3,641 research outputs found
Crystal structures and freezing of dipolar fluids
We investigate the crystal structure of classical systems of spherical
particles with an embedded point dipole at T=0. The ferroelectric ground state
energy is calculated using generalizations of the Ewald summation technique.
Due to the reduced symmetry compared to the nonpolar case the crystals are
never strictly cubic. For the Stockmayer (i.e., Lennard-Jones plus dipolar)
interaction three phases are found upon increasing the dipole moment:
hexagonal, body-centered orthorhombic, and body-centered tetragonal. An even
richer phase diagram arises for dipolar soft spheres with a purely repulsive
inverse power law potential . A crossover between qualitatively
different sequences of phases occurs near the exponent . The results are
applicable to electro- and magnetorheological fluids. In addition to the exact
ground state analysis we study freezing of the Stockmayer fluid by
density-functional theory.Comment: submitted to Phys. Rev.
The three-dimensional structure of the Eta Carinae Homunculus
We investigate, using the modeling code SHAPE, the three-dimensional
structure of the bipolar Homunculus nebula surrounding Eta Carinae, as mapped
by new ESO VLT/X-Shooter observations of the H2 micron
emission line. Our results reveal for the first time important deviations from
the axisymmetric bipolar morphology: 1) circumpolar trenches in each lobe
positioned point-symmetrically from the center and 2) off-planar protrusions in
the equatorial region from each lobe at longitudinal (~55 degrees) and
latitudinal (10-20 degrees) distances from the projected apastron direction of
the binary orbit. The angular distance between the protrusions (~110 degrees)
is similar to the angular extent of each polar trench (~130 degrees) and nearly
equal to the opening angle of the wind-wind collision cavity (~110 degrees). As
in previous studies, we confirm a hole near the centre of each polar lobe and
no detectable near-IR H2 emission from the thin optical skirt seen prominently
in visible imagery. We conclude that the interaction between the outflows
and/or radiation from the central binary stars and their orientation in space
has had, and possibly still has, a strong influence on the Homunculus. This
implies that prevailing theoretical models of the Homunculus are incomplete as
most assume a single star origin that produces an axisymmetric nebula. We
discuss how the newly found features might be related to the Homunculus
ejection, the central binary and the interacting stellar winds. We also include
a 3D printable version of our Homunculus model.Comment: 14 pages, 7 color figures, 1 interactive 3D figure (Figure 5,
requires Adobe Reader), published in MNRAS. A 3D printable version of our
Homunculus model can be downloaded from
http://svs.gsfc.nasa.gov/vis/a010000/a011500/a011568/Eta_Car_Homunuculus_3D_model.zip
or from the 'Supporting Information' link in the electronic version of the
MNRAS articl
Spectroscopic and photometric oscillatory envelope variability during the S Doradus outburst of the Luminous Blue Variable R71
To better understand the LBV phenomenon, we analyze multi-epoch and
multi-wavelength spectra and photometry of R71. Pre-outburst spectra are
analyzed with the radiative transfer code CMFGEN to determine the star's
fundamental stellar parameters. During quiescence, R71 has an effective
temperature of and a luminosity of
log = 5.78 and is thus a classical LBV, but at the lower
luminosity end of this group. We determine its mass-loss rate to yr. We present R71's spectral energy distribution
from the near-ultraviolet to the mid-infrared during its present outburst.
Mid-infrared observations suggest that we are witnessing dust formation and
grain evolution. Semi-regular oscillatory variability in the star's light curve
is observed during the current outburst. Absorption lines develop a second blue
component on a timescale twice that length. The variability may consist of one
(quasi-)periodic component with P ~ 425/850 d with additional variations
superimposed. During its current S Doradus outburst, R71 occupies a region in
the HR diagram at the high-luminosity extension of the Cepheid instability
strip and exhibits similar irregular variations as RV Tau variables. LBVs do
not pass the Cepheid instability strip because of core evolution, but they
develop comparable cool, low-mass, extended atmospheres in which convective
instabilities may occur. As in the case of RV Tau variables, the occurrence of
double absorption lines with an apparent regular cycle may be due to shocks
within the atmosphere and period doubling may explain the factor of two in the
lengths of the photometric and spectroscopic cycles.Comment: 18 pages, 14 figures, submitted to A&
Evaluation of precipitation measurement methods using data from a precision lysimeter network
Accurate precipitation data are essential for assessing the water balance of ecosystems. Methods for point precipitation determination are influenced by wind, precipitation type and intensity and/or technical issues. High-precision weighable lysimeters provide precipitation measurements at ground level that are less affected by wind disturbances and are assumed to be relatively close to actual precipitation. The problem in previous studies was that the biases in precipitation data introduced by different precipitation measurement methods were not comprehensively compared with and quantified on the basis of those obtained by lysimeters in different regions in Germany.
The aim was to quantify measurement errors in standard precipitation gauges
as compared to the lysimeter reference and to analyze the effect of
precipitation correction algorithms on the gauge data quality. Both correction methods rely on empirical constants to account for known external influences on the measurements, following a generic and a site-specific approach. Reference precipitation data were obtained from high-precision weighable lysimeters of the TERrestrial ENvironmental Observatories (TERENO)-SOILCan lysimeter network. Gauge types included tipping bucket gauges (TBs), weighable gauges (WGs), acoustic sensors (ASs) and optical laser disdrometers (LDs). From 2015-2018, data were collected at three locations in Germany, and 1 h aggregated values for precipitation above a threshold of 0.1 mm h−1 were compared.
