Additional Transport Mechanism Research Articles

Spectral Properties and the Influence of Coronal Mass Ejections in 3He-rich Solar Energetic Particle Events

We analyze the spectral properties of 3He and 4He as well as the heavy ions (oxygen, neon, magnesium, silicon, and iron) in 80 3He-rich solar energetic particle (SEP) events observed by the Ultra-Low-Energy Ion Spectrometer on board the Advanced Composition Explorer spacecraft since its launch in 1997 until 2024. We split the spectral analysis into two criteria: events with fast and wide coronal mass ejections (CMEs; called “FW events”) and events with slow, narrow, or no observed CMEs (called “non-FW events”). Overall, we find that events with fast and wide CMEs exhibit more uniform spectra across all species, and their low-energy spectral indices are strongly correlated, suggesting a CME provides an additional reacceleration mechanism for the 3He-rich SEPs. When comparing each species’ low-energy spectral index for events with no associated fast-and-wide CME, we find a primary peak in the spectral hardness of 3He, and a secondary peak in Mg and Si. If we consider a plasma temperature of 1.0–1.3 MK, Mg and Si have a charge-to-mass ratio (Q/M) nearest to one-third (1/3), directly half that of 3He. Thus, our results support the results of Roth & Temerin, which suggest heavy ions resonate with the second harmonic of the same ion cyclotron waves energizing 3He. However, it is unclear why the Fe enhancement is not reflected in its spectral index, and we propose that additional acceleration and/or transport mechanisms are playing a role in the abundance enhancement of Fe and heavier ions.

Navigating Central Oxytocin Transport: Known Realms and Uncharted Territories.

Complex mechanisms govern the transport and action of oxytocin (Oxt), a neuropeptide and hormone that mediates diverse physiologic processes. While Oxt exerts site-specific and rapid effects in the brain via axonal and somatodendritic release, volume transmission via CSF and the neurovascular interface can act as an additional mechanism to distribute Oxt signals across distant brain regions on a slower timescale. This review focuses on modes of Oxt transport and action in the CNS, with particular emphasis on the roles of perivascular spaces, the blood-brain barrier (BBB), and circumventricular organs in coordinating the triadic interaction among circulating blood, CSF, and parenchyma. Perivascular spaces, critical conduits for CSF flow, play a pivotal role in Oxt diffusion and distribution within the CNS and reciprocally undergo Oxt-mediated structural and functional reconstruction. While the BBB modulates the movement of Oxt between systemic and cerebral circulation in a majority of brain regions, circumventricular organs without a functional BBB can allow for diffusion, monitoring, and feedback regulation of bloodborne peripheral signals such as Oxt. Recognition of these additional transport mechanisms provides enhanced insight into the systemic propagation and regulation of Oxt activity.

Strongly magnetized accretion in two ultracompact binary systems

ABSTRACT We present the discoveries of two of AM CVn systems, Gaia14aae and SDSS J080449.49+161624.8, which show X-ray pulsations at their orbital periods, indicative of magnetically collimated accretion. Both also show indications of higher rates of mass transfer relative to the expectations from binary evolution driven purely by gravitational radiation, based on existing optical data for Gaia14aae, which show a hotter white dwarf temperature than expected from standard evolutionary models, and X-ray data for SDSS J080449.49+161624.8 which show a luminosity 10−100 times higher than those for other AM CVn at similar orbital periods. The higher mass transfer rates could be driven by magnetic braking from the disc wind interacting with the magnetosphere of the tidally locked accretor. We discuss implications of this additional angular momentum transport mechanism for evolution and gravitational wave detectability of AM CVn objects.

