1,731 research outputs found
Constraining Radiatively Inefficient Accretion Flows with Polarization
The low-luminosity black hole Sgr A* provides a testbed for models of
Radiatively Inefficient Accretion Flows (RIAFs). Recent sub-millimeter linear
polarization measurements of Sgr A* have provided evidence that the electrons
in the accretion flow are relativistic over a large range of radii. Here, we
show that these high temperatures result in elliptical plasma normal modes.
Thus, polarized millimeter and sub-millimeter radiation emitted within RIAFs
will undergo generalized Faraday rotation, a cyclic conversion between linear
and circular polarization. This effect will not depolarize the radiation even
if the rotation measure is extremely high. Rather, the beam will take on the
linear and circular polarization properties of the plasma normal modes. As a
result, polarization measurements of Sgr A* in this frequency regime will
constrain the temperature, density and magnetic profiles of RIAF models.Comment: 4 pages, 3 figures, accepted by ApJ Letter
Interpreting students' experiences with academic disappointments using resourcefulness scores as a lens
Most postsecondary students have to deal with academic disappointments at some point in time, with many of them succumbing to their anxieties and failing to learn from these lived experiences. Our study aimed to understand why and how disappointments unfolded in a sample of 20 undergraduate students, using a design whereby interview text was concurrently analyzed across the continuum of learned resourcefulness measured using Rosenbaum’s Self-Control Schedule in conjunction with an inductive, data-driven coding and theme-generation perspective. Reasons for attending university, attributional style, coping and learning, and perceptions of others markedly differed for high- and low- resourcefulness scorers. Whereas high-resourceful scorers used academic disappointments as a motivator to engage in more effort and problem-solving strategies, low scorers ruminated and tried to forget about them. Suggestions are provided on ways to effectively help students become more resourceful and in control of their studies
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
Bose Hubbard model in the presence of Ohmic dissipation
We study the zero temperature mean-field phase diagram of the Bose-Hubbard
model in the presence of local coupling between the bosons and an external
bath. We consider a coupling that conserves the on-site occupation number,
preserving the robustness of the Mott and superfluid phases. We show that the
coupling to the bath renormalizes the chemical potential and the interaction
between the bosons and reduces the size of the superfluid regions between the
insulating lobes. For strong enough coupling, a finite value of hopping is
required to obtain superfluidity around the degeneracy points where Mott phases
with different occupation numbers coexist. We discuss the role that such a bath
coupling may play in experiments that probe the formation of the
insulator-superfluid shell structure in systems of trapped atoms.Comment: 5 pages, 2 figures. Error found in v1, now corrected, leads to
qualitative changes in result
Following microscopic motion in a two dimensional glass-forming binary fluid
The dynamics of a binary mixture of large and small discs are studied at
temperatures approaching the glass transition using an analysis based on the
topology of the Voronoi polygon surrounding each atom. At higher temperatures
we find that dynamics is dominated by fluid-like motion that involves particles
entering and exiting the nearest-neighbour shells of nearby particles. As the
temperature is lowered, the rate of topological moves decreases and motion
becomes localised to regions of mixed pentagons and heptagons. In addition we
find that in the low temperature state particles may translate significant
distances without undergoing changes in their nearest neig hbour shell. These
results have implications for dynamical heterogeneities in glass forming
liquids.Comment: 12 pages, 7 figure
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Reductions in the dietary niche of southern sea otters (Enhydra lutris nereis) from the Holocene to the Anthropocene.
The sea otter (Enhydra lutris) is a marine mammal hunted to near extinction during the 1800s. Despite their well-known modern importance as a keystone species, we know little about historical sea otter ecology. Here, we characterize the ecological niche of ancient southern sea otters (E. lutris nereis) using δ13C analysis and δ15N analysis of bones recovered from archaeological sites spanning ~7,000 to 350 years before present (N = 112 individuals) at five regions along the coast of California. These data are compared with previously published data on modern animals (N = 165) and potential modern prey items. In addition, we analyze the δ15N of individual amino acids for 23 individuals to test for differences in sea otter trophic ecology through time. After correcting for tissue-specific and temporal isotopic effects, we employ nonparametric statistics and Bayesian niche models to quantify differences among ancient and modern animals. We find ancient otters occupied a larger isotopic niche than nearly all modern localities; likely reflecting broader habitat and prey use in prefur trade populations. In addition, ancient sea otters at the most southerly sites occupied an isotopic niche that was more than twice as large as ancient otters from northerly regions. This likely reflects greater invertebrate prey diversity in southern California relative to northern California. Thus, we suggest the potential dietary niche of sea otters in southern California could be larger than in central and northern California. At two sites, Año Nuevo and Monterey Bay, ancient otters had significantly higher δ15N values than modern populations. Amino acid δ15N data indicated this resulted from shifting baseline isotope values, rather than a change in sea otter trophic ecology. Our results help in better understanding the contemporary ecological role of sea otters and exemplify the strength of combing zooarchaeological and biological information to provide baseline data for conservation efforts
Sensitivity of the interlayer magnetoresistance of layered metals to intralayer anisotropies
Many of the most interesting and technologically important electronic
materials discovered in the past two decades have two common features: a
layered crystal structure and strong interactions between electrons. Two of the
most fundamental questions about such layered metals concern the origin of
intralayer anisotropies and the coherence of interlayer charge transport. We
show that angle dependent magnetoresistance oscillations (AMRO) are sensitive
to anisotropies around an intralayer Fermi surface. Hence, AMRO can be a probe
of intralayer anisotropies that is complementary to angle-resolved
photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM).
However, AMRO are not very sensitive to the coherence of the interlayer
transport. We illustrate this with comparisons to recent AMRO experiments on an
overdoped cuprate.Comment: 7 pages, 3 figure
Arnotts Blending Project
Established and supported under the Australian Government’s Cooperative Research Centre Progra
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