2,080 research outputs found
Orbital magnetoelectric coupling at finite electric field
We extend the band theory of linear orbital magnetoelectric coupling to treat
crystals under finite electric fields. Previous work established that the
orbital magnetoelectric response of a generic insulator at zero field comprises
three contributions that were denoted as local circulation, itinerant
circulation, and Chern-Simons. We find that the expression for each of them is
modified by the presence of a dc electric field. Remarkably, the sum of the
three correction terms vanishes, so that the total coupling is still given by
the same formula as at zero field. This conclusion is confirmed by numerical
tests on a tight-binding model, for which we calculate the field-induced change
in the linear magnetoelectric coefficient.Comment: 4 pages, 2 figure
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
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
Explosive Disintegration of a Massive Young Stellar System in Orion
Young massive stars in the center of crowded star clusters are expected to
undergo close dynamical encounters that could lead to energetic, explosive
events. However, there has so far never been clear observational evidence of
such a remarkable phenomenon. We here report new interferometric observations
made with the Submillimeter Array (SMA) that indicate the well known enigmatic
wide-angle outflow located in the Orion BN/KL star-forming region to have been
produced by such a violent explosion during the disruption of a massive young
stellar system, and that this was caused by a close dynamical interaction about
500 years ago. This outflow thus belongs to a totally different family of
molecular flows which is not related to the classical bipolar flows that are
generated by stars during their formation process. Our molecular data allow us
to create a 3D view of the debris flow and to link this directly to the well
known Orion H "fingers" farther outComment: Accepted by ApJ Letters The 3D movie can be found in:
ftp://ftp.mpifr-bonn.mpg.de/outgoing/lzapata/movie.gi
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
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
MERLIN radio detection of an interaction zone within a binary Orion proplyd system
Presented here are high angular resolution MERLIN 5 GHz (6 cm) continuum
observations of the binary proplyd system, LV 1 in the Orion nebula, which
consists of proplyd 168--326SE and its binary proplyd companion 168--326NW
(separation 0.4 arcsec). Accurate astrometric alignment allows a detailed
comparison between these data and published HST PC Halpha and [Oiii] images.
Thermal radio sources coincide with the two proplyds and originate in the
ionized photoevaporating flows seen in the optical emission lines. Flow
velocities of approx 50 km/s from the ionized proplyd surfaces and \geq 100
km/s from a possible micro-jet have been detected using the Manchester Echelle
spectrometer.
A third radio source is found to coincide with a region of extended, high
excitation, optical line emission that lies between the binary proplyds
168--326SE/326NW . This is modelled as a bowshock due to the collision of the
photoevaporating flows from the two proplyds. Both a thermal and a non-thermal
origin for the radio emission in this collision zone are considered.Comment: 23 pages, 9 figures, accepted by Ap
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
Abstract
AimsPatients with spinal cord injury (SCI) are at risk of developing renal calculi. This study describes the management of renal calculi among patients with SCI with attention to factors influencing surgical management vs observation.MethodsThis retrospective, cohort study identified patients with SCI and renal calculi between 2009 to 2016 from an institutional neurogenic bladder database and detailed the management of their stones. A stone episode was defined as radiographic evidence of new calculi.ResultsOf 205 patients with SCI, 34 had renal stones, for a prevalence of 17%. The mean age was 50 years (range 22,77) and most had cervical SCI (nâ=â22, 65%). There were 41 stone episodes with 98 individual stones identified with a mean stone size of 4.9âmm (range 1â19).Of the 41 episodes, 10 (24%) underwent surgery after initial diagnosis. Pain was the most common primary indication for surgery (nâ=â9, 60%). The median time from diagnosis to intervention for all patients was 4 months (interquartile range 1,23). Of the 41 episodes, 31 (76%) were initially observed and among these, 5 ultimately required surgery (16%) while 26 (84%) did not. Of these 26, 12 (46%) stones passed spontaneously and 14 (53%) remained unchanged. The need for surgery correlated with more stone episodes (Pâ=â.049).ConclusionIn this cohort of patients with SCI and small, nonobstructing renal stones, 76% (nâ=â31) were offered observation. Of these observed patients, 84% (nâ=â26) did not require further intervention at a median of 4 years of followâup.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151315/1/nau24091.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151315/2/nau24091_am.pd
Towards the field binary population: Influence of orbital decay on close binaries
Surveys of the binary populations in the solar neighbourhood have shown that
the periods of G- and M-type stars are log-normally distributed. However,
observations of young binary populations suggest a log-uniform distribution.
Clearly some process(es) change the period distribution over time. Most stars
form in star clusters, in which two important dynamical processes occur: i)
gas-induced orbital decay of embedded binary systems and ii) destruction of
soft binaries in three-body interactions. The emphasis here is on orbital decay
which has been largely neglected so far. Using a combination of Monte-Carlo and
dynamical nbody modelling it is demonstrated here that the cluster dynamics
destroys the number of wide binaries, but leaves short-period binaries
basically undisturbed even for a initially log-uniform distribution. By
contrast orbital decay significantly reduces the number and changes the
properties of short-period binaries, but leaves wide binaries largely
uneffected. Until now it was unclear whether the short period distribution of
the field is unaltered since its formation. It is shown here, that orbital
decay is a prime candidate for such a task. In combination the dynamics of
these two processes, convert an initial log-uniform distribution to a
log-normal period distribution. The probability is 94% that the evolved and
observed period distribution were sampled from the same parent distribution.
This means binaries can be formed with periods that are sampled from the
log-uniform distribution. As the cluster evolves, short-period binaries are
merged to single stars by the gas-induced orbital decay while the dynamical
evolution in the cluster destroys wide binaries. The combination of these two
equally important processes reshapes a initial log-uniform period distribution
to the log-normal period distribution, that is observed in the field
(abridged).Comment: 9 pages, 9 figure
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