Large mass inflow rates in Saturn’s rings due to ballistic transport and mass loading

The Cassini mission provided key measurements needed to determine the absolute age of Saturn’s rings, including the extrinsic micrometeoroid flux at Saturn, the volume fraction of non-icy pollutants in the rings, and the total ring mass. These three factors constrain the ring age to be no more than a few 100 Myr (Kempf et al., 2022). Observations during the Cassini Grand Finale also showed that the rings are losing mass to the planet at a prodigious rate. Some of the mass flux falls as “ring rain” at high latitudes. However, the influx in ring rain is considerably less than the total measured mass influx of 4800 to 45000 kg s−1 at lower latitudes (Waite et al., 2018).In addition to polluting the rings, micrometeoroid impacts lead to ballistic transport, the mass and angular momentum transport due to net exchanges of meteoroid impact ejecta. Because the ejecta are predominantly prograde, they carry net angular momentum outward. As a result, ring material drifts inward toward the planet. Here, for the first time, we use a simple model to quantify this radial mass inflow rate for dense rings and find that, for plausible choices of parameters, ballistic transport and mass loading by meteoroids can produce a total inward flux of material in the inner B ring and in the C ring that is on the order of a few ⋅103 to a few ⋅104 kg s−1, in agreement with measurements during the Cassini Grand Finale. From these mass inflow rates, we estimate that the remaining ring lifetime is ∼ 15 to 400 Myr. Combining this with a revised pollution age of ∼ 120 Myr, we conclude that Saturn’s rings are not only young but ephemeral and probably started their evolution on a similar timescale to their pollution age with an initial mass of one to a few Mimas masses.In addition to showing that meteoroid impacts can produce a large sustained mass inflow through the B and C rings, this paper addresses various uncertainties and considers possible contributions by additional transport mechanisms and by external torques. We also map out a set of future research projects, including global simulations.

(Electrodeposition Division Early Career Investigator Award Address) Regulating Electrochemical Deposition of Metals at Rechargeable Battery Electrodes

Scalable approaches for precisely manipulating the growth of crystals are of broad-based science and technological interest, and among the most extensively studied fields of research. New research interests have re-emerged in a subgroup of these phenomena—electrochemical deposition of metals in battery electrodes, such as zinc, an anode material that underpins several opportunities for developing cost-competitive technologies for large-scale electrical energy storage. Quantitative assessment shows that a high efficiency/reversibility of the electrochemical plating/stripping process (ie Coulombic efficiency > 99%), is necessary for the successful development practical rechargeable electrochemical energy storage systems that utilize a Zn metal anode. In practice, however, such efficiency values are usually below 80~90% in conventional metal plating/stripping systems.I will show that the inefficiency/irreversibility observed in metal plating/stripping is predominantly attributable to the metals’ susceptibility to parasitic chemical reactions and propensity for producing so-called “orphaned” fragments, which have lost electrical access to the electrode substrate termed the current collector. Both of these mechanisms are found to share the same origin— the non-uniform, rough electrodeposition morphology of metals. This finding motivates the overarching science question in this thesis: can the electrochemical growth of metals be strictly regulated in electrochemical cells? Despite the large volume of recent literature devoted to answering this question, the fundamental crystalline nature of the metal electrodeposits has been largely overlooked. Removing this artifice allows us to borrow knowledge from a rich body of literature on regulating the deposition process of crystalline materials to pursue fresh answers and approaches for achieving highly reversible electrodeposition of metals.Such approaches enable development of multiple strategies that are unprecedentedly effective in regulating metal deposition morphology and in achieving high-enough plating/stripping efficiency to meet the reversibility requirements as set forth. Specifically, I report that heteroepitaxy can be used to direct the shape and orientation of the metal crystallites formed on a rationally designed substrate. This strategy leverages crystals’ inclination for mimicking the atomic arrangement of the substrate surface, especially when the lattice misfit is below a critical value. Secondly, in I consider the fundamental limitation of heteroepitaxy—i.e. the correlation length is finite, meaning that the initial regulation of shape and crystallography of a metal deposit are only preserved for a finite electrodeposits thickness/electrode capacity. For as fundamental reasons, it is shown that this limitation can be overcome by imposing a convective flow normal to the electrode surface. The artificial convective flow serves as an additional solution-state mass transport mechanism and has a heretofore unexplored effect in suppressing chemotaxis-like reorientation of the metal crystallites at high areal capacities. I then demonstrate that solid-state metallurgy strategies for eliminating the built-in crystallographic heterogeneity of commercial foil-type metal electrodes, opens a fruitful path to powerful homoepitaxy regulation processes in which the metal itself is able to reversibly regulate its own electrodeposition morphology. In particular, subjecting a metal electrode to severe plastic deformation in the electrode foil-forming process is used to fabricate strongly-textured metal electrodes where such crystallographic heterogeneities are eliminated.In addition to these physical aspects, it is found that the chemical interaction at the heterointerface between the deposited metal crystallites and the substrate—which is actually a crystal of a different chemistry—also shows nontrivial influences on the electrodeposition process. I will assess consider these effects from two types of perspectives — oxygen- mediated bonding between the metal electrodeposit and substrate and alloying, respectively. The results suggest that the chemical affinity at the heterointerface is necessary to promote a uniform nucleation/growth landscape and to suppress the formation of “orphaned” metal deposits.

