5,894 research outputs found
PAHs and star formation in the HII regions of nearby galaxies M83 and M33
We present mid-infrared (MIR) spectra of HII regions within star-forming
galaxies M83 and M33. Their emission features are compared with Galactic and
extragalactic HII regions, HII-type galaxies, starburst galaxies, and
Seyfert/LINER type galaxies. Our main results are as follows: (i) the M33 and
M83 HII regions lie in between Seyfert/LINER galaxies and HII-type galaxies in
the 7.7/11.3 - 6.2/11.3 plane, while the different sub-samples exhibiting
different 7.7/6.2 ratios; (ii) Using the NASA Ames PAH IR Spectroscopic
database, we demonstrate that the 6.2/7.7 ratio does not effectively track PAH
size, but the 11.3/3.3 PAH ratio does; (iii) variations on the 17 m PAH
band depends on object type; however, there is no dependence on metallicity for
both extragalactic HII regions and galaxies; (iv) the PAH/VSG intensity ratio
decreases with the hardness of the radiation field and galactocentric radius
(Rg), yet the ionization alone cannot account for the variation seen in all of
our sources; (v) the relative strength of PAH features does not change
significantly with increasing radiation hardness, as measured through the
[NeIII]/[NeII] ratio and the ionization index; (vi) We present PAH SFR
calibrations based on the tight correlation between the 6.2, 7.7, and 11.3
m PAH luminosities with the 24 m luminosity and the combination of
the 24 m and H luminosity; (vii) Based on the total luminosity
from PAH and FIR emission, we argue that extragalactic HII regions are more
suitable templates in modeling and interpreting the large scale properties of
galaxies compared to Galactic HII regions.Comment: 26 pages, 24 figures, 6 tables. Accepted for publication in MNRA
Manure Application Timing Drives Energy Absorption for Snowmelt on an Agricultural Soil
Reducing agricultural runoff year-round is important, in particular during snowmelt events on landscapes that receive wintertime applications of manure. To help inform manure guidelines, process-level data are needed that link management scenarios with the complexity of snowmelt, hence runoff. Albedo and radiative energy fluxes are strong drivers of thaw, but applying these mechanistic measurements across multiple, plot-scale management treatments over time presents a logistical challenge. The objective of this study was to first develop a practical field approach to estimate winter albedo in plot-scale field research with multiple management scenarios. The second objective was to quantify the radiative drivers of snowmelt by measuring fluxes after wintertime liquid manure application. Six management treatments were tested in south-central Wisconsin during the winters of 2015–2016 and 2016–2017 with a complete factorial design: three manure application timings (early December, late January, and unmanured) and two tillage treatments (conventional tillage versus no-tillage). A multiple linear regression model was developed to estimate albedo with digital imagery and readily-obtained site characteristics. Manure timing had a significant effect on radiative energy fluxes and tillage was secondary. January applications of liquid manure produced an immediate and lasting decrease in albedo, which resulted in greater net radiation absorbed by snowpack and subsequent energy available for snowmelt. Later applications of liquid manure accelerated snowmelt, which increased runoff losses and posed a challenge for nutrient retention from the liquid manure during thaw
Helical spin-waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14
We have investigated the spin fluctuations in the langasite compound
Ba3NbFe3Si2O14 in both the ordered state and as a function of temperature. The
low temperature magnetic structure is defined by a spiral phase characterized
by magnetic Bragg peaks at q=(0,0,tau ~ 1/7) onset at TN=27 K as previously
reported by Marty et al. The nature of the fluctuations and temperature
dependence of the order parameter is consistent with a classical second order
phase transition for a two dimensional triangular antiferromagnet. We will show
that the physical properties and energy scales including the ordering
wavevector, Curie-Weiss temperature, and the spin-waves can be explained
through the use of only symmetric exchange constants without the need for the
Dzyaloshinskii-Moriya interaction. This is accomplished through a set of
``helical" exchange pathways along the c direction imposed by the chiral
crystal structure and naturally explains the magnetic diffuse scattering which
displays a strong vector chirality up to high temperatures well above the
ordering temperature. This illustrates a strong coupling between magnetic and
crystalline chirality in this compound.Comment: 16 pages, 16 figures, submitted to Physical Review
Spin dynamics near the critical doping in weakly-superconducting underdoped YBa2Cu3O6.35 (Tc=18K)
Using neutron scattering we have determined the magnetic structure and
fluctuations in the YBa2Cu3O6.35 superconductor (Tc=18 K). The long-range
ordered collinear spins of the insulating antiferromagnet are replaced by a
commensurate central mode arising from slow, isotropically polarized,
short-range spin correlations. The inelastic spectrum up to 30 meV is broad in
wave vector and commensurate. In contrast to the the resonance peak of higher
Tc superconductors, the spins exhibit a single overdamped spectrum whose rate
of relaxation decreases on cooling and saturates at 5 meV below 50 K. As the
relaxation rate saturates the quasi-static spin correlations grow and become
resolution limited in energy. The spin susceptibility at high temperatures
illustrates that the dominant energy scale is set by the temperature. At low
temperatures, the scale length is geometric and not linked by velocity to
dynamic widths. There is no observable suppression of the spin fluctuations or
central mode upon the onset of superconductivity. The spins respond not to
coherent charge pairs but to hole doping allowing coexistence of glassy short
range spin order with superconductivity. Since the physics of the weakly
superconducting system YBCO6.35 must connect continuously with that in more
strongly superconducting YBCO6.5, we find that neither incommensurate
stripe-like spin modulations nor a well-defined neutron spin resonance are
essential for the onset with doping of pairing in a high temperature cuprate
superconductor.Comment: 22 pages, 19 figures, accepted for publication in Phys. Rev.
