891 research outputs found
The impact of spin temperature fluctuations on the 21-cm moments
This paper considers the impact of Lyman-alpha coupling and X-ray heating on
the 21-cm brightness-temperature one-point statistics (as predicted by
semi-numerical simulations). The X-ray production efficiency is varied over
four orders of magnitude and the hardness of the X-ray spectrum is varied from
that predicted for high-mass X-ray binaries, to the softer spectrum expected
from the hot inter-stellar medium. We find peaks in the redshift evolution of
both the variance and skewness associated with the efficiency of X-ray
production. The amplitude of the variance is also sensitive to the hardness of
the X-ray SED. We find that the relative timing of the coupling and heating
phases can be inferred from the redshift extent of a plateau that connects a
peak in the variance's evolution associated with Lyman-alpha coupling to the
heating peak. Importantly, we find that late X-ray heating would seriously
hamper our ability to constrain reionization with the variance. Late X-ray
heating also qualitatively alters the evolution of the skewness, providing a
clean way to constrain such models. If foregrounds can be removed, we find that
LOFAR, MWA and PAPER could constrain reionization and late X-ray heating models
with the variance. We find that HERA and SKA (phase 1) will be able to
constrain both reionization and heating by measuring the variance using
foreground-avoidance techniques. If foregrounds can be removed they will also
be able to constrain the nature of Lyman-alpha coupling.Comment: 16 pages, 13 figure, 1 table. Accepted for publication in MNRA
Distinguishing models of reionization using future radio observations of 21-cm 1-point statistics
We explore the impact of reionization topology on 21-cm statistics. Four
reionization models are presented which emulate large ionized bubbles around
over-dense regions (21CMFAST/ global-inside- out), small ionized bubbles in
over-dense regions (local-inside-out), large ionized bubbles around under-dense
regions (global-outside-in) and small ionized bubbles around under-dense
regions (local-outside-in). We show that first-generation instruments might
struggle to distinguish global models using the shape of the power spectrum
alone. All instruments considered are capable of breaking this degeneracy with
the variance, which is higher in outside-in models. Global models can also be
distinguished at small scales from a boost in the power spectrum from a
positive correlation between the density and neutral-fraction fields in
outside-in models. Negative skewness is found to be unique to inside-out models
and we find that pre-SKA instruments could detect this feature in maps smoothed
to reduce noise errors. The early, mid and late phases of reionization imprint
signatures in the brightness-temperature moments, we examine their model
dependence and find pre-SKA instruments capable of exploiting these timing
constraints in smoothed maps. The dimensional skewness is introduced and is
shown to have stronger signatures of the early and mid-phase timing if the
inside-out scenario is correct.Comment: 18 pages, 13 figures, updated to agree with published versio
21-cm signatures of residual HI inside cosmic HII regions during reionization
We investigate the impact of sinks of ionizing radiation on the
reionization-era 21-cm signal, focusing on 1-point statistics. We consider
sinks in both the intergalactic medium and inside galaxies. At a fixed filling
factor of HII regions, sinks will have two main effects on the 21-cm
morphology: (i) as inhomogeneous absorbers of ionizing photons they result in
smaller and more widespread cosmic HII patches; and (ii) as reservoirs of
neutral gas they contribute a non-zero 21-cm signal in otherwise ionized
regions. Both effects damp the contrast between neutral and ionized patches
during reionization, making detection of the epoch of reionization with 21-cm
interferometry more challenging. Here we systematically investigate these
effects using the latest semi-numerical simulations. We find that sinks
dramatically suppress the peak in the redshift evolution of the variance,
corresponding to the midpoint of reionization. As previously predicted,
skewness changes sign at midpoint, but the fluctuations in the residual HI
suppress a late-time rise. Furthermore, large levels of residual HI
dramatically alter the evolution of the variance, skewness and power spectrum
from that seen at lower levels. In general, the evolution of the large-scale
modes provides a better, cleaner, higher signal-to-noise probe of reionization.Comment: Minor edits to agree with MNRAS published versio
