5,705 research outputs found
A note on reducing spurious pressure oscillations in fully conservative discontinuous Galerkin simulations of multicomponent flows
A well-known issue associated with the use of fully conservative schemes in
multicomponent-flow simulations is the generation of spurious pressure
oscillations at contact interfaces. These oscillations can rapidly lead to
solver divergence even in the presence of smooth interfaces that are not fully
resolved. In this note, we compare various strategies for reducing such
oscillations that do not (a) introduce conservation error, (b) rely on
artificial viscosity or limiting, or (c) degrade order of accuracy in smooth
regions of the flow. The considered test case is one-dimensional advection of a
high-pressure nitrogen/n-dodecane thermal bubble using the thermally perfect
gas model. Several results are presented that contradict those corresponding to
the more conventional hydrogen/oxygen thermal-bubble case
The Panchromatic Hubble Andromeda Treasury. VI. The reliability of far-ultraviolet flux as a star formation tracer on sub-kpc scales
We have used optical observations of resolved stars from the Panchromatic
Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star
formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas
ranged from ~ to pc, which allowed us to test the reliability
of FUV flux as a tracer of recent star formation on sub-kpc scales. The star
formation rates (SFRs) derived from the extinction-corrected observed FUV
fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to
within the 1 scatter. Overall, the scatter was larger than the
uncertainties in the SFRs and particularly evident among the smallest regions.
The scatter was consistent with an even combination of discrete sampling of the
initial mass function and high variability in the SFHs. This result
demonstrates the importance of satisfying both the full-IMF and the
constant-SFR assumptions for obtaining precise SFR estimates from FUV flux.
Assuming a robust FUV extinction correction, we estimate that a factor of 2.5
uncertainty can be expected in FUV-based SFRs for regions smaller than
pc, or a few hundred pc. We also examined ages and masses derived from UV
flux under the common assumption that the regions are simple stellar
populations (SSPs). The SFHs showed that most of the regions are not SSPs, and
the age and mass estimates were correspondingly discrepant from the SFHs. For
those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age
and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like
regions from the others based on integrated FUV flux.Comment: Accepted for publication in The Astrophysical Journa
Dietary nitrate increases arginine availability and protects mitochondrial complex I and energetics in the hypoxic rat heart
This is the final version. It was first published by Wiley in The Journal of Physiology at http://onlinelibrary.wiley.com/doi/10.1113/jphysiol.2014.275263/abstract.Hypoxic exposure is associated with impaired cardiac energetics in humans and altered mitochondrial function, with suppressed complex I-supported respiration, in rat heart. This response might limit reactive oxygen species (ROS) generation, but at the cost of impaired electron transport chain (ETC) activity. Dietary nitrate supplementation improves mitochondrial efficiency and can promote tissue oxygenation by enhancing blood flow. We therefore hypothesised that ETC dysfunction, impaired energetics and oxidative damage in the hearts of rats exposed to chronic hypoxia could be alleviated by sustained administration of a moderate dose of dietary nitrate. Male Wistar rats (n=40) were given water supplemented with 0.7 mmol/L NaCl (as control) or 0.7 mmol/L NaNO3, elevating plasma nitrate levels by 80%, and were exposed to 13% O2 (hypoxia) or normoxia (n=10 per group) for 14 days. Respiration rates, ETC protein levels, mitochondrial density, ATP content and protein carbonylation were measured in cardiac muscle. Complex I respiration rates and protein levels were 33% lower in hypoxic/NaCl rats compared with normoxic/NaCl controls. Protein carbonylation was 65% higher in hearts of hypoxic rats compared with controls, indicating increased oxidative stress, whilst ATP levels were 62% lower. Respiration rates, complex I protein and activity, protein carbonylation and ATP levels were all fully protected in the hearts of nitrate-supplemented hypoxic rats. Both in normoxia and hypoxia, dietary nitrate suppressed cardiac arginase expression and activity and markedly elevated cardiac L-arginine concentrations, unmasking a novel mechanism of action by which nitrate enhances tissue NO bioavailability. Dietary nitrate therefore alleviates metabolic abnormalities in the hypoxic heart, improving myocardial energetics
Measuring Galaxy Star Formation Rates From Integrated Photometry: Insights from Color-Magnitude Diagrams of Resolved Stars
We use empirical star formation histories (SFHs), measured from HST-based
resolved star color-magnitude diagrams, as input into population synthesis
codes to model the broadband spectral energy distributions (SEDs) of ~50 nearby
dwarf galaxies (6.5 < log M/M_* < 8.5, with metallicities ~10% solar). In the
presence of realistic SFHs, we compare the modeled and observed SEDs from the
ultraviolet (UV) through near-infrared (NIR) and assess the reliability of
widely used UV-based star formation rate (SFR) indicators. In the FUV through i
bands, we find that the observed and modeled SEDs are in excellent agreement.
In the Spitzer 3.6micron and 4.5micron bands, we find that modeled SEDs
systematically over-predict observed luminosities by up to ~0.2 dex, depending
on treatment of the TP-AGB stars in the synthesis models. We assess the
reliability of UV luminosity as a SFR indicator, in light of independently
constrained SFHs. We find that fluctuations in the SFHs alone can cause factor
of ~2 variations in the UV luminosities relative to the assumption of a
constant SFH over the past 100 Myr. These variations are not strongly
correlated with UV-optical colors, implying that correcting UV-based SFRs for
the effects of realistic SFHs is difficult using only the broadband SED.
Additionally, for this diverse sample of galaxies, we find that stars older
than 100 Myr can contribute from <5% to100% of the present day UV luminosity,
highlighting the challenges in defining a characteristic star formation
timescale associated with UV emission. We do find a relationship between UV
emission timescale and broadband UV-optical color, though it is different than
predictions based on exponentially declining SFH models. Our findings have
significant implications for the comparison of UV-based SFRs across
low-metallicity populations with diverse SFHs.Comment: 22 pages, 15 figures, ApJ accepte
Structural and magnetic properties of E-Fe_{1-x}Co_xSi thin films deposited via pulsed laser deposition
We report pulsed laser deposition synthesis and characterization of
polycrystalline Fe1-xCox Si thin films on Si (111). X-ray diffraction,
transmission electron, and atomic force microscopies reveal films to be dense,
very smooth, and single phase with a cubic B20 crystal structure.
Ferromagnetism with significant magnetic hysteresis is found for all films
including nominally pure FeSi films in contrast to the very weak paramagnetism
of bulk FeSi. For Fe1-xCoxSi this signifies a change from helimagnetism in
bulk, to ferromagnetism in thin films. These ferromagnetic thin films are
promising as a magnetic-silicide/silicon system for polarized current
production, manipulation, and detection.Comment: 12 pages, 4 figures accepted in the Applied Physics Letter
CEEPES: An Evolving System for Agro-environmental Policy
Nonpoint source pollution from agricultural production is now front-and-center in the policy debate over potential sources of environmental degradation (see, for example, Shortie and Dunn 1986; Russell and Shogren 1993). Although partially related to scientific advances in measurement and detection, perceived risks due to chemical loading have intensified the pressure to more closely coordinate agricultural and environmental policy (Cohen, Eiden, and Lorber 1986; Johnson, Wolcott, and Aradhyula 1990). But strategies to coordinate these policies have been impeded by a serious information gap on the explicit environmental and economic trade-offs of various public and private actions. Although most decision makers would agree that securing this information is critical for more effective agro-environmental policy, the question remains as to the best course of action
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