A Universal Phase Diagram for PMN-xPT and PZN-xPT
The phase diagram of the Pb(Mg1/3Nb2/3)O3 and PbTiO3 solid solution (PMN-xPT)
indicates a rhombohedral ground state for x < 0.32. X-ray powder measurements
by Dkhil et al. show a rhombohedrally split (222) Bragg peak for PMN-10%PT at
80 K. Remarkably, neutron data taken on a single crystal of the same compound
with comparable q-resolution reveal a single resolution-limited (111) peak down
to 50 K, and thus no rhombohedral distortion. Our results suggest that the
structure of the outer layer of these relaxors differs from that of the bulk,
which is nearly cubic, as observed in PZN by Xu et al.Comment: Replaced Fig. 3 with better versio
Linking Nutrient Transport to Soil Physical Processes During Freeze/Thaw Events to Promote Wintertime Manure Management, Nutrient Use Efficiency, and Surface Water Quality.
The application of dairy manure to the landscape during winter is a longstanding practice for farms in the Midwestern United States and other temperate regions. Practical motivations behind winter spreading include affordability, availability of time, and the reduced risk of compaction from farm equipment on frozen soils. Wintertime manure applications, however, coincide with environmental conditions that are prone to runoff and accelerate nutrient losses from agricultural fields. Understanding the nutrient dynamics in response to winter-applied manure is especially important to Wisconsin, a leading state in dairy production, where up to 75% of annual runoff volumes occur on frozen and thawing soils. The high potential for winter runoff, hence nutrient transport, has prompted revisions to winter manure regulations, yet little conclusive data exist to guide these changing standards
Competing orders in PZN-xPT and PMN-xPT relaxor ferroelectrics
Neutron and x-ray scattering studies on relaxor ferroelectric systems
Pb(ZnNb)O (PZN), Pb(MgNb)O (PMN), and
their solid solutions with PbTiO (PT) have shown that inhomogeneities and
disorder play important roles in the materials properties. Although a
long-range polar order can be established at low temperature - sometimes with
the help of an external electric field; short-range local structures called the
``polar nano-regions'' (PNR) still persist. Both the bulk structure and the PNR
have been studied in details. The coexistence and competition of long- and
short-range polar orders and how they affect the structural and dynamical
properties of relaxor materials are discussed.Comment: Article submitted for JPSJ Special Topics (Novel States of Matter
Induced by Frustration
Shape oscillation of a rotating Bose-Einstein condensate
We present a theoretical and experimental analysis of the transverse monopole
mode of a fast rotating Bose-Einstein condensate. The condensate's rotation
frequency is similar to the trapping frequency and the effective confinement is
only ensured by a weak quartic potential. We show that the non-harmonic
character of the potential has a clear influence on the mode frequency, thus
making the monopole mode a precise tool for the investigation of the fast
rotation regime
A Climate-Data Record of the "Clear-Sky" Surface Temperature of the Greenland Ice Sheet
We are developing a climate-data record (CDR of daily "clear-sky" ice-surface temperature (IST) of the Greenland Ice Sheet, from 1982 to the present using Advanced Very High Resolution Radiometer (AVHRR) (1982 - present) and Moderate-Resolution Imaging Spectroradiometer (MODIS) data (2000 - present) at a resolution of approximately 5 km. The CDR will be continued in the National Polar-orbiting Operational Environmental Satellite System Visible/Infrared Imager Radiometer Suite era. Two algorithms remain under consideration. One algorithm under consideration is based on the split-window technique used in the Polar Pathfinder dataset (Fowler et al., 2000 & 21007). Another algorithm under consideration, developed by Comiso (2006), uses a single channel of AVHRR data (channel 4) in conjunction with meteorological-station data to account for atmospheric effects and drift between AVHRR instruments. Known issues being addressed in the production of the CDR are: tune-series bias caused by cloud cover (surface temperatures can be different under clouds vs. clear areas) and cross-calibration in the overlap period between AVHRR instruments, and between AVHRR and MODIS instruments. Because of uncertainties, mainly due to clouds (Stroeve & Steffen, 1998; Wang and Key, 2005; Hall et al., 2008 and Koenig and Hall, submitted), time-series of satellite 1S'1" do not necessarily correspond to actual surface temperatures. The CDR will be validated by comparing results with automatic-,",eather station (AWS) data and with satellite-derived surface-temperature products. Regional "clear-sky" surface temperature increases in the Arctic, measured from AVHRR infrared data, range from 0.57+/-0.02 deg C (Wang and Key, 2005) to 0.72+/-0.10 deg C (Comiso, 2006) per decade since the early 1980s. Arctic warming has important implications for ice-sheet mass balance because much of the periphery of the Greenland Ice Sheet is already near 0 deg C during the melt season, and is thus vulnerable to rapid melting if temperatures continue to increase. Reference
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