Reconnaissance of cumulative risk of pesticides and pharmaceuticals in Great Smoky Mountains National Park streams

The United States (US) National Park Service (NPS) manages protected public lands to preserve biodiversity. Exposure to and effects of bioactive organic contaminants in NPS streams are challenges for resource managers. Recent assessment of pesticides and pharmaceuticals in protected-streams within the urbanized NPS Southeast Region (SER) indicated the importance of fluvial inflows from external sources as drivers of aquatic contaminant-mixture exposures. Great Smoky Mountains National Park (GRSM), lies within SER, has the highest biodiversity and annual visitation of NPS parks, but, in contrast to the previously studied systems, straddles a high-elevation hydrologic divide; this setting limits fluvial-inflows of contaminants but potentially increases visitation-driven contaminant deliveries. We leveraged the unique characteristics of GRSM to test further the importance of fluvial contaminant inflows as drivers of protected-stream exposures and to inform the relative importance of potential additional contaminant transport mechanisms, by comparing the estimated risks of 328 pesticides and pharmaceuticals in water at 16 GRSM stream locations to those estimated previously in SER streams. Extensive mixtures (31 compounds) were only observed in an atypical reach on the boundary of GRSM downstream of a wastewater discharge, while limited mixtures (2–5 compounds) were observed in one stream with elevated visitation pressure (recreational “tube floating”). The insecticide, imidacloprid, used to eradicate hemlock woolly adelgid, was detected in 8 (50%) streams. Infrequent exceedances of a cumulative ToxCast-based, exposure-activity ratio (ΣEAR) 0.001 screening-level of concern suggested limited risk to non-target, aquatic vertebrates, whereas exceedances of a cumulative benchmark-based, invertebrate toxicity quotient (ΣTQ) 0.1 screening level at 8 locations indicated generally high risk to invertebrates. The results are consistent with the importance of fluvial transport from extra-park sources as a driver of bioactive-contaminant mixture exposures in protected streams and illustrate the potential additional risks from visitation-driven and tactical-use-pesticides.

Thermal-nonthermal energy partition in solar flares derived from X-ray, EUV, and bolometric observations

