6,817 research outputs found
Saturation Effects and the Concurrency Hypothesis: Insights from an Analytic Model
Sexual partnerships that overlap in time (concurrent relationships) may play
a significant role in the HIV epidemic, but the precise effect is unclear. We
derive edge-based compartmental models of disease spread in idealized dynamic
populations with and without concurrency to allow for an investigation of its
effects. Our models assume that partnerships change in time and individuals
enter and leave the at-risk population. Infected individuals transmit at a
constant per-partnership rate to their susceptible partners. In our idealized
populations we find regions of parameter space where the existence of
concurrent partnerships leads to substantially faster growth and higher
equilibrium levels, but also regions in which the existence of concurrent
partnerships has very little impact on the growth or the equilibrium.
Additionally we find mixed regimes in which concurrency significantly increases
the early growth, but has little effect on the ultimate equilibrium level.
Guided by model predictions, we discuss general conditions under which
concurrent relationships would be expected to have large or small effects in
real-world settings. Our observation that the impact of concurrency saturates
suggests that concurrency-reducing interventions may be most effective in
populations with low to moderate concurrency
Optical properties of carbon grains: Influence on dynamical models of AGB stars
For amorphous carbon several laboratory extinction data are available, which
show quite a wide range of differences due to the structural complexity of this
material. We have calculated self-consistent dynamic models of circumstellar
dust-shells around carbon-rich asymptotic giant branch stars, based on a number
of these data sets. The structure and the wind properties of the dynamical
models are directly influenced by the different types of amorphous carbon. In
our test models the mass loss is not severely dependent on the difference in
the optical properties of the dust, but the influence on the degree of
condensation and the final outflow velocity is considerable. Furthermore, the
spectral energy distributions and colours resulting from the different data
show a much wider spread than the variations within the models due to the
variability of the star. Silicon carbide was also considered in the radiative
transfer calculations to test its influence on the spectral energy
distribution.Comment: 12 pages, 6 figures. To appear in A&
Reconsidering the origin of the 21 micron feature: Oxides in carbon-rich PPNe?
The origin of the so-called "21 micron" feature which is especially prominent
in the spectra of some carbon-rich protoplanetary nebulae (PPNe}) is the matter
of a lively debate. A large number of potential band carriers have been
presented and discarded within the past decade. The present paper gives an
overview of the problems related to the hitherto proposed feature
identifications, including the recently suggested candidate carrier silicon
carbide. We also discuss the case for spectroscopically promising oxides.
SiC is shown to produce a strong resonance band at 20-21 micron if coated by
a layer of silicon dioxide. At low temperatures, core-mantle particles composed
of SiC and amorphous SiO indeed have their strongest spectral signature at
a position of 20.1 micron, which coincides with the position of the "21 micron"
emission band.
The optical constants of another candidate carrier that has been relatively
neglected so far -- iron monoxide -- are proven to permit a fairly accurate
reproduction of the "21 micron" feature profile as well, especially when
low-temperature measurements of the infrared properties of FeO are taken into
account. As candidate carrier of the "21 micron" emission band, FeO has the
advantage of being stable against further oxidation and reduction only in a
narrow range of chemical and physical conditions, coinciding with the fact that
the feature, too, is detected in a small group of objects only. However, it is
unclear how FeO should form or survive particularly in carbon-rich PPNe.Comment: 28 pages, 15 figures, accepted for publication in ApJ (December
Stellar abundances and presolar grains trace the nucleosynthetic origin of molybdenum and ruthenium
This work presents a large consistent study of molybdenum (Mo) and ruthenium
(Ru) abundances in the Milky Way. These two elements are important
nucleosynthetic diagnostics. In our sample of 71 Galactic metal-poor field
stars, we detect Ru and/or Mo in 51 of these (59 including upper limits). The
sample consists of high-resolution, high signal-to-noise spectra covering both
dwarfs and giants from [Fe/H]=-0.63 down to -3.16. Thus we provide information
on the behaviour of Mo I and Ru I at higher and lower metallicity than is
currently known. We find a wide spread in the Mo and Ru abundances, which is
typical of heavy elements. This indicates that several formation processes, in
addition to high entropy winds, can be responsible for the formation of Mo and
Ru. The formation processes are traced by comparing Mo and Ru to elements (Sr,
Zr, Pd, Ag, Ba, and Eu) with known formation processes. We find contributions
from different formation channels, namely p-, slow (s-), and rapid (r-)
neutron-capture processes. Molybdenum is a highly convolved element that
receives contributions from several processes, whereas Ru is mainly formed by
the weak r-process as is silver. We also compare our absolute elemental stellar
abundances to relative isotopic abundances of presolar grains extracted from
meteorites. Their isotopic abundances can be directly linked to the formation
process (e.g. r-only isotopes) providing a unique comparison between
observationally derived abundances and the nuclear formation process. The
comparison to abundances in presolar grains shows that the r-/s-process ratios
from the presolar grains match the total elemental chemical composition derived
from metal-poor halo stars with [Fe/H]~ -1.5 to -1.1 dex. This indicates that
both grains and stars around and above [Fe/H]=-1.5 are equally (well) mixed and
therefore do not support a heterogeneous presolar nebula... Abridged.Comment: 18 pages, 12 figures, accepted by A&
Hong-Ou-Mandel-like two-droplet correlations
We present a numerical study of two-droplet pair correlations for in-phase
droplets walking on a vibrating bath. Two such walkers are launched towards a
common origin. As they approach, their carrier waves may overlap and the
droplets have a non-zero probability of forming a two-droplet bound state. The
likelihood of such pairing is quantified by measuring the probability of
finding the droplets in a bound state at late times. Three generic types of
two-droplet correlations are observed: promenading, orbiting and chasing pair
of walkers. For certain parameters, the droplets may become correlated for
certain initial path differences and remain uncorrelated for others, while in
other cases the droplets may never produce droplet pairs. These observations
pave the way for further studies of strongly correlated many-droplet behaviors
in the hydrodynamical quantum analogs of bouncing and walking droplets.Comment: 8 pages, 5 figure
From flux to dust mass: Does the grain-temperature distribution matter for estimates of cold dust masses in supernova remnants?
