Shallow Gradient Research Articles

An Assessment of Mobile Predator Populations along Shallow and Mesophotic Depth Gradients in the Hawaiian Archipelago

Large-bodied coral reef roving predators (sharks, jacks, snappers) are largely considered to be depleted around human population centers. In the Hawaiian Archipelago, supporting evidence is primarily derived from underwater visual censuses in shallow waters (≤30 m). However, while many roving predators are present or potentially more abundant in deeper strata (30–100 m+), distributional information remains sparse. To partially fill that knowledge gap, we conducted surveys in the remote Northwestern Hawaiian Islands (NWHI) and populated Main Hawaiian Islands (MHI) from 2012–2014 using baited remote underwater stereo-video. Surveys between 0–100 m found considerable roving predator community dissimilarities between regions, marked conspicuous changes in species abundances with increasing depth, and largely corroborated patterns documented during shallow water underwater visual censuses, with up to an order of magnitude more jacks and five times more sharks sampled in the NWHI compared to the MHI. Additionally, several species were significantly more abundant and larger in mesophotic versus shallow depths, which remains particularly suggestive of deep-water refugia effects in the MHI. Stereo-video extends the depth range of current roving predator surveys in a more robust manner than was previously available, and appears to be well-suited for large-scale roving predator work in the Hawaiian Archipelago.

Evidence that differentiation-inducing factor-1 controls chemotaxis and cell differentiation, at least in part, via mitochondria in D. discoideum.

ABSTRACTDifferentiation-inducing factor-1 [1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one (DIF-1)] is an important regulator of cell differentiation and chemotaxis in the development of the cellular slime mold Dictyostelium discoideum. However, the entire signaling pathways downstream of DIF-1 remain to be elucidated. To characterize DIF-1 and its potential receptor(s), we synthesized two fluorescent derivatives of DIF-1, boron-dipyrromethene (BODIPY)-conjugated DIF-1 (DIF-1-BODIPY) and nitrobenzoxadiazole (NBD)-conjugated DIF-1 (DIF-1-NBD), and investigated their biological activities and cellular localization. DIF-1-BODIPY (5 µM) and DIF-1 (2 nM) induced stalk cell differentiation in the DIF-deficient strain HM44 in the presence of cyclic adenosine monosphosphate (cAMP), whereas DIF-1-NBD (5 µM) hardly induced stalk cell differentiation under the same conditions. Microscopic analyses revealed that the biologically active derivative, DIF-1-BODIPY, was incorporated by stalk cells at late stages of differentiation and was localized to mitochondria. The mitochondrial uncouplers carbonyl cyanide m-chlorophenylhydrazone (CCCP), at 25–50 nM, and dinitrophenol (DNP), at 2.5–5 µM, induced partial stalk cell differentiation in HM44 in the presence of cAMP. DIF-1-BODIPY (1–2 µM) and DIF-1 (10 nM), as well as CCCP and DNP, suppressed chemotaxis in the wild-type strain Ax2 in shallow cAMP gradients. These results suggest that DIF-1-BODIPY and DIF-1 induce stalk cell differentiation and modulate chemotaxis, at least in part, by disturbing mitochondrial activity.

Provenance of Lower Cretaceous clastic reservoirs in the Middle East

Some of the world's largest oilfields contain reservoirs in Lower Cretaceous (Hauterivian–Albian) clastic successions of the Middle East. They comprise sandstone-prone intervals of hundreds of metres thickness deposited in non-marine to paralic settings on a very shallow gradient, otherwise carbonate-dominated, passive margin. This study integrates petrography, X-ray powder diffraction, heavy mineral analysis, mineral chemistry, zircon geochronology and palaeodrainage basin scale calculations to determine the likely provenance of the clastic deposits. Regional geological considerations are then used to suggest the likely driving mechanisms for the clastic influx. The sediments are highly texturally and chemically mature with low-diversity heavy mineral assemblages, suggesting extensive weathering and sediment recycling. The mixture of heavy minerals derived from metapelitic, metamafic and granitic domains at variable metamorphic grades is consistent with erosion of a complex heterogeneous basement or recycling of multiple earlier cycle sediments. Comparison with published data suggests an Arabian Shield provenance, with input from Precambrian basement and Palaeozoic sedimentary cover both possible. This is consistent with drainage basin scaling calculations. Large-scale tectonic reorganization associated with opening of the South and Equatorial Atlantic together with mantle plume related uplift in the northern Arabian Shield is postulated as the main driving mechanism of clastic influx.