Context.In solar flares, energy is released impulsively and is partly converted into thermal energy of hot plasmas and kinetic energy of accelerated nonthermal particles. It is crucial to constrain the partition of these two energy components to understand energy release and transport as well as particle acceleration in solar flares. Despite numerous efforts, no consensus on quantifying this energy balance has yet been reached.Aims.We aim to understand the reasons for the contradicting results on energy partition obtained by various recent studies. The overarching question we address is whether there is sufficient energy in nonthermal particles to account for the thermal flare component.Methods.We considered five recent studies that address the thermal-nonthermal energy partition in solar flares. Their results are reviewed, and their methods are compared and discussed in detail.Results.The main uncertainties in deriving the energy partition are identified as (a) the derivation of the differential emission measure distribution and (b) the role of the conductive energy loss for the thermal component, as well as (c) the determination of the low-energy cutoff for the injected electrons. The bolometric radiated energy, as a proxy for the total energy released in the flare, is a useful independent constraint on both thermal and nonthermal energetics. In most of the cases, the derived energetics are consistent with this constraint. There are indications that the thermal-nonthermal energy partition changes with flare strength: in weak flares, there appears to be a deficit of energetic electrons, while the injected nonthermal energy is sufficient to account for the thermal component in strong flares. This behavior is identified as the main cause of the dissimilar results in the studies we considered. The changing partition has two important consequences: (a) an additional direct (i.e. non-beam) heating mechanism has to be present, and (b) considering that the bolometric emission originates mainly from deeper atmospheric layers, conduction or waves are required as additional energy transport mechanisms.

Phase-field simulations of grain boundary grooving under diffusive-convective conditions

Grain boundaries are potential sites for the development of morphological instabilities along a solidifying interface, and play an important role in the microstructural properties of cast alloys. In the present study, we employ a multiphase-field model to address the grain boundary grooving phenomenon under the cooperative effect of convection and volume diffusion in the liquid phase. Under pure diffusive conditions, we first benchmark our phase-field model by illustrating and comparing the formation of symmetric groove profiles across the grain boundary with Mullins’s groove. In the presence of an additional convective transport mechanism, we systematically depict that the asymmetricity of groove profile increases with the increase in the convection velocity. Particularly, it is identified that the groove kinetics as well as the grain boundary grooving mechanism is significantly modified in the diffusive-convective regime. The present two-dimensional simulations, while providing considerable insights into the mechanism of grain boundary grooving, also closes the gap with the experimental and theoretical observations reported earlier. In addition, the role of relative surface energies on the groove deepening along with the ridge morphology is discussed in detail. The simulated grain boundary features are of practical significance for understanding and controlling the nanocrystalline thin film experiments.

Provenance of sub-aerial surface sediments in the Tarim Basin, Western China

Provenance of surface sediments in the Tarim Basin is important for understanding the aridification of the Asian interior and the interplay between the Westerlies and the Asian monsoon. Although the desert sands in the Taklamakan Desert have been studied intensively, there is no consensus regarding their provenance, namely, the dispute exists between various sand sources in different parts of the desert and homogenization of the sands in the entire desert. Moreover, other surface sediments in the basin are poorly investigated. Here we examine the particle-size-specific rare earth element (REE) and trace element characteristics of various surface sediments from different regions of the basin. The results reveal that the <2 µm fraction has multiple sources from Central Asia as well as from the adjoining arid/semi-arid areas to the east, together with the abrasion of coarser particles from multiple regional sources. They are completely homogenized prior to deposition by the action of the Westerlies and the local wind systems. The 2–16 µm fraction of most of the sediments in the basin have a higher proportion of Central Asian dust and a relatively high degree of homogeneity. Westerlies and local wind systems remain a more important transport mechanism than the fluvial activity. In addition, the desert sands of 2–16 µm fraction in the Taklamakan Desert has a substantial contribution from material derived from the basement rocks. The 32–63 µm and >63 µm fraction are dominated by regional-derived materials, mainly from the adjacent mountains (e.g. Pamirs, Kunlun, Kuruktag, Altun, Tianshan Mountains), and the fluvial systems are an important additional transport mechanism. Our findings potentially provide an improved understanding of the dust cycle and atmospheric circulation patterns in Central Asia, and of the provenance of loess in North China.

Sheared Amorphous Packings Display Two Separate Particle Transport Mechanisms.

