2,041 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
Modelling the Warm H2 Infrared Emission of the Helix Nebula Cometary Knots
Molecular hydrogen emission is commonly observed in planetary nebulae. Images
taken in infrared H2 emission lines show that at least part of the molecular
emission is produced inside the ionised region. In the best-studied case, the
Helix nebula, the H2 emission is produced inside cometary knots (CKs),
comet-shaped structures believed to be clumps of dense neutral gas embedded
within the ionised gas. Most of the H2 emission of the CKs seems to be produced
in a thin layer between the ionised diffuse gas and the neutral material of the
knot, in a mini photodissociation region (PDR). However, PDR models published
so far cannot fully explain all the characteristics of the H2 emission of the
CKs. In this work, we use the photoionisation code \textsc{Aangaba} to study
the H2 emission of the CKs, particularly that produced in the interface H^+/H^0
of the knot, where a significant fraction of the H2 1-0S(1) emission seems to
be produced. Our results show that the production of molecular hydrogen in such
a region may explain several characteristics of the observed emission,
particularly the high excitation temperature of the H2 infrared lines. We find
that the temperature derived from H2 observations even of a single knot, will
depend very strongly on the observed transitions, with much higher temperatures
derived from excited levels. We also proposed that the separation between the
H_alpha and NII peak emission observed in the images of CKs may be an effect of
the distance of the knot from the star, since for knots farther from the
central star the NII line is produced closer to the border of the CK than
H_alpha.Comment: Accepted by MNRAS. The paper contains 12 pages, 9 figures, and 3
table
Interference scheme to measure light-induced nonlinearities in Bose-Einstein condensates
Light-induced nonlinear terms in the Gross-Pitaevskii equation arise from the
stimulated coherent exchange of photons between two atoms. For atoms in an
optical dipole trap this effect depends on the spatial profile of the trapping
laser beam. Two different laser beams can induce the same trapping potential
but very different nonlinearities. We propose a scheme to measure light-induced
nonlinearities which is based on this observation.Comment: 2 figure
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
Terahertz spectroscopy of electromagnons in Eu_{1-x}Y_xMnO_3
Dielectric permittivity spectra of yttrium-doped EuMnO in the composition
range 0 =< x =< 0.5 have been investigated in the terahertz frequency range.
Magnetoelectric contributions to the permittivity were observed in all
compositions for ac electric fields parallel to the crystallographic a-axis.
Well defined electromagnons exist for x >= 0.2 close to \nu ~ 20 cm^{-1} and
with dielectric strength strongly increasing on doping. In addition to
electromagnons, a broad contribution of magnetoelectric origin is observed for
all compositions. For Eu_{0.8}Y_{0.2}MnO_3 the electromagnons can be suppressed
by external magnetic fields which induce a canted antiferromagnetic phase.
Magnetoelectric effects in the different doping regimes are discussed in
detail.Comment: 7 pages, 9 figures include
A Chandra Search for Coronal X Rays from the Cool White Dwarf GD 356
We report observations with the Chandra X-ray Observatory of the single,
cool, magnetic white dwarf GD 356. For consistent comparison with other X-ray
observations of single white dwarfs, we also re-analyzed archival ROSAT data
for GD 356 (GJ 1205), G 99-47 (GR 290 = V1201 Ori), GD 90, G 195-19 (EG250 = GJ
339.1), and WD 2316+123 and archival Chandra data for LHS 1038 (GJ 1004) and GD
358 (V777 Her). Our Chandra observation detected no X rays from GD 356, setting
the most restrictive upper limit to the X-ray luminosity from any cool white
dwarf -- L_{X} < 6.0 x 10^{25} ergs/s, at 99.7% confidence, for a 1-keV
thermal-bremsstrahlung spectrum. The corresponding limit to the electron
density is n_{0} < 4.4 x 10^{11} cm^{-3}. Our re-analysis of the archival data
confirmed the non-detections reported by the original investigators. We discuss
the implications of our and prior observations on models for coronal emission
from white dwarfs. For magnetic white dwarfs, we emphasize the more stringent
constraints imposed by cyclotron radiation. In addition, we describe (in an
appendix) a statistical methodology for detecting a source and for constraining
the strength of a source, which applies even when the number of source or
background events is small.Comment: 27 pages, 4 figures, submitted to the Astrophysical Journa