Continental climate gradients in North America and Western Eurasia before and after the closure of the Central American Seaway

The Gulf Stream, as part of the Atlantic Meridional Overturning Circulation (AMOC), is known as a major driver of latitudinal energy transport in the North Atlantic presently causing mild winters over northwestern Eurasia. The intensity of the AMOC throughout the Neogene, prior to the final closure of the Central American Seaway (CAS) in the early Pliocene, is still poorly known, but most authors assume that the circulation was considerably weaker than present. Here we address this issue from a continental point of view. We studied the past AMOC intensity by analyzing Neogene continental climate patterns along North American and Western Eurasian transects. Based on a total of 317 palaeofloras thermal latitudinal gradients are reconstructed for three Neogene time slices, namely the middle Miocene, late Miocene, and late Pliocene using the Coexistence Approach to obtain quantitative climate data. The obtained proxy-based, continental temperature gradients are evaluated against data from a selection of published General Circulation Model (GCM) simulations for the three time slices studied.Our study suggests that shallow thermal latitudinal gradients existed in North America and Western Eurasia throughout the Miocene but became strongly steepened in the late Pliocene. In both Miocene time slices studied, the higher latitudes were by up to 30 °C warmer than present (cold month mean), also at times with presumed pre-industrial CO2 such as the late Miocene. In the late Pliocene high-latitude, the temperature difference with respect to the present had decreased by up to 10 °C (cold month mean). Both mean annual temperatures and cold month means of the lower mid and low latitudes were at the present-day level throughout all three time slices, or even slightly below. In both Miocene time slices, zonal temperature means at both continental transects were similar in the mid and higher latitudes. However, several northwest European sites reveal very mild winter condition suggesting the early existence of a probably less intense Palaeo-Gulf Stream. The distinct thermal anomaly (annual and cold month means) today existing between North America and Western Eurasia appeared for the first time in the late Pliocene, attaining about 50% of the present-day magnitude. This supports the assumption that the AMOC intensified after the final closure of the CAS during the early Pliocene. The results obtained from the palaeobotanical proxies are in line with data from coeval marine archives, particularly with North Atlantic sea surface temperatures (SSTs) inferred from oxygen isotopes. However, the proxy-based thermal gradients are not well reproduced by a selection of GCM simulations, due to a well-known systematic underestimation of high latitude warming by GCMs for the Miocene and Pliocene time slices.

Synthesis and materialization of a reaction-diffusion French flag pattern.

During embryo development, patterns of protein concentration appear in response to morphogen gradients. These patterns provide spatial and chemical information that directs the fate of the underlying cells. Here, we emulate this process within non-living matter and demonstrate the autonomous structuration of a synthetic material. First, we use DNA-based reaction networks to synthesize a French flag, an archetypal pattern composed of three chemically distinct zones with sharp borders whose synthetic analogue has remained elusive. A bistable network within a shallow concentration gradient creates an immobile, sharp and long-lasting concentration front through a reaction-diffusion mechanism. The combination of two bistable circuits generates a French flag pattern whose 'phenotype' can be reprogrammed by network mutation. Second, these concentration patterns control the macroscopic organization of DNA-decorated particles, inducing a French flag pattern of colloidal aggregation. This experimental framework could be used to test reaction-diffusion models and fabricate soft materials following an autonomous developmental programme.