Shearing granular materials induces nonaffine displacements. Such nonaffine displacements have been studied extensively, and are known to correlate with plasticity and other mechanical features of amorphous packings. A well known example is shear transformation zones as captured by the local deviation from affine deformation, D_{min}^{2}, and their relevance to failure and stress fluctuations. We analyze sheared frictional athermal disc packings and show that there exists at least one additional mesoscopic transport mechanism that superimposes itself on top of local diffusive motion. We evidence this second transport mechanism in a homogeneous system via a diffusion tensor analysis and show that the trace of the diffusion tensor equals the classic D_{min}^{2} when this second mesoscopic transport is corrected for. The new transport mechanism is consistently observed over a wide range of volume fractions and even for particles with different friction coefficients and is consistently observed also upon shear reversal, hinting at its relevance for memory effects.

Computational and experimental investigation of particulate matter deposition in cerebral side aneurysms.

Intracranial aneurysms frequently develop blood clots, plaque and inflammations, which are linked to enhanced particulate mass deposition. In this work, we propose a computational model for particulate deposition, that accounts for the influence of field forces, such as gravity and electrostatics, which produce an additional flux of particles perpendicular to the fluid motion and towards the wall. This field-mediated flux can significantly enhance particle deposition in low-shear environments, such as in aneurysm cavities. Experimental investigation of particle deposition patterns in in vitro models of side aneurysms, demonstrated the ability of the model to predict enhanced particle adhesion at these sites. Our results showed a significant influence of gravity and electrostatic forces (greater than 10%), indicating that the additional terms presented in our models may be necessary for modelling a wide range of physiological flow conditions and not only for ultra-low shear regions. Spatial differences between the computational model and the experimental results suggested that additional transport and fluidic mechanisms affect the deposition pattern within aneurysms. Taken together, the presented findings may enhance our understanding of pathological deposition processes at cardiovascular disease sites, and facilitate rational design and optimization of cardiovascular particulate drug carriers.

Digital holographic interferometry investigation of liquid hydrocarbon vapor cloud above a circular well.

The current study investigates evaporation of liquid hydrocarbons from a circular well cavity of small depth. Gravimetric analysis is performed to measure the evaporation rate and digital holographic interferometry is used for the measurement of normalized mole fraction profile inside the vapor cloud above the well. Phase unwrapping has been implemented to obtain continuous phase distribution in the image plane. The Fourier-Hankel tomographic inversion algorithm is implemented to obtain the refractive index change distribution inside the object plane, i.e., vapor cloud. Four liquid hydrocarbons, i.e., pentane, hexane, cyclohexane, and heptane, are studied. The radius of circular well cavities is varied in the range of 1.5 to 12.5 mm. Results using a quasi-steady, diffusion-controlled model are compared with the experimental evaporation rate. Measured evaporation rates are higher than the diffusion-limited model calculation for all working fluids and well sizes. This difference is attributed to natural convection occurring inside the vapor cloud due to the density difference between the gas-vapor mixture and the surrounding air. Holographic analysis confirms the presence of natural convection by revealing the formation of a flat disk-shaped vapor cloud above the well surface. Experimentally obtained vapor cloud shape is different from the hemispherical vapor cloud obtained using the pure diffusion-limited evaporation model. The gradient of vapor mole fraction at the liquid-vapor interface is higher compared to that of the diffusion-limited model because of the additional transport mechanism due to natural convection. Transient analysis of the vapor cloud reveals time invariant overall shape of the vapor cloud with a reduction in average magnitude of vapor concentration inside the vapor cloud during evaporation. The existing correlation for sessile droplet cannot successfully predict the evaporation rate from a liquid well. A new correlation is proposed for evaporation rate prediction, which can predict the evaporation rate within a root mean square error of 5.6% for a broad size range of well cavity.

Charge-state independent anomalous transport for a wide range of different impurity species observed at Wendelstein 7-X

In this paper, the plasma volume averaged impurity confinement of selected charge states and impurity species has been characterized for the Stellarator Wendelstein 7-X (W7-X), covering a wide range of atomic charges (Z = 12–44) and atomic masses (M = 28–184). A comparison of the experimental findings to theoretical neoclassical and turbulent transport expectations suggests, aside from/in addition to the neoclassical transport, an additional significant anomalous transport mechanism, which is not inconsistent with the predictions of a turbulence dominated impurity transport and is in agreement with the experimental results from recent transport studies based on the direct measurements of impurity diffusion profiles, performed at W7-X.

