108 yr, are near the end of the core
hydrogen burning phase.Comment: 26 pages, 5 Encapsulated Postscript figure
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