Initiation of modern-style plate tectonics recorded in Mesoarchean marine chemical sediments

The chemistry of the oceans in part reflects a balance between inputs from the continents and mantle. Traditionally, it has been thought that Archean ocean chemistry was dominated by mantle sources, but recent work has suggested that continental weathering during the Archean provided a much higher flux to the oceans than previously recognized. Here, we present new Rb-Sr and Sm-Nd isotope compositions on carbonate (dolomite and limestone) from the 2.94Ga Red Lake and 2.80Ga Steep Rock groups in the Superior Province, Canada to assess the potential impact continental weathering had on ocean chemistry during the Mesoarchean, a time when initiation of modern-style plate tectonics has been proposed to have occurred. The low Rb contents of all carbonate samples suggest that clastic contamination does not affect the Sr isotope compositions. Using O and Sr isotope modeling, we identified unaltered samples and estimate a 87Sr/86Sr ratio of 0.70173 for seawater at 2.94Ga and 0.70182 at 2.80Ga. Strontium isotope compositions from both Red Lake and Steep Rock indicate that seawater was significantly more radiogenic than contemporaneous mantle, and suggests that weathering of evolved continental crust was an important input to seawater. Continental weathering likely affected seawater chemistry through uplift of continental lithosphere during the initiation of modern-style plate tectonics at 3.2Ga, a model that is contrary to those that suggest the Archean continents were small in extent and largely submerged. Initiation of modern-style plate tectonics and associated continental weathering had an important effect on the biosphere, including increased nutrient delivery, as well as creation of ecological niches that allowed development of the first biologically produced shallow marine redox gradients.

Yeast chemotropism: A paradigm shift in chemical gradient sensing

The ability of cells to direct their movement and growth in response to shallow chemical gradients is essential in the life cycles of all eukaryotic organisms. The signaling mechanisms underlying directional sensing in chemotactic cells have been well studied; however, relatively little is known about how chemotropic cells interpret chemical gradients. Recent studies of chemotropism in budding and fission yeast have revealed 2 quite different mechanisms-biased wandering of the polarity complex, and differential internalization of the receptor and G protein. Each of these mechanisms has been proposed to play a key role in decoding mating pheromone gradients. Here we explore how they may work together as 2 essential components of one gradient sensing machine.

Coupled excitable Ras and F-actin activation mediates spontaneous pseudopod formation and directed cell movement.

Many eukaryotic cells regulate their mobility by external cues. Genetic studies have identified >100 components that participate in chemotaxis, which hinders the identification of the conceptual framework of how cells sense and respond to shallow chemical gradients. The activation of Ras occurs during basal locomotion and is an essential connector between receptor and cytoskeleton during chemotaxis. Using a sensitive assay for activated Ras, we show here that activation of Ras and F-actin forms two excitable systems that are coupled through mutual positive feedback and memory. This coupled excitable system leads to short-lived patches of activated Ras and associated F-actin that precede the extension of protrusions. In buffer, excitability starts frequently with Ras activation in the back/side of the cell or with F-actin in the front of the cell. In a shallow gradient of chemoattractant, local Ras activation triggers full excitation of Ras and subsequently F-actin at the side of the cell facing the chemoattractant, leading to directed pseudopod extension and chemotaxis. A computational model shows that the coupled excitable Ras/F-actin system forms the driving heart for the ordered-stochastic extension of pseudopods in buffer and for efficient directional extension of pseudopods in chemotactic gradients.

Characterizing the Role of Yeast Cyclin Pcl1 in the Establishment of Pheromone Receptor Polarity

