745 research outputs found
Collisional dust avalanches in debris discs
We quantitatively investigate how collisional avalanches may developin debris
discs as the result of the initial break-up of a planetesimal or comet-like
object, triggering a collisional chain reaction due to outward escaping small
dust grains. We use a specifically developed numerical code that follows both
the spatial distribution of the dust grains and the evolution of their
size-frequency distribution due to collisions. We investigate how strongly
avalanche propagation depends on different parameters (e.g., amount of dust
released in the initial break-up, collisional properties of dust grains and
their distribution in the disc). Our simulations show that avalanches evolve on
timescales of ~1000 years, propagating outwards following a spiral-like
pattern, and that their amplitude exponentially depends on the number density
of dust grains in the system. We estimate a probability for witnessing an
avalanche event as a function of disc densities, for a gas-free case around an
A-type star, and find that features created by avalanche propagation can lead
to observable asymmetries for dusty systems with a beta Pictoris-like dust
content or higher. Characteristic observable features include: (i) a brightness
asymmetry of the two sides for a disc viewed edge-on, and (ii) a one-armed open
spiral or a lumpy structure in the case of face-on orientation. A possible
system in which avalanche-induced structures might have been observed is the
edge-on seen debris disc around HD32297, which displays a strong luminosity
difference between its two sides.Comment: 18 pages, 19 figures; has been accepted for publication in Astronomy
and Astrophysics, section 6. Interstellar and circumstellar matter. The
official date of acceptance is 29/08/200
Acute Oxygen-Sensing via Mitochondria-Generated Temperature Transients in Rat Carotid Body Type I Cells
The Carotid Bodies (CB) are peripheral chemoreceptors that detect changes in arterial oxygenation and, via afferent inputs to the brainstem, correct the pattern of breathing to restore blood gas homeostasis. Herein, preliminary evidence is presented supporting a novel oxygen-sensing hypothesis which suggests CB Type I cell “hypoxic signaling” may in part be mediated by mitochondria-generated thermal transients in TASK-channel-containing microdomains. Distances were measured between antibody-labeled mitochondria and TASK-potassium channels in primary rat CB Type I cells. Sub-micron distance measurements (TASK-1: 0.33 ± 0.04 µm, n = 47 vs TASK-3: 0.32 ± 0.03 µm, n = 54) provided evidence for CB Type I cell oxygen-sensing microdomains. A temperature-sensitive dye (ERthermAC) indicated that inhibition of mitochondrial activity in isolated cells caused a rapid and reversible inhibition of mitochondrial thermogenesis and thus temperature in these microdomains. Whole-cell perforated-patch current-clamp electrophysiological recordings demonstrated sensitivity of resting membrane potential (Vm) to temperature: lowering bath temperature from 37°C to 24°C induced consistent and reversible depolarizations (Vm at 37°C: -48.4 ± 4.11 mV vs 24°C: -31.0 ± 5.69 mV; n = 5; p \u3c 0.01). These data suggest that hypoxic inhibition of mitochondrial thermogenesis may play an important role in oxygen chemotransduction in the CB. A reduction in temperature within cellular microdomains will inhibit plasma membrane ion channels, influence the balance of cellular phosphorylation–dephosphorylation, and may extend the half-life of reactive oxygen species. The characterization of a thermosensory chemotransduction mechanism, that may also be used by other oxygen-sensitive cell types and may impact multiple other chemotransduction mechanisms is critical if we are to fully understand how the CBs, and potentially other oxygen-sensitive cells, respond to hypoxia
Shadows Aren't So Dangerous After All: A Fast and Robust Defense Against Shadow-Based Adversarial Attacks
Robust classification is essential in tasks like autonomous vehicle sign
recognition, where the downsides of misclassification can be grave. Adversarial
attacks threaten the robustness of neural network classifiers, causing them to
consistently and confidently misidentify road signs. One such class of attack,
shadow-based attacks, causes misidentifications by applying a natural-looking
shadow to input images, resulting in road signs that appear natural to a human
observer but confusing for these classifiers. Current defenses against such
attacks use a simple adversarial training procedure to achieve a rather low
25\% and 40\% robustness on the GTSRB and LISA test sets, respectively. In this
paper, we propose a robust, fast, and generalizable method, designed to defend
against shadow attacks in the context of road sign recognition, that augments
source images with binary adaptive threshold and edge maps. We empirically show
its robustness against shadow attacks, and reformulate the problem to show its
similarity to perturbation-based attacks. Experimental results
show that our edge defense results in 78\% robustness while maintaining 98\%
benign test accuracy on the GTSRB test set, with similar results from our
threshold defense. Link to our code is in the paper.Comment: This is a draft version - our core results are reported, but
additional experiments for journal submission are still being ru
Ethanol and Opioids Do Not Act Synergistically To Depress Excitation in Carotid Body Type I Cells
The combination of opioids and ethanol can synergistically depress breathing and the acute ventilatory response to hypoxia. Multiple studies have shown that the underlying mechanisms for this may involve calcium channel inhibition in central neurons. But we have previously identified opioid receptors in the carotid bodies and shown that their activation inhibits calcium influx into the chemosensitive cells. Given that the carotid bodies contribute to the drive to breathe and underpin the acute hypoxic ventilatory response, we hypothesized that ethanol and opioids may act synergistically in these peripheral sensory organs to further inhibit calcium influx and therefore inhibit ventilation. Methods
Carotid bodies were removed from 56 Sprague–Dawley rats (1021 days old) and then enzymatically dissociated to allow calcium imaging of isolated chemosensitive type I cells. Cells were stimulated with high K+ in the presence and absence of the µ-opioid agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO) (10 µM), a maximal sublethal concentration of ethanol (3 g L-1, 65.1 mM) or a combination of both. Results
DAMGO alone significantly inhibited Ca2+ influx but this effect was not potentiated by the high concentration of ethanol. Conclusion
