Context. Theory predicts that low-mass protoplanets in a protostellar disc migrate into the central star on a time scale that is short compared with the disc lifetime or the giant planet formation ...time scale. Protoplanet eccentricities of $e\ga H/r$ can slow or reverse migration, but previous 2D studies of multiple protoplanets embedded in a protoplanetary disc have shown that gravitational scattering cannot maintain significant planet eccentricities against disc-induced damping. The eventual fate of these systems was migration into the central star. Aims. Here we simulate the evolution of low-mass protoplanetary swarms in three dimensions. The aim is to examine both protoplanet survival rates and the dynamical structure of the resulting planetary systems, and to compare them with 2D simulations. Methods. We present results from a 3D hydrodynamic simulation of eight protoplanets embedded in a protoplanetary disc. We also present a suite of simulations performed using an N-body code, modified to include prescriptions for planetary migration and for eccentricity and inclination damping. These prescriptions were obtained by fitting analytic formulae to hydrodynamic simulations of planets embedded in discs with initially eccentric and/or inclined orbits. Results. As was found in two dimensions, differential migration produces groups of protoplanets in stable, multiple mean-motion resonances that migrate in lockstep, preventing prolonged periods of gravitational scattering. In almost all simulations, this leads to large-scale migration of the protoplanet swarm into the central star in the absence of a viable stopping mechanism. The evolution involves mutual collisions, occasional instances of large-scale scattering, and the frequent formation of the long-lived, co-orbital planet systems that arise in >30% of all runs. Conclusions. Disc-induced damping overwhelms eccentricity and inclination growth due to planet-planet interactions, leading to large-scale migration of protoplanet swarms. Co-orbital planets are a natural outcome of dynamical relaxation in a strongly dissipative environment, and if observed in nature would imply that such a period of evolution commonly arises during planetary formation.
Context.Theory predicts that low mass protoplanets in a laminar protostellar disc will migrate into the central star prior to disc dispersal. It is known that protoplanets on orbits with eccentricity ...$e \ga H/r$, where H is the disc scale height and r is the radius, can halt or reverse their migration. Aims.We examine whether a system of interacting protoplanetary cores can excite and sustain significant eccentricity of the population, allowing some planetary cores to survive in the disc over its lifetime. Methods.We employ two distinct numerical schemes: an N-body code, adapted to include migration and eccentricity damping due to the gas disc via analytic prescriptions, and a hydrodynamics code that explicitly evolves a 2D protoplanetary disc model with embedded protoplanets. The former allows us to study the long term evolution, the latter to model the systems with greater fidelity but for shorter times. Results.After a brief period of chaotic interaction between the protoplanets that involves scattering, orbital exchange, collisions and the formation of co-orbital planets, we find that the system settles into a quiescent state of inward migration. Differential migration causes the protoplanets to form a series of mean motion resonances, such that a planet is often in resonance with both its interior and exterior neighbours. This helps prevent close encounters and leads to the protoplanetary swarm, or subgroups within it, migrating inward at a uniform rate. In about $2 \%$ of runs a single planet is scattered onto a distant orbit with significant eccentricity, allowing it to survive in the disc for ${\sim} 10^6$ years. Over $20 \%$ of runs produce co-orbital planets that survive for the duration of the simulation, occupying mutual horseshoe or tadpole orbits. Conclusions.Disc-induced damping overwhelms eccentricity growth through planet-planet interactions, such that a protoplanetary swarm migrates inward. We suggest co-orbital planets may be observed in future exoplanet searches.
In response to the increasingly complex social–ecological issues facing society, there is a growing trend to conduct environmental research in large collaborative programs. This approach is described ...as transdisciplinary research as it transcends formal disciplinary boundaries, explicitly acknowledges that many different perspectives are relevant to the resolution of complex problems, and actively involves the users of research. This poses challenges for the evaluation of “impact” as any evaluation process must take into consideration the different expectations, values, culture, language and reward structures of the main participating groups, the funders, researchers and end users. How can these participating groups learn about the progress of a transdisciplinary research program in a way that is purposeful and structured, continues through the life of the program, and includes explicit feedback mechanisms that facilitate adaptation during the course of the program? This paper presents a framework for co-reflecting on the accomplishment of transdisciplinary research programs. The framework incorporates the perspectives of funders, researchers and users, and recognizes that while they place different emphasis on measures of achievement such as efficiency, rigor and relevance, ultimate accomplishment in terms of translating knowledge into practice requires that the needs and expectations of all three groups are adequately addressed. What emerges from the framework is the importance of early investment in processes, behaviors and relationships that foster social learning and the co-production of the knowledge and understanding that are required to ensure relevance; while maintaining emphasis in the traditional areas of formally testing evidence and mentoring young researchers to ensure rigor and build confidence and capacity in transdisciplinary approaches.