Quantifying the non-Gaussianity in the EoR 21-cm signal through bispectrum
The epoch of reionization (EoR) 21-cm signal is expected to be highly
non-Gaussian in nature and this non-Gaussianity is also expected to evolve with
the progressing state of reionization. Therefore the signal will be correlated
between different Fourier modes (). The power spectrum will not be able
capture this correlation in the signal. We use a higher-order estimator -- the
bispectrum -- to quantify this evolving non-Gaussianity. We study the
bispectrum using an ensemble of simulated 21-cm signal and with a large variety
of triangles. We observe two competing sources driving the non-Gaussianity
in the signal: fluctuations in the neutral fraction () field and
fluctuations in the matter density field. We find that the non-Gaussian
contribution from these two sources vary, depending on the stage of
reionization and on which modes are being studied. We show that the sign of
the bispectrum works as a unique marker to identify which among these two
components is driving the non-Gaussianity. We propose that the sign change in
the bispectrum, when plotted as a function of triangle configuration
and at a certain stage of the EoR can be used as a confirmative
test for the detection of the 21-cm signal. We also propose a new consolidated
way to visualize the signal evolution (with evolving or
redshift), through the trajectories of the signal in a power spectrum and
equilateral bispectrum i.e. space.Comment: 18 pages, 11 figures. Accepted for publication in MNRAS. Replaced to
match the accepted versio
Fouling Characteristics of a Light Australian Crude Oil
Australian crude oils, which generally contain little asphaltenes, nevertheless give rise to fouling in refinery pre-heat trains. In this research, fouling of a series of such crude oils and their blends is being assessed. The present work focuses on thermal fouling resulting from heating Gippsland crude oil at moderate temperatures. The oil is maintained under nitrogen at a pressure of 379 kPa, and re-circulated at bulk temperatures of 80-120°C through an electrically heated annular probe at velocities in the range 0.25 to 0.65 m/s with surface temperatures from 180-260°C. Experiments are run for periods up to 90 hours at constant heat flux. Fouling is detected by the increase of wall temperature of the probe. The oil is characterized by its filterable solids content, density and viscosity both before and after the fouling run. The trends in fouling rates are compared to predictions of the threshold-fouling model proposed by Ebert and Panchal (1995). Data on deposit composition are presented, and the fouling mechanism discussed
Determining the probability of cyanobacterial blooms: the application of Bayesian networks in multiple lake systems
A Bayesian network model was developed to assess the combined influence of nutrient conditions and climate on the occurrence of cyanobacterial blooms within lakes of diverse hydrology and nutrient supply. Physicochemical, biological, and meteorological observations were collated from 20 lakes located at different latitudes and characterized by a range of sizes and trophic states. Using these data, we built a Bayesian network to (1) analyze the sensitivity of cyanobacterial bloom development to different environmental factors and (2) determine the probability that cyanobacterial blooms would occur. Blooms were classified in three categories of hazard (low, moderate, and high) based on cell abundances. The most important factors determining cyanobacterial bloom occurrence were water temperature, nutrient availability, and the ratio of mixing depth to euphotic depth. The probability of cyanobacterial blooms was evaluated under different combinations of total phosphorus and water temperature. The Bayesian network was then applied to quantify the probability of blooms under a future climate warming scenario. The probability of the "high hazardous" category of cyanobacterial blooms increased 5% in response to either an increase in water temperature of 0.8°C (initial water temperature above 24°C) or an increase in total phosphorus from 0.01 mg/L to 0.02 mg/L. Mesotrophic lakes were particularly vulnerable to warming. Reducing nutrient concentrations counteracts the increased cyanobacterial risk associated with higher temperatures
Analysing Habitat Connectivity and Home Ranges of Bigmouth Buffalo and Channel Catfish Using a Large-Scale Acoustic Receiver Network
