1,822 research outputs found
Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium
Orion's Veil is an absorbing screen that lies along the line of sight to the
Orion H II region. It consists of two or more layers of gas that must lie
within a few parsecs of the Trapezium cluster. Our previous work considered the
Veil as a whole and found that the magnetic field dominates the energetics of
the gas in at least one component. Here we use high-resolution STIS UV spectra
that resolve the two velocity components in absorption and determine the
conditions in each. We derive a volume hydrogen density, 21 cm spin
temperature, turbulent velocity, and kinetic temperature, for each. We combine
these estimates with magnetic field measurements to find that magnetic energy
significantly dominates turbulent and thermal energies in one component, while
the other component is close to equipartition between turbulent and magnetic
energies. We observe molecular hydrogen absorption for highly excited v, J
levels that are photoexcited by the stellar continuum, and detect blueshifted S
III and P III. These ions must arise from ionized gas between the mostly
neutral portions of the Veil and the Trapezium and shields the Veil from
ionizing radiation. We find that this layer of ionized gas is also responsible
for He I absorption in the Veil, which resolves a 40-year-old debate on the
origin of He I absorption towards the Trapezium. Finally, we determine that the
ionized and mostly atomic layers of the Veil will collide in less than 85,000
years.Comment: 43 pages, 15 figures, to be published in Ap
Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lying between
us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I
absorption and in optical and UV absorption lines of H I and other species.
Toward the Trapezium, the veil has two remarkable properties, high magnetic
field (~100 microGauss) and a surprising lack of molecular hydrogen given its
total hydrogen column density. Here we compute photoionization models of the
veil to establish its gas density and its distance from the Trapezium. We use a
greatly improved model of the hydrogen molecule that determines level
populations in ~1e5 rotational/vibrational levels and provides improved
estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our
best fit photoionization models place the veil 1-3 pc in front of the star at a
density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of
non-thermal motions in at least one of the 21cm H I velocity components.
Therefore, the veil is the first interstellar environment where magnetic
dominance appears to exist. We find that the low ratio of molecular to atomic
hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due
to its proximity to the Trapezium stars and the absence of small grains in the
region.Comment: 45 pages, 20 figures, accepted for publication in Ap
Self-Binding Transition in Bose Condensates with Laser-Induced ``Gravitation''
In our recent publication (D. O'Dell, et al, Phys. Rev. Lett. 84, 5687
(2000)) we proposed a scheme for electromagnetically generating a self-bound
Bose-Einstein condensate with 1/r attractive interactions: the analog of a Bose
star. Here we focus upon the conditions neccessary to observe the transition
from external trapping to self-binding. This transition becomes manifest in a
sharp reduction of the condensate radius and its dependence on the laser
intensity rather that the trap potential.Comment: 5 pages, 2 figures: slightly enhanced text: more explanatio
Macroscopic Quantum Tunneling of Ferromagnetic Domain Walls
Quantum tunneling of domain walls out of an impurity potential in a
mesoscopic ferromagnetic sample is investigated. Using improved expressions for
the domain wall mass and for the pinning potential, we find that the cross-over
temperature between thermal activation and quantum tunneling is of a different
functional form than found previously. In materials like Ni or YIG, the
crossover temperatures are around 5 mK. We also find that the WKB exponent is
typically two orders of magnitude larger than current estimates. The sources
for these discrepancies are discussed, and precise estimates for the transition
from three-dimensional to one-dimensional magnetic behavior of a wire are
given. The cross-over temperatures from thermal to quantum transitions and
tunneling rates are calculated for various materials and sample sizes.Comment: 10 pages, 2 postscript figures, REVTe
Fermi systems with long scattering lengths
Ground state energies and superfluid gaps are calculated for degenerate Fermi
systems interacting via long attractive scattering lengths such as cold atomic
gases, neutron and nuclear matter. In the intermediate region of densities,