Chemotaxis (directed cell movement) and chemotropism (directed cell growth) are vital mechanisms essential to a variety of biological processes, including cell development, cancer metastasis, angiogenesis, and axon guidance. Both chemotaxis and chemotropism depend on the ability of the cell to interpret shallow and complex chemical gradients. One of the best‐studied models of eukaryotic directional sensing is the mating response of Saccharomyces cerevisiae (budding yeast). Yeast exist as two haploid mating types, MATa and MATα, that can sense pheromone gradients, chemotrop toward the closest mating partner, and eventually fuse at their tips to form diploid cells. In vegetative cells, the G protein coupled receptor (GPCR) that binds pheromone and its associated heterotrimeric G protein are uniformly distributed on the plasma membrane. Upon receptor activation, Gβγ is released from Gα and Gβ is rapidly phosphorylated on multiple residues. Free Gβγ signals through a MAP kinase cascade to trigger cell‐cycle arrest and changes in gene expression. Pheromone‐activated receptors are globally internalized and then reappear as polarized crescents that mark the chemotropic growth site, where the cell ultimately forms its mating projection. Previous studies have shown that the receptor crescents are visible before detectable actin polarization and that the receptors can polarize without actin‐dependent directed secretion [1]. On the contrary, inhibiting receptor internalization confers a moderate defect in gradient sensing [2]. A key question raised by these observations is how does the yeast cell interpret the external signals and establish the chemotropic growth site? From a directed genetic screen, we found that Pcl1 is critical for the establishment of receptor polarity. Pcl1 is a cyclin required by the cyclin dependent kinase Pho85, which has been implicated in polarization during budding. It is known that Gβ phosphorylation is crucial to receptor polarization, and Gβ is predicted to be a substrate of Pho85‐Pcl1 [3]. Additionally, our data indicated a genetic interaction between Gβ and Pcl1. Here, we show that Pcl1 localizes to the mating projection during the mating response. Moreover, Pcl1 and Gβ interacted directly on the plasma membrane of pheromone‐treated cells, as indicated by a Bimolecular fluorescence complementation assay. Lastly, Pho85 inactivation also appears to affect receptor polarity. Taken together, these results suggest that Pho85‐Pcl1 regulates polarization of the receptor by phosphorylating Gβ.Support or Funding InformationNational Science Foundation

Monolithic stationary phases with a longitudinal gradient of porosity.

The duration of the hypercrosslinking reaction has been used to control the extent of small pores formation in polymer-based monolithic stationary phases. Segments of five columns hypercrosslinked for 30-360min were coupled via zero-volume unions to prepare columns with segmented porosity gradients. The steepness of the porosity gradient affected column efficiency, mass transfer resistance, and separation of both small-molecule alkylbenzenes and high-molar-mass polystyrene standards. In addition, the segmented column with the steepest porosity gradient was prepared as a single column with a continuous porosity gradient. The steepness of porosity gradient in this type column was tuned. Compared to a completely hypercrosslinked column, the column with the shallower gradient produced comparable size-exclusion separation of polystyrene standards but allowed higher column permeability. The completely hypercrosslinked column and the column with porosity gradient were successfully coupled in online two-dimensional liquid chromatography of polymers.

Pedestal structure and inter-ELM evolution for different main ion species in ASDEX Upgrade

In tokamak plasmas with different main ion species, a change in confinement occurs, known as the isotope effect. Experiments comparing hydrogen (H), deuterium (D), and helium (4He) plasmas have been performed to identify processes that define the pedestal structure and evolution in between the crashes of edge localized modes (ELMs). The pedestal top electron densities and temperatures have been matched to compare the pedestal shape and stability. In the D and H discharges, the pedestal electron temperature profiles do not differ, whereas the density profile in H has shallower gradients. Furthermore, the heat flux across the pedestal in H is roughly a factor of two higher than in D. In 4He plasmas at similar stored energy, the pedestal top electron density is roughly a factor of 1.5 larger than in the references owing to the larger effective charge. The peeling-ballooning theory, which is independent of the main ion species mass, can sufficiently describe the pedestal stability in the hydrogenic plasmas. The inter-ELM pedestal evolution has the same sequence of recovery phases for all investigated species, giving evidence that similar mechanisms are acting in the pedestals. This is further supported by a similar evolution of the inter-ELM magnetic signature and the corresponding toroidal structure.