Effect of surface treatments on electrical properties of β-Ga2O3

The effect of various combinations of gaseous (ultraviolet/O3), liquid (HCl, buffered oxide etch, and H2O2), or plasma (CF4 and O2) treatments of the surface of β-Ga2O3 was quantified by current–voltage and capacitance–voltage measurements of rectifier structures. Plasma exposure (13.56 MHz, 24 kW/cm2) always led to significant degradation of the surface, as evidenced by large increases in rectifier reverse current and ideality factor (from 1.01 in control samples to ∼3.8 in plasma exposed samples, indicating additional defect-related carrier transport mechanisms) and lowering of the Schottky barrier height (from 1.21 eV in control samples to 0.75–0.86 eV in plasma exposed samples) and diode rectification ratio, with degraded reverse recovery characteristics. This was true of both CF4 and O2, even though it is known that fluorine incorporation in the near-surface leads to donor compensation and an increase in barrier height. Damage from the plasma exposure was not fully recovered by annealing at 500 °C. The O3 and liquid chemical cleans did lead to reduced reverse current in rectifiers, with no measurable decrease in barrier height, increase in ideality factor, or degradation of reverse recovery characteristics. Surfaces treated in this manner did not significantly change for anneals up to 500 °C; however, the Ni/Au contacts already show degradation after annealing at 350 °C.

Enhanced Chemical Separation by Freestanding CNT-Polyamide/Imide Nanofilm Synthesized at the Vapor-Liquid Interface.

In chemical separation, thin membranes exhibit high selectivity, but often require a support at the expense of permeance. Here, we report a pinhole-free polymeric layer synthesized within freestanding carbon nanotube buckypaper through vapor-liquid interfacial polymerization (VLIP). The VLIP process results in thin, smooth and uniform polyamide and imide films. The scaffold reinforces the nanofilm, defines the membrane thickness, and introduces an additional transport mechanism. Our membranes exhibit superior gas selectivity and osmotic semipermeability. Plasticization resistance and high permeance in hydrocarbon separation together with a considerable improvement in water-salt permselectivity highlight their potential as new membrane architecture for chemical separation.

Surfactant effects on interfacial flow and thermal transport processes during phase change in film boiling

The presence of surfactants in two-phase flows results in the transport and adsorption of surfactants to the interface, and the resulting local interfacial concentration significantly influences the surface tension between the liquid and vapor phases in a fluid undergoing phase change. This computational study is aimed at understanding and elucidating the mechanisms of enhanced flows and thermal transport processes in film boiling due to the addition of surfactants. A change in surface tension results in a change in the critical Rayleigh-Taylor wavelength leading to different bubble release patterns and a change in the overall heat transfer rates. Due to the presence of surfactants, an additional transport mechanism of the Marangoni convection arises from the resulting tangential gradients in the surfactant concentration along the phase interface. Our computational approach to study such phenomena consists of representing the interfacial motion by means of the coupled level set-volume-of-fluid method, the fluid motion via the classical marker-and-cell approach, as well as representations for the bulk transport of energy and surfactants, in conjunction with a phase change model and an interfacial surfactant model. Using such an approach, we perform numerical simulations of surfactant-laden single mode as well as multiple mode film boiling and study the effect of surfactants on the transport processes in film boiling, including bubble release patterns, vapor generation rates, and heat transfer rates at different surfactant concentrations. The details of the underlying mechanisms will be investigated and interpreted.

The Combined Action of Duplicated Boron Transporters Is Required for Maize Growth in Boron-Deficient Conditions.

