The light scattered from dust grains in debris disks is typically modeled as compact spheres using the Lorenz-Mie theory or as porous spheres by incorporating an effective medium theory. In this work ...we examine the effect of incorporating a more realistic particle morphology on estimated radiation-pressure blowout sizes. To calculate the scattering and absorption cross-sections of irregularly shaped dust grains, we use the discrete dipole approximation. These cross-sections are necessary to calculate the β-ratio, which determines whether dust grains can remain gravitationally bound to their star. We calculate blowout sizes for a range of stellar spectral types corresponding with stars known to host debris disks. As with compact spheres, more luminous stars blow out larger irregularly shaped dust grains. We also find that dust grain composition influences blowout size such that absorptive grains are more readily removed from the disk. Moreover, the difference between blowout sizes calculated assuming spherical particles versus particle morphologies more representative of real dust particles is compositionally dependent as well, with blowout size estimates diverging further for transparent grains. We find that the blowout sizes calculated have a strong dependence on the particle model used, with differences in the blowout size calculated being as large as an order of magnitude for particles of similar porosities.
Abstract
We explore whether assumptions about dust grain shape affect the resulting estimates of the composition and grain size distribution of the AU Microscopii (AU Mic) debris disk from ...scattered-light data collected by Lomax et al. The near edge-on orientation of the AU Mic debris disk makes it ideal for studying the effect of the scattering phase function on the measured flux ratios as a function of wavelength and projected distance. Previous efforts to model the AU Mic debris disk have invoked a variety of dust grain compositions and explored the effect of porosity, but did not undertake a systematic effort to explore a full range of size distributions and compositions to understand possible degeneracies in fitting the data. The degree to which modeling dust grains with more realistic shapes compounds these degeneracies has also not previously been explored. We find differences in the grain properties retrieved depending on the grain shape model used. We also present here our calculations of porous grains of size parameters
x
= 0.1 to 48 and complex refractive indices (
m
=
n
+
iκ
) ranging from
n
= 1.1 to 2.43 and
k
= 0 to 1.0, covering multiple compositions at visible and near-infrared wavelengths such as ice, silicates, amorphous carbon, and tholins.
We quantitatively investigate the contribution of large dust particles to the polarimetric response in comets using the light-scattering properties of model agglomerated debris particles. We ...demonstrate that large, supermicron-sized particles have a decreasing role on the degree of linear polarization at phase angle ≤ 80°, and the effect of particles greater than 10 m is minimal. At larger phase angles, they may only slightly increase the measured percent of polarization by up to 1%. Omitting the effects of these particles in modeling the observations only slightly affects the retrievals of the microphysical properties of dust in comets and could lead to a small underestimation of the index in a power-law size distribution and population of weakly absorbing dust particles.
Cue competition effects are pervasive in young adults' learning, but evidence for these effects in older adults' learning is mixed. For example, although older adults show strong forward blocking, ...they do not show recovery from overshadowing. We examined whether this could be due to problems with associative binding using a rapid, streamed trial contingency learning task to minimize long-term memory retrieval demands. In a forward blocking paradigm , target cues gained less predictive value when the competing companion cues had high predictive value and this forward blocking effect was similar for younger and older adults. In a backward blocking paradigm, target cues lost more predictive value when the competing companion cues had high predictive value, but this backward blocking effect was greater for younger than older adults. These findings, together with evidence that within-compound associations for companion and target cues mediate backward, but not forward cue competition effects, suggest that a decline in associative binding may be responsible for the absence of backward cue competition effects in older adults' contingency learning
.
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BFBNIB, NUK, PILJ, SAZU, UL, UM, UPUK
Background
Three flaviviruses (equine pegivirus EPgV; Theiler's disease–associated virus TDAV; non‐primate hepacivirus NPHV) and equine parvovirus (EqPV‐H) are present in equine blood products; the ...TDAV, NPHV, and EqPV‐H have been suggested as potential causes of serum hepatitis.
Objective
To determine the prevalence of these viruses in horses with equine serum hepatitis.
Animals
Eighteen horses diagnosed with serum hepatitis, enrolled from US referral hospitals.
Methods
In the prospective case study, liver, serum, or both samples were tested for EPgV, TDAV, NPHV, and EqPV‐H by PCR.
Results
Both liver tissue and serum were tested for 6 cases, serum only for 8 cases, and liver only for 4 cases. Twelve horses received tetanus antitoxin (TAT) 4‐12.7 weeks (median = 8 weeks), 3 horses received commercial equine plasma 6‐8.6 weeks, and 3 horses received allogenic stem cells 6.4‐7.6 weeks before the onset of hepatic failure. All samples were TDAV negative. Two of 14 serum samples were NPHV‐positive. Six of 14 serum samples were EPgV‐positive. All liver samples were NPHV‐negative and EPgV‐negative. EqPV‐H was detected in the serum (N = 8), liver (N = 4), or both samples (N = 6) of all 18 cases. The TAT of the same lot number was available for virologic testing in 10 of 12 TAT‐associated cases, and all 10 samples were EqPV‐H positive.