The Grism Lens-Amplified Survey from Space (GLASS). X. Sub-kiloparsec Resolution Gas-phase Metallicity Maps at Cosmic Noon behind the Hubble Frontier Fields Cluster MACS1149.6+2223

Abstract We combine deep Hubble Space Telescope grism spectroscopy with a new Bayesian method to derive maps of gas-phase metallicity for 10 star-forming galaxies at high redshift ( ). Exploiting lensing magnification by the foreground cluster MACS1149.6+2223, we reach sub-kiloparsec spatial resolution and push the limit of stellar mass associated with such high-z spatially resolved measurements below for the first time. Our maps exhibit diverse morphologies, indicative of various effects such as efficient radial mixing from tidal torques, rapid accretion of low-metallicity gas, and other physical processes that can affect the gas and metallicity distributions in individual galaxies. Based upon an exhaustive sample of all existing sub-kiloparesec resolution metallicity gradient measurements at high z, we find that predictions given by analytical chemical evolution models assuming a relatively extended star-formation profile in the early disk-formation phase can explain the majority of observed metallicity gradients, without involving galactic feedback or radial outflows. We observe a tentative correlation between stellar mass and metallicity gradients, consistent with the “downsizing” galaxy formation picture that more massive galaxies are more evolved into a later phase of disk growth, where they experience more coherent mass assembly at all radii and thus show shallower metallicity gradients. In addition to the spatially resolved analysis, we compile a sample of homogeneously cross-calibrated integrated metallicity measurements spanning three orders of magnitude in stellar mass at z ∼ 1.8. We use this sample to study the mass–metallicity relation (MZR) and find that the slope of the observed MZR can rule out the momentum-driven wind model at a 3σ confidence level.

Pennypack Creek Drainage Basin Erosion History: Bucks, Montgomery, and Philadelphia Counties, PA, USA

Topographic map evidence is used to interpret Pennypack Creek drainage basin erosion history in and north of the City of Philadelphia, Pennsylvania (USA). Southwest and west-southwest oriented through valleys crossing the south oriented Pennypack Creek drainage basin, barbed Pennypack Creek tributaries, and significant valley direction changes are used to determine that the Pennypack Creek valley eroded headward across massive southwest oriented floods. Initially floodwaters flowed on a low gradient topographic surface at least as high, if not higher, than the highest Pennypack Creek drainage basin elevations today. Shallow low gradient diverging and converging flow channels were eroded into the underlying bedrock surface predominantly along fault lines and other zones of easier to erode materials. Headward erosion of the much deeper Pennypack Creek valley across this anastomosing channel complex captured southwest oriented floodwaters and flow on northeast ends of beheaded channels was reversed so as to move toward the newly eroded and deeper Pennypack Creek valley. These reversed flow channels captured southwest oriented floodwaters still moving north of the actively eroding Pennypack Creek valley head. This captured water then moved in a northeast direction and eroded deep northeast oriented valleys headward from the newly eroded Pennypack Creek valley. These valleys today account for northeast and east oriented Pennypack Creek valley segments and northeast oriented (barbed) tributaries flowing to south oriented Pennypack Creek. The floodwater source cannot be determined from Pennypack Creek drainage basin evidence, but was from the northeast. Melting of a continental ice sheet could produce floods of sufficient volume and duration to overwhelm whatever drainage system previously existed and to erode new drainage basins in a manner similar to how the Pennypack Creek drainage basin was eroded.

Testing the limits of gradient sensing.

The ability to detect a chemical gradient is fundamental to many cellular processes. In multicellular organisms gradient sensing plays an important role in many physiological processes such as wound healing and development. Unicellular organisms use gradient sensing to move (chemotaxis) or grow (chemotropism) towards a favorable environment. Some cells are capable of detecting extremely shallow gradients, even in the presence of significant molecular-level noise. For example, yeast have been reported to detect pheromone gradients as shallow as 0.1 nM/μm. Noise reduction mechanisms, such as time-averaging and the internalization of pheromone molecules, have been proposed to explain how yeast cells filter fluctuations and detect shallow gradients. Here, we use a Particle-Based Reaction-Diffusion model of ligand-receptor dynamics to test the effectiveness of these mechanisms and to determine the limits of gradient sensing. In particular, we develop novel simulation methods for establishing chemical gradients that not only allow us to study gradient sensing under steady-state conditions, but also take into account transient effects as the gradient forms. Based on reported measurements of reaction rates, our results indicate neither time-averaging nor receptor endocytosis significantly improves the cell’s accuracy in detecting gradients over time scales associated with the initiation of polarized growth. Additionally, our results demonstrate the physical barrier of the cell membrane sharpens chemical gradients across the cell. While our studies are motivated by the mating response of yeast, we believe our results and simulation methods will find applications in many different contexts.