The results show that all investigated measurement methods underestimated
the precipitation amounts relative to the lysimeter references for long-term
precipitation totals with catch ratios (CRs) of between 33 %–92 %. Data from ASs had overall biases of −0.25 to −0.07 mm h−1, while data from WGs and LDs showed the lowest measurement bias (−0.14 to −0.06 mm h−1 and −0.01 to −0.02 mm h−1). Two TBs showed systematic deviations with biases of −0.69 to −0.61 mm h−1, while other TBs were in the previously reported range with biases of −0.2 mm h−1. The site-specific and generic correction schemes reduced the hourly measurement bias by 0.13 and 0.08 mm h−1 for the TBs and by 0.09 and 0.07 mm h−1 for the WGs and increased long-term CRs by 14 % and 9 % and by 10 % and 11 %, respectively.
It could be shown that the lysimeter reference operated with minor uncertainties in long-term measurements under different site and weather
conditions. The results indicate that considerable precipitation measurement
errors can occur even at well-maintained and professionally operated stations equipped with standard precipitation gauges. This generally leads
to an underestimation of the actual precipitation amounts. The results
suggest that the application of relatively simple correction schemes, manual
or automated data quality checks, instrument calibrations, and/or an adequate
choice of observation period can help improve the data quality of
gauge-based measurements for water balance calculations, ecosystem modeling, water management, assessment of agricultural irrigation needs, or
radar-based precipitation analyses.</p
Inhomogeneous magnetization in dipolar ferromagnetic liquids
At high densities fluids of strongly dipolar spherical particles exhibit
spontaneous long-ranged orientational order. Typically, due to demagnetization
effects induced by the long range of the dipolar interactions, the
magnetization structure is spatially inhomogeneous and depends on the shape of
the sample. We determine this structure for a cubic sample by the free
minimization of an appropriate microscopic density functional using simulated
annealing. We find a vortex structure resembling four domains separated by four
domain walls whose thickness increases proportional to the system size L. There
are indications that for large L the whole configuration scales with the system
size. Near the axis of the mainly planar vortex structure the direction of the
magnetization escapes into the third dimension or, at higher temperatures, the
absolute value of the magnetization is strongly reduced. Thus the orientational
order is characterized by two point defects at the top and the bottom of the
sample, respectively. The equilibrium structure in an external field and the
transition to a homogeneous magnetization for strong fields are analyzed, too.Comment: 17 postscript figures included, submitted to Phys. Rev.
Ferromagnetic Liquid Thin Films Under Applied Field
Theoretical calculations, computer simulations and experiments indicate the
possible existence of a ferromagnetic liquid state, although definitive
experimental evidence is lacking. Should such a state exist, demagnetization
effects would force a nontrivial magnetization texture. Since liquid droplets
are deformable, the droplet shape is coupled with the magnetization texture. In
a thin-film geometry in zero applied field, the droplet has a circular shape
and a rotating magnetization texture with a point vortex at the center. We
calculate the elongation and magnetization texture of such ferromagnetic thin
film liquid droplet confined between two parallel plates under a weak applied
magnetic field. The vortex stretches into a domain wall and exchange forces
break the reflection symmetry. This behavior contrasts qualitatively and
quantitatively with the elongation of paramagnetic thin films.Comment: 10 pages, 4 figures, Submitted to Phys. Rev.
Constraining the Absolute Orientation of Eta Carinae's Binary Orbit: A 3-D Dynamical Model for the Broad [Fe III] Emission
We present a three-dimensional (3-D) dynamical model for the broad [Fe III]
emission observed in Eta Carinae using the Hubble Space Telescope/Space
Telescope Imaging Spectrograph (HST/STIS). This model is based on full 3-D
Smoothed Particle Hydrodynamics (SPH) simulations of Eta Car's binary colliding
winds. Radiative transfer codes are used to generate synthetic spectro-images
of [Fe III] emission line structures at various observed orbital phases and
STIS slit position angles (PAs). Through a parameter study that varies the
orbital inclination i, the PA {\theta} that the orbital plane projection of the
line-of-sight makes with the apastron side of the semi-major axis, and the PA
on the sky of the orbital axis, we are able, for the first time, to tightly
constrain the absolute 3-D orientation of the binary orbit. To simultaneously
reproduce the blue-shifted emission arcs observed at orbital phase 0.976, STIS
slit PA = +38 degrees, and the temporal variations in emission seen at negative
slit PAs, the binary needs to have an i \approx 130 to 145 degrees, {\theta}
\approx -15 to +30 degrees, and an orbital axis projected on the sky at a PA
\approx 302 to 327 degrees east of north. This represents a system with an
orbital axis that is closely aligned with the inferred polar axis of the
Homunculus nebula, in 3-D. The companion star, Eta B, thus orbits clockwise on
the sky and is on the observer's side of the system at apastron. This
orientation has important implications for theories for the formation of the
Homunculus and helps lay the groundwork for orbital modeling to determine the
stellar masses.Comment: 23 pages, 12 color figures, plus 2 online-only appendices (available
in the /anc folder of the Source directory). Accepted for publication in
MNRA
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