Context.Young planets embedded in their protoplanetary disk interact gravitationally with it leading to energy and angular momentum exchange. This interaction determines the evolution of the planet ...through changes to the orbital parameters. Aims.We investigate changes in the orbital elements of a 20 Earth-mass planet due to the torques from the disk. We focus on the non-linear evolution of initially non-vanishing eccentricity, e, and/or inclination, i. Methods.We treat the disk as a two- or three-dimensional viscous fluid and perform hydrodynamical simulations using finite difference methods. The planetary orbit is updated according to the gravitational torque exerted by the disk. We monitor the time evolution of the orbital elements of the planet. Results.We find rapid exponential decay of the planet orbital eccentricity and inclination for small initial values of e and i, in agreement with linear theory. For larger values of $e > 0.1$ the decay time increases and the decay rate scales as $\dot{e} \propto e^{-2}$, consistent with existing theoretical models. For large inclinations ($i > 6^\circ$) the inclination decay rate shows an identical scaling d$i/{\rm d}t \propto i^{-2}$. We find an interesting dependence of the migration on the eccentricity. In a disk with aspect ratio $H/r=0.05$ the migration rate is enhanced for small non-zero eccentricities ($e < 0.1$), while for larger values we see a significant reduction by a factor of ~4. We find no indication for a reversal of the migration for large e, although the torque experienced by the planet becomes positive when $e \simeq 0.3$. This inward migration is caused by the persisting energy loss of the planet. Conclusions.For non gap forming planets, eccentricity and inclination damping occurs on a time scale that is very much shorter than the migration time scale. The results of non linear hydrodynamic simulations are in very good agreement with linear theory for values of e and i for which the theory is applicable (i.e. e and $i \le H/r$).
Little is known about the mechanism by which IFNs inhibit human cytomegalovirus (HCMV) replication. Indeed, infection of fibroblasts with HCMV initiates the expression of a subset of type I ...IFN-inducible genes whose role in the infectious process is unclear. We describe here the identification of a cytoplasmic antiviral protein that is induced by IFNs, by HCMV infection, and by the HCMV envelope protein, glycoprotein B (gB). Stable expression of the protein in fibroblasts inhibits productive HCMV infection, down-regulating several HCMV structural proteins (gB, pp28, and pp65) known to be indispensable for viral assembly and maturation. We have named the protein viperin (for virus inhibitory protein, endoplasmic reticulum-associated, interferon-inducible). HCMV infection causes the redistribution of the induced viperin from its normal endoplasmic reticulum association, first to the Golgi apparatus and then to cytoplasmic vacuoles containing gB and pp28. Expression before HCMV infection reduces viperin redistribution from the endoplasmic reticulum to the Golgi apparatus and prevents vacuolar localization, perhaps reflecting the mechanism used by HCMV to evade the antiviral function.
Glycosylation and the Immune System Rudd, Pauline M.; Elliott, Tim; Cresswell, Peter ...
Science (American Association for the Advancement of Science),
03/2001, Letnik:
291, Številka:
5512
Journal Article
Recenzirano
Almost all of the key molecules involved in the innate and adaptive immune response are glycoproteins. In the cellular immune system, specific glycoforms are involved in the folding, quality control, ...and assembly of peptide-loaded major histocompatibility complex (MHC) antigens and the T cell receptor complex. Although some glycopeptide antigens are presented by the MHC, the generation of peptide antigens from glycoproteins may require enzymatic removal of sugars before the protein can be cleaved. Oligosaccharides attached to glycoproteins in the junction between T cells and antigen-presenting cells help to orient binding faces, provide protease protection, and restrict nonspecific lateral protein-protein interactions. In the humoral immune system, all of the immunoglobulins and most of the complement components are glycosylated. Although a major function for sugars is to contribute to the stability of the proteins to which they are attached, specific glycoforms are involved in recognition events. For example, in rheumatoid arthritis, an autoimmune disease, agalactosylated glycoforms of aggregated immunoglobulin G may induce association with the mannose-binding lectin and contribute to the pathology.
A comparative study of disc-planet interaction De Val-Borro, M.; Edgar, R. G.; Artymowicz, P. ...
Monthly Notices of the Royal Astronomical Society,
August 2006, Letnik:
370, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We perform numerical simulations of a disc-planet system using various grid-based and smoothed particle hydrodynamics (SPH) codes. The tests are run for a simple setup where Jupiter and Neptune mass ...planets on a circular orbit open a gap in a protoplanetary disc during a few hundred orbital periods. We compare the surface density contours, potential vorticity and smoothed radial profiles at several times. The disc mass and gravitational torque time evolution are analysed with high temporal resolution. There is overall consistency between the codes. The density profiles agree within about 5 per cent for the Eulerian simulations. The SPH results predict the correct shape of the gap although have less resolution in the low-density regions and weaker planetary wakes. The disc masses after 200 orbital periods agree within 10 per cent. The spread is larger in the tidal torques acting on the planet which agree within a factor of 2 at the end of the simulation. In the Neptune case, the dispersion in the torques is greater than for Jupiter, possibly owing to the contribution from the not completely cleared region close to the planet.