The micronutrient boron is essential in maintaining the structure of plant cell walls and is critical for high yields in crop species. Boron can move into plants by diffusion or by active and facilitated transport mechanisms. We recently showed that mutations in the maize boron efflux transporter ROTTEN EAR (RTE) cause severe developmental defects and sterility. RTE is part of a small gene family containing five additional members (RTE2-RTE6) that show tissue-specific expression. The close paralogous gene RTE2 encodes a protein with 95% amino acid identity with RTE and is similarly expressed in shoot and root cells surrounding the vasculature. Despite sharing a similar function with RTE, mutations in the RTE2 gene do not cause growth defects in the shoot, even in boron-deficient conditions. However, rte2 mutants strongly enhance the rte phenotype in soils with low boron content, producing shorter plants that fail to form all reproductive structures. The joint action of RTE and RTE2 is also required in root development. These defects can be fully complemented by supplying boric acid, suggesting that diffusion or additional transport mechanisms overcome active boron transport deficiencies in the presence of an excess of boron. Overall, these results suggest that RTE2 and RTE function are essential for maize shoot and root growth in boron-deficient conditions.

Constraining the efficiency of angular momentum transport with asteroseismology of red giants: the effect of stellar mass

Context: Constraints on the internal rotation of red giants are now available thanks to asteroseismic observations. Preliminary comparisons with rotating stellar models indicate that an undetermined additional process for the internal transport of angular momentum is required in addition to purely hydrodynamic processes. Aims: We investigate how asteroseismic measurements of red giants can help us characterize the additional transport mechanism. Methods: We first determine the efficiency of the missing transport mechanism for the low-mass red giant KIC 7341231 by computing rotating models that include an additional viscosity corresponding to this process. We then discuss the change in the efficiency of this transport of angular momentum with the mass, metallicity and evolutionary stage. Results: In the case of the low-mass red giant KIC 7341231, we find that the viscosity corresponding to the additional mechanism is constrained to the range 1 x 10^3 - 1.3 x 10^4 cm^2/s. This constraint on the efficiency of the unknown additional transport mechanism during the post-main sequence is obtained independently of any specific assumption about the modelling of rotational effects during the pre-main sequence and the main sequence (in particular, the braking of the surface by magnetized winds and the efficiency of the internal transport of angular momentum before the post-main-sequence phase). When we assume that the additional transport mechanism is at work during the whole evolution of the star together with a solar-calibrated braking of the surface by magnetized winds, the range of nu_add is reduced to 1 - 4 x 10^3 cm^2/s. In addition to being sensitive to the evolutionary stage of the star, we show that the efficiency of the unknown process for internal transport of angular momentum increases with the stellar mass.

Insights from Asteroseismology of Massive Stars: The Need for Additional Angular Momentum Transport Mechanisms

In massive stars, rotation and oscillatory waves can have a tight interplay. In order to assess the importance of additional angular momentum transport mechanisms other than rotation, we compare the asteroseismic properties of a uniformly rotating model and a differentially rotating one. Accordingly, we employ the observed period spacing of 36 dipole g-modes in the Kepler $\sim3.2$ M$_\odot$ target KIC 7760680 to discriminate between these two models. We favor the uniformly rotating model, which fully satisfies all observational constraints. Therefore, efficient angular momentum transport by additional mechanisms such as internal gravity waves, heat-driven modes and magnetic field is needed during early main sequence evolution of massive stars.

Short‐range, overpressure‐driven methane migration in coarse‐grained gas hydrate reservoirs

AbstractTwo methane migration mechanisms have been proposed for coarse‐grained gas hydrate reservoirs: short‐range diffusive gas migration and long‐range advective fluid transport from depth. Herein, we demonstrate that short‐range fluid flow due to overpressure in marine sediments is a significant additional methane transport mechanism that allows hydrate to precipitate in large quantities in thick, coarse‐grained hydrate reservoirs. Two‐dimensional simulations demonstrate that this migration mechanism, short‐range advective transport, can supply significant amounts of dissolved gas and is unencumbered by limitations of the other two end‐member mechanisms. Short‐range advective migration can increase the amount of methane delivered to sands as compared to the slow process of diffusion, yet it is not necessarily limited by effective porosity reduction as is typical of updip advection from a deep source.