SG1120-1202: Mass-quenching as Tracked by UV Emission in the Group Environment at z = 0.37

Abstract We use the Hubble Space Telescope to obtain WFC3/F390W imaging of the supergroup SG1120-1202 at , mapping the UV emission of 138 spectroscopically confirmed members. We measure total (F390W–F814W) colors and visually classify the UV morphology of individual galaxies as “clumpy” or “smooth.” Approximately 30% of the members have pockets of UV emission (clumpy) and we identify for the first time in the group environment galaxies with UV morphologies similar to the “jellyfish” galaxies observed in massive clusters. We stack the clumpy UV members and measure a shallow internal color gradient, which indicates that unobscured star formation is occurring throughout these galaxies. We also stack the four galaxy groups and measure a strong trend of decreasing UV emission with decreasing projected group distance ( ). We find that the strong correlation between decreasing UV emission and increasing stellar mass can fully account for the observed trend in (F390W–F814W)– , i.e., mass-quenching is the dominant mechanism for extinguishing UV emission in group galaxies. Our extensive multi-wavelength analysis of SG1120-1202 indicates that stellar mass is the primary predictor of UV emission, but that the increasing fraction of massive (red/smooth) galaxies at R 200 and existence of jellyfish candidates is due to the group environment.

Sensitivity and Robustness in an Axon Guidance Signaling System

Correct functioning of the brain requires the orderly wiring of billions of cells during development. A major mechanism that mediates this is the guidance of axons to their targets by extracellular chemical gradients. Although well studied, the computational rules by which even simple guidance decisions are made are not yet understood at a biophysical level. The task of a growing axon to estimate a direction signal is immense, as unavoidable biological noise corrupts the measurement at all levels of processing. Here, we show how the modulation of growth by a balance of positive and negative feedback can explain the remarkable chemotactic sensitivity observed in vitro. We perform a detailed analysis of the experimental data of ref. [1] which characterizes the neurite growth of ∼2500 rat dorsal root ganglia explants in very shallow gradients of nerve growth factor. Constrained by these data, we construct a model chemical signaling system for growth and guidance in the developing brain. In the model, amplification of the gradient signal occurs via paracrine signalling between cell bodies within the ganglion, while robustness is conferred by the dynamics of receptor trafficking. The model gives a unified and quantitative account of experimentally observed behavior, and yields testable predictions with implications for understanding brain development and repair after injury.[1] Mortimer, D. et al (2009) A Bayesian model predicts the response of axons to molecular gradients. Proc. Natl. Acad. Sci. U.S.A, 106(25), 10296-10301.

Macroscopic Limits of Pathway-Based Kinetic Models for E. coli Chemotaxis in Large Gradient Environments

It is of great biological interest to understand the molecular origins of chemotactic behavior of E. coli by developing population-level models based on the underlying signaling pathway dynamics. We derive macroscopic models for E. coli chemotaxis that quantitatively match with the agent-based model for all ranges of the spatial gradient, in particular when the chemical gradient is large such that the standard Keller--Segel model is no longer valid. These equations are derived both formally and rigorously as asymptotic limits for pathway-based kinetic equations. We also present numerical results that show good agreement between the macroscopic models and SPECS. Our work provides an answer to the question of how to determine the population-level diffusion coefficient and drift velocity from the molecular mechanisms of chemotaxis, for both shallow gradient and large gradient environments.