The amount of dust estimated from infrared to sub-millimetre (submm)
observations strongly depends on assumptions of different grain sizes,
compositions and optical properties. Here we use a simple model of thermal
emission from cold silicate/carbon dust at a range of dust grain temperatures
and fit the spectral energy distribution (SED) of the Crab Nebula as a test.
This can lower the derived dust mass for the Crab by ~50% and 30-40% for
astronomical silicates and amorphous carbon grains compared to recently
published values (0.25M_sun -> 0.12M_sun and 0.12M_sun -> 0.072M_sun,
respectively), but the implied dust mass can also increase by as much as almost
a factor of six (0.25M_sun -> 1.14M_sun and 0.12M_sun -> 0.71M_sun) depending
on assumptions regarding the sizes/temperatures of the coldest grains. The
latter values are clearly unrealistic due to the expected metal budget, though.
Furthermore, we show by a simple numerical experiment that if a cold-dust
component does have a grain-temperature distribution, it is almost unavoidable
that a two-temperature fit will yield an incorrect dust mass estimate. But we
conclude that grain temperatures is not a greater uncertainty than the often
poorly constrained emissivities (i.e., material properties) of cosmic dust,
although there is clearly a need for improved dust emission models. The
greatest complication associated with deriving dust masses still arises in the
uncertainty in the dust composition.Comment: 13 pages, 7 figures, to appear in MNRA
On the (in)variance of the dust-to-metals ratio in galaxies
Recent works have demonstrated a surprisingly small variation of the
dust-to-metals ratio in different environments and a correlation between dust
extinction and the density of stars. Naively, one would interpret these
findings as strong evidence of cosmic dust being produced mainly by stars. But
other observational evidence suggest there is a significant variation of the
dust-to-metals ratio with metallicity. As we demonstrate in this paper, a
simple star-dust scenario is problematic also in the sense that it requires
that destruction of dust in the interstellar medium (e.g., due to passage of
supernova shocks) must be highly inefficient. We suggest a model where stellar
dust production is indeed efficient, but where interstellar dust growth is
equally important and acts as a replenishment mechanism which can counteract
the effects of dust destruction. This model appears to resolve the seemingly
contradictive observations, given that the ratio of the effective (stellar)
dust and metal yields is not universal and thus may change from one environment
to another, depending on metallicity.Comment: 10 pages, 4 figures. Accepted for publication in MNRA
A Dynamically Diluted Alignment Model Reveals the Impact of Cell Turnover on the Plasticity of Tissue Polarity Patterns
The polarisation of cells and tissues is fundamental for tissue morphogenesis
during biological development and regeneration. A deeper understanding of
biological polarity pattern formation can be gained from the consideration of
pattern reorganisation in response to an opposing instructive cue, which we
here consider by example of experimentally inducible body axis inversions in
planarian flatworms. Our dynamically diluted alignment model represents three
processes: entrainment of cell polarity by a global signal, local cell-cell
coupling aligning polarity among neighbours and cell turnover inserting
initially unpolarised cells. We show that a persistent global orienting signal
determines the final mean polarity orientation in this stochastic model.
Combining numerical and analytical approaches, we find that neighbour coupling
retards polarity pattern reorganisation, whereas cell turnover accelerates it.
We derive a formula for an effective neighbour coupling strength integrating
both effects and find that the time of polarity reorganisation depends linearly
on this effective parameter and no abrupt transitions are observed. This allows
to determine neighbour coupling strengths from experimental observations. Our
model is related to a dynamic -Potts model with annealed site-dilution and
makes testable predictions regarding the polarisation of dynamic systems, such
as the planarian epithelium.Comment: Preprint as prior to first submission to Journal of the Royal Society
Interface. 25 pages, 6 figures, plus supplement (18 pages, contains 1 table
and 7 figures). A supplementary movie is available from
https://dx.doi.org/10.6084/m9.figshare.c388781
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