The determination if fish movement of potadromous species is impeded in a river system is often difficult, particularly when timing and extent of movements are unknown. Furthermore, evaluating river connectivity poses additional challenges. Here, we used large-scale, long-term fish movement to study and identify anthropogenic barriers to movements in the Lake Winnipeg basin including the Red, Winnipeg, and Assiniboine rivers. In the frame of the project, 80 Bigmouth Buffalo (Ictiobus cyprinellus) and 161 Channel Catfish (Ictalurus punctatus) were tagged with acoustic transmitters. Individual fish were detected with an acoustic telemetry network. Movements were subsequently analyzed using a continuous-time Markov model (CTMM). The study demonstrated large home ranges in the Lake Winnipeg basin and evidence of frequent transborder movements between Canada and the United States. The study also highlighted successful downstream fish passage at some barriers, whereas some barriers limited or completely blocked upstream movement. This biological knowledge on fish movements in the Lake Winnipeg basin highlights the need for fish passage solutions at different obstructions
The 21-cm bispectrum as a probe of non-Gaussianities due to X-ray heating
We present analysis of the normalized 21-cm bispectrum from fully-numerical simulations of intergalactic-medium heating by stellar sources and high-mass X-ray binaries (HMXBs) during the cosmic dawn. Ly-α coupling is assumed to be saturated, we therefore probe the nature of non-Gaussianities produced by X-ray heating processes. We find the evolution of the normalized bispectrum to be very different from that of the power spectrum. It exhibits a turnover whose peak moves from large to small scales with decreasing redshift, and corresponds to the typical separation of emission regions. This characteristic scale reduces as more and more regions move into emission with time. Ultimately, small-scale fluctuations within heated regions come to dominate the normalized bispectrum, which at the end of the simulation is almost entirely driven by fluctuations in the density field. To establish how generic the qualitative evolution of the normalized bispectrum we see in the stellar + HMXB simulation is, we examine several other simulations – two fully numerical simulations that include quasi-stellar object (QSO) sources, and two with contrasting source properties produced with the semi-numerical simulation 21CMFAST. We find the qualitative evolution of the normalized bispectrum during X-ray heating to be generic, unless the sources of X-rays are, as with QSOs, less numerous and so exhibit more distinct isolated heated profiles. Assuming mitigation of foreground and instrumental effects are ultimately effective, we find that we should be sensitive to the normalized bispectrum during the epoch of heating, so long as the spin temperature has not saturated by z≈19
Simulating Intestinal Transporter and Enzyme Activity in a Physiologically Based Pharmacokinetic Model for Tenofovir Disoproxil Fumarate
Tenofovir disoproxil fumarate (TDF), a prodrug of tenofovir, has oral bioavailability (25%) limited by intestinal transport (P-glycoprotein), and intestinal degradation (carboxylesterase). However, the influence of luminal pancreatic enzymes is not fully understood. Physiologically based pharmacokinetic (PBPK) modeling has utility for estimating drug exposure from in vitro data. This study aimed to develop a PBPK model that included luminal enzyme activity to inform dose reduction strategies. TDF and tenofovir stability in porcine pancrelipase concentrations was assessed (0, 0.48, 4.8, 48, and 480 U/ml of lipase; 1 mM TDF; 37°C; 0 to 30 min). Samples were analyzed using mass spectrometry. TDF stability and permeation data allowed calculation of absorption rates within a human PBPK model to predict plasma exposure following 6 days of once-daily dosing with 300 mg of TDF. Regional absorption of drug was simulated across gut segments. TDF was degraded by pancrelipase (half-lives of 0.07 and 0.62 h using 480 and 48 U/ml, respectively). Previously reported maximum concentration (Cmax; 335 ng/ml), time to Cmax (Tmax; 2.4 h), area under the concentration-time curve from 0 to 24 h (AUC0–24; 3,045 ng · h/ml), and concentration at 24 h (C24; 48.3 ng/ml) were all within a 0.5-fold difference from the simulated Cmax (238 ng/ml), Tmax (3 h), AUC0–24 (3,036 ng · h/ml), and C24 (42.7 ng/ml). Simulated TDF absorption was higher in duodenum and jejunum than in ileum (p<0.05). These data support that TDF absorption is limited by the action of intestinal lipases. Our results suggest that bioavailability may be improved by protection of drug from intestinal transporters and enzymes, for example, by coadministration of enzyme-inhibiting agents or nanoformulation strategies
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