where the interparticle spacing is longer than the range of the
interaction but shorter than the scattering length, the superfluid gaps and the
energy per particle are found to be proportional to the Fermi energy and thus
differs from the dilute and high density limits. The attractive potential
increase linearly with the spin-isospin or hyperspin statistical factor such
that, e.g., symmetric nuclear matter undergoes spinodal decomposition and
collapses whereas neutron matter and Fermionic atomic gases with two hyperspin
states are mechanically stable in the intermediate density region. The regions
of spinodal instabilities in the resulting phase diagram are reduced and do not
prevent a superfluid transition.Comment: extended and revised version, 7 pages including new phase diagra
Quantum Vacuum Contribution to the Momentum of the Dielectric Media
Momentum transfer between matter and electromagnetic field is analyzed. The
related equations of motion and conservation laws are derived using
relativistic formalism. Their correspondence to various, at first sight
self-contradicting, experimental data (the so called Abraham-Minkowski
controversy) is demonstrated. A new, Casimir like, quantum phenomenon is
predicted: contribution of vacuum fluctuations to the motion of dielectric
liquids in crossed electric and magnetic fields. Velocities about can
be expected due to the contribution of high frequency vacuum modes
Extended axion electrodynamics: Optical activity induced by nonstationary dark matter
We establish a new self-consistent Einstein-Maxwell-axion model based on the
Lagrangian, which is linear in the pseudoscalar (axion) field and its
four-gradient and includes the four-vector of macroscopic velocity of the axion
system as a whole. We consider extended equations of the axion electrodynamics,
modified gravity field equations, and discuss nonstationary effects in the
phenomenon of optical activity induced by axions.Comment: 6 pages, 0 figures, accepted for publication in the Journal
Gravitation and Cosmology, reported at the 14th Russian Gravitational
Conference (Ulyanovsk, 2011
Bivariate genetic modelling of the response to an oral glucose tolerance challenge: A gene x environment interaction approach
AIMS/HYPOTHESIS: Twin and family studies have shown the importance of genetic factors influencing fasting and 2 h glucose and insulin levels. However, the genetics of the physiological response to a glucose load has not been thoroughly investigated. METHODS: We studied 580 monozygotic and 1,937 dizygotic British female twins from the Twins UK Registry. The effects of genetic and environmental factors on fasting and 2 h glucose and insulin levels were estimated using univariate genetic modelling. Bivariate model fitting was used to investigate the glucose and insulin responses to a glucose load, i.e. an OGTT. RESULTS: The genetic effect on fasting and 2 h glucose and insulin levels ranged between 40% and 56% after adjustment for age and BMI. Exposure to a glucose load resulted in the emergence of novel genetic effects on 2 h glucose independent of the fasting level, accounting for about 55% of its heritability. For 2 h insulin, the effect of the same genes that already influenced fasting insulin was amplified by about 30%. CONCLUSIONS/INTERPRETATION: Exposure to a glucose challenge uncovers new genetic variance for glucose and amplifies the effects of genes that already influence the fasting insulin level. Finding the genes acting on 2 h glucose independently of fasting glucose may offer new aetiological insight into the risk of cardiovascular events and death from all causes
Robust isothermal electric switching of interface magnetization: A route to voltage-controlled spintronics
Roughness-insensitive and electrically controllable magnetization at the
(0001) surface of antiferromagnetic chromia is observed using magnetometry and
spin-resolved photoemission measurements and explained by the interplay of
surface termination and magnetic ordering. Further, this surface in placed in
proximity with a ferromagnetic Co/Pd multilayer film. Exchange coupling across
the interface between chromia and Co/Pd induces an electrically controllable
exchange bias in the Co/Pd film, which enables a reversible isothermal (at room
temperature) shift of the global magnetic hysteresis loop of the Co/Pd film
along the magnetic field axis between negative and positive values. These
results reveal the potential of magnetoelectric chromia for spintronic
applications requiring non-volatile electric control of magnetization.Comment: Single PDF file: 27 pages, 6 figures; version of 12/30/09; submitted
to Nature Material
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