16,796 research outputs found
3-D Models of Embedded High-Mass Stars: Effects of a Clumpy Circumstellar Medium
We use 3-D radiative transfer models to show the effects of clumpy
circumstellar material on the observed infrared colors of high mass stars
embedded in molecular clouds. We highlight differences between 3-D clumpy and
1-D smooth models which can affect the interpretation of data. We discuss
several important properties of the emergent spectral energy distribution
(SED): More near-infrared light (scattered and direct from the central source)
can escape than in smooth 1-D models. The near- and mid-infrared SED of the
same object can vary significantly with viewing angle, depending on the clump
geometry along the sightline. Even the wavelength-integrated flux can vary with
angle by more than a factor of two. Objects with the same average circumstellar
dust distribution can have very different near-and mid-IR SEDs depending on the
clump geometry and the proximity of the most massive clump to the central
source.
Although clumpiness can cause similar objects to have very different SEDs,
there are some observable trends. Near- and mid-infrared colors are sensitive
to the weighted average distance of clumps from the central source and to the
magnitude of clumpy density variations (smooth-to-clumpy ratio). Far-infrared
emission remains a robust measure of the total dust mass. We present simulated
SEDs, colors, and images for 2MASS and Spitzer filters. We compare to
observations of some UCHII regions and find that 3-D clumpy models fit better
than smooth models. In particular, clumpy models with fractal dimensions in the
range 2.3-2.8, smooth to clumpy ratios of <50%, and density distributions with
shallow average radial density profiles fit the SEDs best.Comment: accepted to ApJ; version with full-res figures:
http://www.astro.virginia.edu/~ri3e/clumpy3d.pd
MacWilliams Identities for -tuple Weight Enumerators
Since MacWilliams proved the original identity relating the Hamming weight
enumerator of a linear code to the weight enumerator of its dual code there
have been many different generalizations, leading to the development of
-tuple support enumerators. We prove a generalization of theorems of Britz
and of Ray-Chaudhuri and Siap, which build on earlier work of Kl{\o}ve,
Shiromoto, Wan, and others. We then give illustrations of these -tuple
weight enumerators.Comment: 17 pages. Accepted to SIAM Journal on Discrete Mathematic
The tensor structure on the representation category of the triplet algebra
We study the braided monoidal structure that the fusion product induces on
the abelian category -mod, the category of representations of
the triplet -algebra . The -algebras are a
family of vertex operator algebras that form the simplest known examples of
symmetry algebras of logarithmic conformal field theories. We formalise the
methods for computing fusion products, developed by Nahm, Gaberdiel and Kausch,
that are widely used in the physics literature and illustrate a systematic
approach to calculating fusion products in non-semi-simple representation
categories. We apply these methods to the braided monoidal structure of
-mod, previously constructed by Huang, Lepowsky and Zhang, to
prove that this braided monoidal structure is rigid. The rigidity of
-mod allows us to prove explicit formulae for the fusion product
on the set of all simple and all projective -modules, which were
first conjectured by Fuchs, Hwang, Semikhatov and Tipunin; and Gaberdiel and
Runkel.Comment: 58 pages; edit: added references and revisions according to referee
reports. Version to appear on J. Phys.
Effects of the COVID-19 lockdown on mental health in a UK student sample
BACKGROUND: The COVID-19 pandemic and the resulting restrictions placed upon society have had a profound impact on both physical and mental health, particularly for young people. AIMS: The current study assesses the impact of COVID-19 on student mental health. METHOD: Four hundred and thirty four first year Undergraduate students completed a battery of self-report questionnaires (PHQ-P, GAD-7 and SAS-SV) to assess for depression, anxiety and mobile phone addiction respectively with data being collected over a 2 year period. The data from each year was compared (216 and 218 students respectively). RESULTS: A MANOVA revealed that COVID-19 had a significant impact on self-reported levels of depression, anxiety and smartphone addictionâwhich all significantly increased from the 2020 to the 2021 group. The percentage of students who had a score which warranted a classification of clinical depression increased from 30 to 44%, and for anxiety increased from 22 to 27%âthose students who showed a comorbidity across the two rose from 12 to 21%. Smartphone addiction levels rose from 39 to 50%. Correlational analysis showed a significant relationship between Smartphone usage and depression and anxiety. CONCLUSIONS: This research suggests that COVID-19 has had a major impact upon student mental health, and smartphone addiction. The importance of identifying predictive factors of depression and anxiety is emphasised, and suggestions for intervention are discussed
Photometry of SN 2002ic and Implications for the Progenitor Mass-Loss History
We present new pre-maximum and late-time optical photometry of the Type
Ia/IIn supernova 2002ic. These observations are combined with the published
V-band magnitudes of Hamuy et al. (2003) and the VLT spectrophotometry of Wang
et al. (2004) to construct the most extensive light curve to date of this
unusual supernova. The observed flux at late time is significantly higher
relative to the flux at maximum than that of any other observed Type Ia
supernova and continues to fade very slowly a year after explosion. Our
analysis of the light curve suggests that a non-Type Ia supernova component
becomes prominent days after explosion. Modeling of the non-Type Ia
supernova component as heating from the shock interaction of the supernova
ejecta with pre-existing circumstellar material suggests the presence of a