This study evaluated the accuracy of pressure plate apparatus for measuring soil water retention at -0.5 and -1.5 MPa matric potential. Samples from 35 contrasting Australian soils were wetted with ...distilled water and drained on pressure plate apparatus at -0.5 and -1.5 MPa. The soil matric potential of each sample was then determined using a thermocouple psychrometer, and water content was measured. Water content at exactly -0.5 and -1.5 MPa matric potential was determined independently by interpolating between replicates of matric potential-water content data measured using a thermocouple psychrometer. Water content of the soil samples at apparent equilibrium on pressure plates was compared with these "target" water contents. The 35 samples on pressure plates at -1.5 MPa equilibrated, on average, to 0.3% (w/w) wetter than the target water content, with mean matric potential of -1.10 MPa. Fifteen samples were significantly wetter than the target values. Soil samples on pressure plates at -0.5 MPa equilibrated, on average, to 0.2% (w/w) wetter than the target water content, attaining a mean matric potential of -0.48 MPa. Mean error in water content at -1.5 MPa on pressure plates was reduced from >0.5 to <0.1% (w/w) in a subset of 10 samples prone to dispersion by wetting with 0.01 mol L-1 CaCl2. Water contents of samples equilibrated on pressure plates at -1.5 MPa were good estimates of "true" -1.5 MPa water content for the nonswelling soils tested, provided CaCl2 was used to minimize dispersion. Vapor equilibrium measurement methods are recommended for swelling soils.
Major histocompatibility complex (MHC) class II molecules go to extraordinary lengths to make sure that they bind to peptides generated in the endosomal-lysosomal system (reviewed by Cresswell, ...1994). First, in the endoplasmic reticulum (ER), newly synthesized class II alpha and beta chains associate with the invariant (Ii) chain to form a complex that itself is incapable of binding peptides. This complex contains three class II alpha beta dimers associated with an Ii chain trimer. Following transit of the alpha beta Ii complex through the Golgi apparatus, targeting signals in the cytoplasmic domain of the Ii chain drive the complex into the endosomal-lysosomal pathway. Once in this pathway, the Ii chain is proteolytically degraded, leaving a small fragment (the class II-associated Ii chain peptide CLIP) bound to the released alpha beta dimers. Interaction of alpha beta CLIP complexes in a specialized lysosome-like compartment with another class II-related alpha beta dimer, called HLA-DM in the human system and H2-M in mice, drives out the residual CLIP and allows the class II molecules to bind lysosomally generated peptides. The peptide-loaded class II molecules, the majority of which are not substrates for HLA-DM, then leave this compartment, designated the MHC class II compartment, by an unknown route to be expressed on the cell surface and surveyed by CD4 super(+) T lymphocytes. The model as set out above raises some interesting structural and mechanistic questions. First, what are the structural properties of the Ii chain that allow it to interact with class II molecules in a way that prevents them from binding peptides? Second, what are the structural properties of the Ii chain that make it susceptible to proteolysis, so as to generate alpha beta CLIP from alpha beta Ii? Third, where is CLIP bound to class II molecules: is it in the binding groove like a conventional peptide or elsewhere? Finally, what is it about the structure of alpha beta CLIP that is responsible for HLA-DM-induced CLIP dissociation? The first three questions have been at least partially answered by several recent papers, including two structurally oriented papers from the group led by Don Wiley. The answer to the fourth question remains enigmatic following the recent publication of the three-dimensional structure of the HLA-DR3-CLIP complex by the Wiley group in collaboration with a group led by Elizabeth Mellins.
Spatiotemporal behavior of soil water is essential to understand the science of hydrodynamics. Data intensive measurement of surface soil water using remote sensing has established that the spatial ...variability of soil water can be described using the principle of self-similarity (scaling properties) or fractal theory. This information can be used in determining land management practices provided the surface scaling properties are kept at deep layers. The current study examined the scaling properties of sub-surface soil water and their relationship to surface soil water, thereby serving as supporting information for plant root and vadose zone models. Soil water storage (SWS) down to 1.4 m depth at seven equal intervals was measured along a transect of 576 m for 5 years in Saskatchewan. The surface SWS showed multifractal nature only during the wet period (from snowmelt until mid- to late June) indicating the need for multiple scaling indices in transferring soil water variability information over multiple scales. However, with increasing depth, the SWS became monofractal in nature indicating the need for a single scaling index to upscale/downscale soil water variability information. In contrast, all soil layers during the dry period (from late June to the end of the growing season in early November) were monofractal in nature, probably resulting from the high evapotranspirative demand of the growing vegetation that surpassed other effects. This strong similarity between the scaling properties at the surface layer and deep layers provides the possibility of inferring about the whole profile soil water dynamics using the scaling properties of the easy-to-measure surface SWS data.
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