417 research outputs found
Plume Development of the Shoemaker-Levy 9 Comet Impact
We have studied plume formation after a Jovian comet impact using the ZEUS-MP
2 hydrodynamics code. The three-dimensional models followed objects with 500,
750, and 1000 meter diameters. Our simulations show the development of a fast,
upward-moving component of the plume in the wake of the impacting comet that
"pinches off" from the bulk of the cometary material ~50 km below the 1 bar
pressure level, ~100 km above the depth of greatest mass and energy deposition.
The fast-moving component contains about twice the mass of the initial comet,
but consists almost entirely (>99.9%) of Jovian atmosphere rather than cometary
material. The ejecta rise mainly along the impact trajectory, but an additional
vertical velocity component due to buoyancy establishes itself within seconds
of impact, leading to an asymmetry in the ejecta with respect to the entry
trajectory. The mass of the upward-moving component follows a velocity
distribution M(>v) approximately proportional to v^-1.4 (v^-1.6 for the 750 m
and 500 m cases) in the velocity range 0.1 < v < 10 km/s.Comment: 5 pages, 4 figures. Accepted for publication in The Astrophysical
Journa
Crystal invariant theory I: Geometric RSK
Berenstein and Kazhdan's theory of geometric crystals gives rise to two
commuting families of geometric crystal operators acting on the space of
complex matrices. These are birational actions, which we view as a
crystal-theoretic analogue of the usual action of on matrices. We prove that the field of rational invariants
(and ring of polynomial invariants) of each family of geometric crystal
operators is generated by a set of algebraically independent polynomials, which
are generalizations of the elementary symmetric polynomials in (or )
variables. We also give a set of algebraically independent generators for the
intersection of these fields, and we explain how these fields are situated
inside the larger fields of geometric -matrix invariants, which were studied
by Lam and the third-named author under the name loop symmetric functions. The
key tool in our proof is the geometric RSK correspondence of Noumi and Yamada,
which we show to be an isomorphism of geometric crystals.
In an appendix jointly written with Thomas Lam, we prove the fundamental
theorem of loop symmetric functions, which says that the polynomial invariants
of the geometric -matrix are generated by the loop elementary symmetric
functions.Comment: 36 pages. v2: 39 pages, 4 figures. Added part (2) of the main theorem
(Theorem 1.1), added Appendix
Blue light regenerates functional visual pigments in mammals through a retinyl-phospholipid intermediate.
The light absorbing chromophore in opsin visual pigments is the protonated Schiff base of 11-cis-retinaldehyde (11cRAL). Absorption of a photon isomerizes 11cRAL to all-trans-retinaldehyde (atRAL), briefly activating the pigment before it dissociates. Light sensitivity is restored when apo-opsin combines with another 11cRAL to form a new visual pigment. Conversion of atRAL to 11cRAL is carried out by enzyme pathways in neighboring cells. Here we show that blue (450-nm) light converts atRAL specifically to 11cRAL through a retinyl-phospholipid intermediate in photoreceptor membranes. The quantum efficiency of this photoconversion is similar to rhodopsin. Photoreceptor membranes synthesize 11cRAL chromophore faster under blue light than in darkness. Live mice regenerate rhodopsin more rapidly in blue light. Finally, whole retinas and isolated cone cells show increased photosensitivity following exposure to blue light. These results indicate that light contributes to visual-pigment renewal in mammalian rods and cones through a non-enzymatic process involving retinyl-phospholipids.It is currently thought that visual pigments in vertebrate photoreceptors are regenerated exclusively through enzymatic cycles. Here the authors show that mammalian photoreceptors also regenerate opsin pigments in light through photoisomerization of N-ret-PE (N-retinylidene-phosphatidylethanolamine
Study of GaN LED ITO nano-gratings with standing wave analysis
This study reveals the effect of nanoscale ITO transmission gratings on light emission from the top, sides, and bottom of a GaN light-emitting diode (LED), based on the substrate standing wave analysis. First, we show that sapphire substrate thickness affects the standing wave pattern in the LED and find the best- and worst-case sapphire thicknesses. Second, we find that adding nanoscale ITO transmission gratings can improve light extraction by 222% or 253%, depending on the reference chosen. Third, we observe that maximizing top light emission with the nano-grating can significantly reduce bottom and side light emissions. Finally, we study grating performance over different wavelengths and generate the LED spectrum
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Collision Chains among the Terrestrial Planets. II. An Asymmetry between Earth and Venus
During the late stage of terrestrial planet formation, hit-and-run collisions are about as common as accretionary mergers, for expected velocities and angles of giant impacts. Average hit-and-runs leave two major remnants plus debris: the target and impactor, somewhat modified through erosion, escaping at lower relative velocity. Here we continue our study of the dynamical effects of such collisions. We compare the dynamical fates of intact runners that start from hit-and-runs with proto-Venus at 0.7 au and proto-Earth at 1.0 au. We follow the orbital evolutions of the runners, including the other terrestrial planets, Jupiter, and Saturn, in an N-body code. We find that the accretion of these runners can take ≳10 Myr (depending on the egress velocity of the first collision) and can involve successive collisions with the original target planet or with other planets. We treat successive collisions that the runner experiences using surrogate models from machine learning, as in previous work, and evolve subsequent hit-and-runs in a similar fashion. We identify asymmetries in the capture, loss, and interchange of runners in the growth of Venus and Earth. Hit-and-run is a more probable outcome at proto-Venus, being smaller and faster orbiting than proto-Earth. But Venus acts as a sink, eventually accreting most of its runners, assuming typical events, whereas proto-Earth loses about half, many of those continuing to Venus. This leads to a disparity in the style of late-stage accretion that could have led to significant differences in geology, composition, and satellite formation at Earth and Venus