cm gap or trough between the progenitor system and the
surrounding circumstellar material. This gap could be due to significantly
lower mass-loss years prior to explosion or
evacuation of the circumstellar material by a low-density fast wind. The latter
is consistent with observed properties of proto-planetary nebulae and with
models of white-dwarf + asymptotic giant branch star progenitor systems with
the asymptotic giant branch star in the proto-planetary nebula phase.Comment: accepted for publication in Ap
2-D Radiative Transfer in Protostellar Envelopes: I. Effects of Geometry on Class I Sources
We present 2-D radiation transfer models of Class I Protostars and show the
effect of including more realistic geometries on the resulting spectral energy
distributions and images. We begin with a rotationally flattened infalling
envelope as our comparison model, and add a flared disk and bipolar cavity. The
disk affects the spectral energy distribution most strongly at edge-on
inclinations, causing a broad dip at about 10 um (independent of the silicate
feature) due to high extinction and low scattering albedo in this wavelength
region. The bipolar cavities allow more direct stellar+disk radiation to emerge
into polar directions, and more scattering radiation to emerge into all
directions. The wavelength-integrated flux, often interpreted as luminosity,
varies with viewing angle, with pole-on viewing angles seeing 2-4 times as much
flux as edge-on, depending on geometry. Thus, observational estimates of
luminosity should take into account the inclination of a source. The envelopes
with cavities are significantly bluer in near-IR and mid-IR color-color plots
than those without cavities. Using 1-D models to interpret Class I sources with
bipolar cavities would lead to an underestimate of envelope mass and an
overestimate of the implied evolutionary state. We compute images at near-,
mid-, and far-IR wavelengths. We find that the mid-IR colors and images are
sensitive to scattering albedo, and that the flared disk shadows the midplane
on large size scales at all wavelengths plotted. Finally, our models produce
polarization spectra which can be used to diagnose dust properties, such as
albedo variations due to grain growth. Our results of polarization across the
3.1 um ice feature agree well with observations for ice mantles covering 5% of
the radius of the grains.Comment: Accepted for publication in ApJ, 37 pages, 13 figures (several
figures reduced in quality; find original version at
http://gemelli.colorado.edu/~bwhitney/preprints.html
Nonequilibrium molecular dynamics simulation of rapid directional solidification
We present the results of non-equilibrium molecular dynamics simulations for
the growth of a solid binary alloy from its liquid phase. The regime of high
pulling velocities, , for which there is a progressive transition from
solute segregation to solute trapping, is considered. In the segregation
regime, we recover the exponential form of the concentration profile within the
liquid phase. Solute trapping is shown to settle in progressively as is
increased and our results are in good agreement with the theoretical
predictions of Aziz [J. Appl. Phys. {\bf 53}, 1158 (1981)]. In addition, the
fluid advection velocity is shown to remain directly proportional to , even
at the highest velocities considered here (ms).Comment: Submitted to Phys. Rev.
Van der Waals epitaxy of Bi2Se3 on Si(111) vicinal surface: An approach to prepare high-quality thin films of topological insulator
Epitaxial growth of topological insulator Bi2Se3 thin films on nominally flat
and vicinal Si(111) substrates is studied. In order to achieve planner growth
front and better quality epifilms, a two-step growth method is adopted for the
van der Waal epitaxy of Bi2Se3 to proceed. By employing vicinal Si(111)
substrate surfaces, the in-pane growth rate anisotropy of Bi2Se3 is explored to
achieve single crystalline Bi2Se3 epifilms, in which threading defects and
twins are effectively suppressed. Optimization of the growth parameters has
resulted in vicinal Bi2Se3 films showing a carrier mobility of ~ 2000 cm2V-1s-1
and the background doping of ~ 3 x 1018 cm-3 of the as-grown layers. Such
samples not only show relatively high magnetoresistance but also a linear
dependence on magnetic field.Comment: 18 pages, 4 figure
Exploring the Monero Peer-to-Peer Network
As of September 2019, Monero is the most capitalized privacy-
preserving cryptocurrency, and is ranked tenth among all cryptocurren-
cies. Moneroâs on-chain data privacy guarantees, i.e., how mixins are
selected in each transaction, have been extensively studied. However, de-
spite Moneroâs prominence, the network of peers running Monero clients
has not been analyzed. Such analysis is of prime importance, since po-
tential vulnerabilities in the peer-to-peer network may lead to attacks on
the blockchainâs safety (e.g., by isolating a set of nodes) and on usersâ
privacy (e.g., tracing transactions flow in the network).
This paper provides the first step study on understanding Moneroâs peer-
to-peer (P2P) network. In particular, we deconstruct Moneroâs P2P pro-
tocol based on its source code, and develop a toolset to explore Moneroâs
network, which allows us to infer its topology, size, node distribution,
and node connectivity. During our experiments, we collected 510 GB of
raw data, from which we extracted 21,678 IP addresses of Monero nodes
distributed in 970 autonomous systems. We show that Moneroâs network
is highly centralized â 13.2% of the nodes collectively maintain 82.86%
of the network connections. We have identified approximately 2,758 ac-
tive nodes per day, which is 68.7% higher than the number reported by
the MoneroHash mining pool. We also identified all concurrent outgoing
connections maintained by Monero nodes with very high probability (on
average 97.98% for nodes with less than 250 outgoing connections, and
93.79% for nodes with more connections)
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