Conclusions and Clinical Importance
We demonstrated EqPV‐H in 18 consecutive cases of serum hepatitis. EPgV, TDAV, and NPHV were not consistently present. This information should encourage blood product manufacturers to test for EqPV‐H and eliminate EqPV‐H–infected horses from their donor herds.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
We obtained direct global measurements of the lunar surface using multispectral thermal emission mapping with the Lunar Reconnaissance Orbiter Diviner Lunar Radiometer Experiment. Most lunar terrains ...have spectral signatures that are consistent with known lunar anorthosite and basalt compositions. However, the data have also revealed the presence of highly evolved, silica-rich lunar soils in kilometer-scale and larger exposures, expanded the compositional range of the anorthosites that dominate the lunar crust, and shown that pristine lunar mantle is not exposed at the lunar surface at the kilometer scale. Together, these observations provide compelling evidence that the Moon is a complex body that has experienced a diverse set of igneous processes.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Lunar swirls are high-albedo markings on the Moon that occur in both mare and highland terrains; their origin remains a point of contention. Here, we use data from the Lunar Reconnaissance Orbiter ...Diviner Lunar Radiometer to support the hypothesis that the swirls are formed as a result of deflection of the solar wind by local magnetic fields. Thermal infrared data from this instrument display an anomaly in the position of the silicate Christiansen Feature consistent with reduced space weathering. These data also show that swirl regions are not thermophysically anomalous, which strongly constrains their formation mechanism. The results of this study indicate that either solar wind sputtering and implantation are more important than micrometeoroid bombardment in the space-weathering process, or that micrometeoroid bombardment is a necessary but not sufficient process in space weathering, which occurs on airless bodies throughout the solar system.
Summary
Background
Central tarsal bone fractures in non‐racehorses are uncommon and their MRI appearance and follow‐up have not been described.
Objectives
To describe the clinical and MRI ...characteristics and follow‐up of central tarsal bone fractures in non‐racehorses.
Study design
Retrospective descriptive case series.
Methods
The low‐field MRI appearance of central tarsal bone fractures in sports horses identified over a 9‐year period, was reviewed. Follow‐up MRI examinations, treatment and clinical outcome were included.
Results
Four horses with five vertical fractures of the central tarsal bone were included. There were three Warmbloods, of which two were showjumpers and one a dressage horse, and one Quarter Horse used for western performance. All fractures could consistently be visualised on low‐field standing MRI and were associated with marked sclerosis in the surrounding bone, with little to no associated bone marrow lesion of the central tarsal bone. One horse had bilateral fractures. Three fractures were complete (full‐thickness) slab fractures and two were incomplete (partial‐thickness) slab fractures. Fractures occurred in a consistent dorsomedial to plantarolateral oblique orientation and were non‐displaced. Incomplete fractures occurred at the dorsomedial proximal aspect of the central tarsal bone. All horses were treated conservatively initially. The dressage horse with bilateral fractures and the Quarter Horse remained lame after conservative treatment and were subsequently treated surgically. Three horses returned to their previous level of performance, one horse in spite of persistent MRI evidence of a fracture line (non‐union), while the Quarter Horse returned to a lower level of reining.
Main limitations
Small number of horses.
Conclusions
Standing low‐field MRI is helpful in the diagnosis of central tarsal bone fractures. Central tarsal bone fractures occur in a dorsomedial to plantarolateral orientation in sports horses, can be complete or incomplete, unilateral or bilateral and remain non‐displaced. The prognosis is good for return to performance.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
•We find that the lunar Christiansen Feature is influenced by space weathering.•Albedo-dependent thermal gradients are the likely cause of the observed effect.•Composition is also demonstrated to ...influence the Christiansen Feature.•Space weathering can be partly removed from Diviner data using an independent maturity index.
Multispectral infrared measurements by the Diviner Lunar Radiometer Experiment on the Lunar Renaissance Orbiter enable the characterization of the position of the Christiansen Feature, a thermal infrared spectral feature that laboratory work has shown is proportional to the bulk silica content of lunar surface materials. Diviner measurements show that the position of this feature is also influenced by the changes in optical and physical properties of the lunar surface with exposure to space, the process known as space weathering. Large rayed craters and lunar swirls show corresponding Christiansen Feature anomalies. The space weathering effect is likely due to differences in thermal gradients in the optical surface imposed by the space weathering control of albedo. However, inspected at high resolution, locations with extreme compositions and Christiansen Feature wavelength positions – silica-rich and olivine-rich areas – do not have extreme albedos, and fall off the albedo- Christiansen Feature wavelength position trend occupied by most of the Moon. These areas demonstrate that the Christiansen Feature wavelength position contains compositional information and is not solely dictated by albedo. An optical maturity parameter derived from near-IR measurements is used to partly correct Diviner data for space weathering influences.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
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