Realistic On-the-fly Outcomes of Planetary Collisions: Machine Learning Applied to Simulations of Giant Impacts
Planet formation simulations are capable of directly integrating the evolution of hundreds to thousands of planetary embryos and planetesimals as they accrete pairwise to become planets. In principle, these investigations allow us to better understand the final configuration and geochemistry of the terrestrial planets, and also to place our solar system in the context of other exosolar systems. While these simulations classically prescribe collisions to result in perfect mergers, recent computational advances have begun to allow for more complex outcomes to be implemented. Here we apply machine learning to a large but sparse database of giant impact studies, which allows us to streamline the simulations into a classifier of collision outcomes and a regressor of accretion efficiency. The classifier maps a four-dimensional (4D) parameter space (target mass, projectile-to-target mass ratio, impact velocity, impact angle) into the four major collision types: merger, graze-and-merge, hit-and-run, and disruption. The definition of the four regimes and their boundary is fully data-driven. The results do not suffer from any model assumption in the fitting. The classifier maps the structure of the parameter space and it provides insights into the outcome regimes. The regressor is a neural network that is trained to closely mimic the functional relationship between the 4D space of collision parameters, and a real-variable outcome, the mass of the largest remnant. This work is a prototype of a more complete surrogate model, that will be based on extended sets of simulations (big data), that will quickly and reliably predict specific collision outcomes for use in realistic N-body dynamical studies of planetary formation.NASA Planetary Science Division; University of ArizonaThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
A-to-I editing on tRNAs: Biochemical, biological and evolutionary implications
AbstractInosine on transfer RNAs (tRNAs) are post-transcriptionally formed by a deamination mechanism of adenosines at positions 34, 37 and 57 of certain tRNAs. Despite its ubiquitous nature, the biological role of inosine in tRNAs remains poorly understood. Recent developments in the study of nucleotide modifications are beginning to indicate that the dynamics of such modifications are used in the control of specific genetic programs. Likewise, the essentiality of inosine-modified tRNAs in genome evolution and animal biology is becoming apparent. Here we review our current understanding on the role of inosine in tRNAs, the enzymes that catalyze the modification and the evolutionary link between such enzymes and other deaminases
Determinants of The Fear of The Pandemic and Its Effect on Voting Behavior Among Young Adult Filipinos in The Next Presidential Election
With COVID-19 severely impacting several aspects of society, the upcoming 2022 Philippine Presidential Elections will be the first to take place under such unique circumstances. This study provides information on how various determinants of fear of COVID-19 affect the voting behavior of young adult Filipinos. This study utilized a survey consisting of five sections composed of sociodemographic questionnaire, Multidimensional Scale of Perceived Social Support, Core Dimensions of Spirituality Questionnaire, Fear of COVID-19 Scale and a question about political participation. The results showed that individuals with a higher level of social support and higher level of spirituality were more likely to conform to the political ideals of their respective environments (i.e. family & religious institutions) and were more likely to participate in the elections, along with individuals with higher levels of fea
Non-photopic and photopic visual cycles differentially regulate immediate, early and late-phases of cone photoreceptor-mediated vision
Cone photoreceptors in the retina enable vision over a wide range of light intensities. However, the processes enabling cone vision in bright light (i.e. photopic vision) are not adequately understood. Chromophore regeneration of cone photopigments may require the retinal pigment epithelium (RPE) and/or retinal Müller glia. In the RPE, isomerization of all-trans-retinyl esters (atRE) to 11-cis-retinol (11cROL) is mediated by the retinoid isomerohydrolase Rpe65. A putative alternative retinoid isomerase, dihydroceramide desaturase-1 (DES1), is expressed in RPE and Müller cells. The retinol-isomerase activities of Rpe65 and Des1 are inhibited by emixustat and fenretinide, respectively. Here, we tested the effects of these visual cycle inhibitors on immediate, early and late phases of cone photopic vision. In zebrafish larvae raised under cyclic light conditions, fenretinide impaired late cone photopic vision, whereas emixustat-treated zebrafish unexpectedly had normal vision. In contrast, emixustat-treated larvae raised under extensive dark-adaption displayed significantly attenuated immediate photopic vision concomitant with significantly reduced 11-cis-retinaldehyde (11cRAL). Following 30 minutes of light, early photopic vision recovered, despite 11cRAL levels remaining significantly reduced. Defects in immediate cone photopic vision were rescued in emixustat- or fenretinide-treated larvae following exogenous 9-cis-retinaldehyde (9cRAL) supplementation. Genetic knockout of Des1 (degs1) or retinaldehyde-binding protein 1b (rlbp1b) did not eliminate photopic vision in zebrafish. Our findings define molecular and temporal requirements of the non-photopic or photopic visual cycles for mediating vision in bright light.European Commission Horizon 2020Irish Research CouncilNational Institutes of Health12 month embargo limited to 6 months due to H2020 - A
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