These results indicate for the first time that while opioids may suppress breathing via an action at the level of the carotid bodies, ethanol is unlikely to potentiate inhibition via this pathway. Thus, the synergistic effects of ethanol and opioids on ventilatory parameters are likely mediated by central rather than peripheral actions
UNITE CubeSat: From Inception to Early Orbital Operations
On January 31, 2019 the University of Southern Indiana UNITE CubeSat was deployed from the International Space Station and has transmitted data every day since. The missions of UNITE [Undergraduate Nano Ionospheric Temperature Explorer] are to measure plasma properties in the lower ionosphere using a Langmuir Plasma Probe, to measure its internal and skin temperatures to compare with a student-developed thermal model, and to track the orbital decay of the CubeSat, particularly near re-entry. The 3U UNITE CubeSat is passively magnetically and aerodynamically stabilized. This paper summarizes the design, build, integration, test and operations phases of the UNITE project since its inception in August 2016. The all-undergraduate team designed, fabricated, tested and integrated the command board, solar panels, and temperature sensor array. In addition, the team integrated a magnetometer and GPS. A commercially purchased Electric Power/Communication Subsystem provides Maximum Power Point Tracking and Simplex and Duplex communication through the Globalstar satellite network allowing nearly 24/7 contact with UNITE. The team wrote and tested the flight software which is divided into five primary modes. Some results from the first several months of flight are summarized and lessons learned are shared, with the intent of assisting future CubeSat teams
Vein-type gold formation during late extensional collapse of the Eastern Desert, Egypt: the Gidami deposit
Orogenic gold deposits, though construed to focused fluid flow during orogenesis, commonly post-date the main accretionary events. Several lines of evidence indicate that orogenic gold formation in the Arabian–Nubian Shield continued through the orogen collapse stage and associated rapid exhumation and thermal re-equilibration. The Gidami gold deposit in the Eastern Desert of Egypt is associated with post-foliation, brittle-ductile shear zones that deformed a weakly foliated tonalite-trondhjemite massif dated as ~ 704 Ma (U–Pb zircon age). Gold-sulfide quartz veins exhibit textural features indicative of repeated mylonitization, recrystallization, and muscovite crystallization. New Ar/Ar ages of muscovite flakes from the auriferous quartz veins and from the altered wallrock overlap within analytical uncertainty at ~ 583 Ma, which corresponds to the climax of extension-related wrenching and rapid exhumation in the region (~ 596 to 582 Ma). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) data coupled with microtextural characteristics suggest that early formed pyrite generations experienced substantial fluid-mediated recrystallization, and that a set of metals was remobilized by later fluids. A late generation of fibrous pyrite, ubiquitous in microfractures, deposited while the veins re-opened and deformed. The occurrence of free gold particles along with a late-paragenetic assemblage of galena-sphalerite-chalcopyrite(± hessite ± cervelleite) was related to influx of low salinity, metalliferous HO-NaCl-CO-CH fluids as indicated by the fluid inclusion laser Raman spectroscopy and microthermometry results. Au-mobilization and redeposition at T ≤ 350 °C and P ~ 1 to 1.7 kbar, triggered by intermittent fluid pluses and thermal re-equilibration, were most likely stimulated by extensional structures and within-plate magmatism. Coincident province- and deposit-scale pressure–temperature-time data highlight the pivotal role of the orogenic collapse tectonics in gold endowment in the Central Eastern Desert’s crust
Position Paper On Use Of Stereoscopy To Support Science Learning: Ten Years Of Research
Stereoscopys potential as a tool for science education has been largely eclipsed by its popularity as an entertainment platform and marketing gimmick. Dozens of empirical papers have been published in the last decade about the impact of stereoscopy on learning. As a result, a corpus of research now points to a coherent message about how, when, and where stereoscopy can be most effective in supporting science education. This position paper synthesizes that research with examples from three studies recently completed and published by the authors of this paper. Results of the synthesis point towards generally limited successful uses of stereoscopic media in science education with a pocket of potentially beneficial applications. Our position is that stereoscopy should be used only where its unique properties can accommodate specific requirements of understanding topics and tasks namely visualizations where the spatial sense of depth is germane to conveying core ideas and cognitive load is high. Stereoscopys impact on learning is also related to the spatial ability of the viewer. More research is needed on the effect of novelty, long-term learning and possible learning differences between the various methods of implementing stereoscopy
Novel approach to semi-supervised relation extraction in medical language
Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 67-69).We address the problem of weakly-supervised relation extraction in hospital discharge summaries. Sentences with pre-identified concept types (for example: medication, test, problem, symptom) are labeled with the relationship between the concepts. We present a novel technique for weakly-supervised bootstrapping of a classifier for this task: Groundtruth Budgeting. In the case of highly-overlapping, self-similar datasets as is the case with the 2010 i2b2/VA challenge corpus, the performance of classifiers on the minority classes is often poor. To address this we set aside a random portion of the groundtruth at the beginning of bootstrapping which will be gradually added as the classifier is bootstrapped. The classifier chooses groundtruth samples to be added by measuring the confidence of its predictions on them and choosing samples for which it has the least confident predictions. By adding samples in this fashion, the classifier is able to increase its coverage of the decision space while not adding too many majority-class examples. We evaluate this approach on the 2010 i2b2/VA challenge corpus containing of 477 patient discharge summaries and show that with a training corpus of 349 discharge summaries, budgeting 10% of the corpus achieves equivalent results to a bootstrapping classifier starting with the entire corpus. We compare our results to those of other papers published in the proceedings of the 2010 Fourth i2b2/VA Shared-Task and Workshop.by Russell J. Ryan.M.Eng
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