A highly sensitive high performance liquid chromatography-electrochemical detection method for the determination of five phenolic compounds from Salvia miltiorrhiza

The determination of antioxidants continues to be interested, since the oxidative damage is thought to be one of the main mechanisms involved in nearly all chronic renal pathologies. A highly sensitive high performance liquid chromatography-electrochemical detection (HPLC-ECD) method was developed for evaluating the antioxidant properties of Salvia miltiorrhiza (Dan Shen). The method was optimized with respect to selectivity and sensitivity. Chromatographic conditions, including mobile phase pH value, buffer concentration, buffer type, organic solvent type, gradient profile and flow rate, were systematically investigated. Low pH value (2.8), low buffer concentration (20 mmol/L NaH2PO4), a shallow water-acetonitrile gradient, and a flow rate of 0.2 mL/min were the determined optimal conditions for the quantitative analysis of aimed five antioxidants from 14 batches of Dan Shen samples. The described method provided a good recovery (>95%), a very wide linear range (up to 104 for all analytes), a good precision (RSDs<4.01%), and a high sensitivity (LOQ of caffeic acid, 1.5 μ g/L). Compared with UV detection, the described ECD method was also more effective for evaluating the antioxidant properties of Dan Shen, as it provided highly selective detection of electro-active antioxidants.

ON HELIUM MIXING IN QUASI-GLOBAL SIMULATIONS OF THE INTRACLUSTER MEDIUM

ABSTRACT The assumption of a spatially uniform helium distribution in the intracluster medium (ICM) can lead to biases in the estimates of key cluster parameters if composition gradients are present. The helium concentration profile in galaxy clusters is unfortunately not directly observable. Current models addressing the putative sedimentation are one-dimensional and parametrize the presence of magnetic fields in a crude way, ignoring the weakly collisional, magnetized nature of the medium. When these effects are considered, a wide variety of instabilities can play an important role in the plasma dynamics. In a series of recent papers, we have developed the local, linear theory of these instabilities and addressed their nonlinear development with a modified version of Athena. Here, we extend our study by developing a quasi-global approach that we use to simulate the mixing of helium as induced by generalizations of the heat-flux-driven buoyancy instability (HBI) and the magnetothermal instability, which feed off thermal and composition gradients. In the inner region of the ICM, mixing can occur over a few gigayears, after which the average magnetic field inclination angle is ∼30°–50°, resulting in an averaged Spitzer parameter higher by about 20% than the value obtained in homogeneous simulations. In the cluster outskirts the instabilities are rather inefficient, due to the shallow gradients. This suggests that composition gradients in cluster cores might be shallower than one-dimensional models predict. More quantitative statements demand more refined models that can incorporate the physics driving the sedimentation process and simultaneously account for the weakly collisional nature of the plasma.

Potassium detection in the clear atmosphere of a hot-Jupiter

We present FORS2 (attached to ESO's Very Large Telescope) observations of the exoplanet WASP-17b during its primary transit, for the purpose of differential spectrophotometry analysis. We use the instrument in its Mask eXchange Unit (MXU) mode to simultaneously obtain low resolution spectra of the planet hosting star, as well as several reference stars in the field of view. The integration of these spectra within broadband and smaller 100$\AA$ bins provides us with 'white' and spectrophotometric light curves, from 5700 to 8000$\AA$. Through modelling the white light curve, we obtain refined bulk and transit parameters of the planet, as well as wavelength-dependent variations of the planetary radius from smaller spectral bins through which the transmission spectrum is obtained. The inference of transit parameters, as well as the noise statistics, is performed using a Gaussian Process model. We achieve a typical precision in the transit depth of a few hundred parts per million from various transit light curves. From the transmission spectra we rule out a flat spectrum at >3$\sigma$ and detect marginal presence of the pressure-broadened sodium wings. Furthermore, we detect the wing of the potassium absorption line in the upper atmosphere of the planet with 3$\sigma$ confidence, both facts pointing to a relatively shallow temperature gradient of the atmosphere. These conclusions are mostly consistent with previous studies of this exo-atmosphere, although previous potassium measurements have been inconclusive.