A Wide-Field Survey of the Orion Nebula Cluster in the Near-Infrared
We present J, H and K photometry of the Orion Nebula Cluster obtained at the
CTIO/Blanco 4 m telescope in Cerro Tololo with the ISPI imager. From the
observations we have assembled a catalog of about 7800 sources distributed over
an area of approximately 30'x40', the largest of any survey deeper than 2MASS
in this region. The catalog provides absolute coordinates accurate to about
0.15 arcseconds and 3sigma photometry in the 2MASS system down to J 19.5mag, H
18.0mag, K 18.5mag, enough to detect planetary size objects 1 Myr old under Av
10mag of extinction at the distance of the Orion Nebula. We present a
preliminary analysis of the catalog, done comparing the (J-H, H-K) color-color
diagram, the (H, J-H) and (K, H-K) color-magnitude diagrams and the JHK
luminosity functions of three regions at increasing projected distance from the
Trapezium. Sources in the inner region typically show IR colors compatible with
reddened T Tauri stars, whereas the outer fields are dominated by field stars
seen through an amount of extinction which decreases with the distance from the
center. The color-magnitude diagrams make it possible to clearly distinguish
between the main ONC population, spread across the full field, and background
sources. The luminosity functions of the inner region, corrected for
completeness, remain relatively flat in the sub-stellar regime regardless of
the strategy adopted to remove background contamination.Comment: Astronomical Journal, Accepted Oct. 1, 200
Electromagnetic field angular momentum in condensed matter systems
Various electromagnetic systems can carry an angular momentum in their {\bf
E} and {\bf B} fields. The electromagnetic field angular momentum (EMAM) of
these systems can combine with the spin angular momentum to give composite
fermions or composite bosons. In this paper we examine the possiblity that an
EMAM could provide an explanation of the fractional quantum Hall effect (FQHE)
which is complimentary to the Chern-Simons explanation. We also examine a toy
model of a non-BCS superconductor (e.g. high superconductors) in terms of
an EMAM. The models presented give a common, simple picture of these two
systems in terms of an EMAM. The presence of an EMAM in these systems might be
tested through the observation of the decay modes of a charged, spin zero
unstable particle inside one of these systems.Comment: 17 pages, no figures, to be published in Phys. Rev.
Oscillations of rotating trapped Bose-Einstein condensates
The tensor-virial method is applied for a study of oscillation modes of
uniformly rotating Bose-Einstein condensed gases, whose rigid body rotation is
supported by an vortex array. The second order virial equations are derived in
the hydrodynamic regime for an arbitrary external harmonic trapping potential
assuming that the condensate is a superfluid at zero temperature. The
axisymmetric equilibrium shape of the condensate is determined as a function of
the deformation of the trap; its domain of stability is bounded by the
constraint on the rotation rate (measured in units of the trap
frequency .) The oscillations of the axisymmetric condensate are
stable with respect to the transverse-shear, toroidal and quasi-radial modes of
oscillations, corresponding to the , surface
deformations. In non-axisymmetric traps, the equilibrium constrains the
(dimensionless) deformation in the plane orthogonal to the rotation to the
domain with . The second harmonic oscillation modes
in non-axisymmetric traps separate into two classes which have even or odd
parity with respect to the direction of the rotation axis. Numerical solutions
show that these modes are stable in the parameter domain where equilibrium
figures exist.Comment: 16 pages, including 4 figures, uses Revtex; v2 includes a treatment
of modes in unisotropic traps; PRA in pres
Using vignette methodology as a tool for exploring cultural identity positions of language brokers
This paper examines how vignette methodology can aid understanding of cultural identity. This is demonstrated through a study of child language brokers where a child is engaged in the cultural contexts of both the host culture and the home culture and must therefore negotiate new cultural identities. Participants were young people aged 15-18 years; some of whom were brokers while others were not. Drawing on notions of adequacy and inadequacy, visibility and invisibility, theoretical ideas around cultural identity theory and dialogical self theory can provide an understanding of how the young people moved through different (often conflicting) identity positions
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