The streaming instability is a promising mechanism to overcome the barriers in direct dust growth and lead to the formation of planetesimals. Most previous studies of the streaming instability, ...however, were focused on a local region of a protoplanetary disk with a limited simulation domain such that only one filamentary concentration of solids has been observed. The characteristic separation between filaments is therefore not known. To address this, we conduct the largest-scale simulations of the streaming instability to date, with computational domains up to 1.6 gas scale heights both horizontally and vertically. The large dynamical range allows the effect of vertical gas stratification to become prominent. We observe more frequent merging and splitting of filaments in simulation boxes of high vertical extent. We find multiple filamentary concentrations of solids with an average separation of about 0.2 local gas scale heights, much higher than the most unstable wavelength from linear stability analysis. This measures the characteristic separation of planetesimal forming events driven by the streaming instability and thus the initial feeding zone of planetesimals.
ABSTRACT
Recent observations have revealed a gallery of substructures in the dust component of nearby protoplanetary discs, including rings, gaps, spiral arms, and lopsided concentrations. One ...interpretation of these substructures is the existence of embedded planets. Not until recently, however, most of the modelling effort to interpret these observations ignored the dust back reaction to the gas. In this work, we conduct local-shearing-sheet simulations for an isothermal, inviscid, non-self-gravitating, razor-thin dusty disc with a planet on a fixed circular orbit. We systematically examine the parameter space spanned by planet mass (0.1Mth ≤ Mp ≤ 1Mth, where Mth is the thermal mass), dimensionless stopping time (10−3 ≤ τs ≤ 1), and solid abundance (0 < Z ≤ 1). We find that when the dust particles are tightly coupled to the gas (τs < 0.1), the spiral arms are less open and the gap driven by the planet becomes deeper with increasing Z, consistent with a reduced speed of sound in the approximation of a single dust–gas mixture. By contrast, when the dust particles are marginally coupled (0.1 ≲ τs ≲ 1), the spiral structure is insensitive to Z and the gap structure in the gas can become significantly skewed and unidentifiable. When the latter occurs, the pressure maximum radially outside of the planet is weakened or even extinguished, and hence dust filtration by a low-mass (Mp < Mth) planet could be reduced or eliminated. Finally, we find that the gap edges where the dust particles are accumulated as well as the lopsided large-scale vortices driven by a massive planet, if any, are unstable, and they are broken into numerous small-scale dust–gas vortices.
ABSTRACT
Rings and gaps are commonly observed in the dust continuum emission of young stellar discs. Previous studies have shown that substructures naturally develop in the weakly ionized gas of ...magnetized, non-ideal MHD discs. The gas rings are expected to trap large mm/cm-sized grains through pressure gradient-induced radial dust–gas drift. Using 2D (axisymmetric) MHD simulations that include ambipolar diffusion and dust grains of three representative sizes (1 mm, 3.3 mm, and 1 cm), we show that the grains indeed tend to drift radially relative to the gas towards the centres of the gas rings, at speeds much higher than in a smooth disc because of steeper pressure gradients. However, their spatial distribution is primarily controlled by meridional gas motions, which are typically much faster than the dust–gas drift. In particular, the grains that have settled near the mid-plane are carried rapidly inwards by a fast accretion stream to the inner edges of the gas rings, where they are lifted up by the gas flows diverted away from the mid-plane by a strong poloidal magnetic field. The flow pattern in our simulation provides an attractive explanation for the meridional flows recently inferred in HD 163296 and other discs, including both ‘collapsing’ regions where the gas near the disc surface converges towards the mid-plane and a disc wind. Our study highlights the prevalence of the potentially observable meridional flows associated with the gas substructure formation in non-ideal MHD discs and their crucial role in generating rings and gaps in dust.
•The change of fatty acid profile of almond kernel oils during roasting is revealed.•Roasting temperature and duration affect phenolic profile in almond kernels.•Antioxidant effects of roasted almond ...kernels correlated well with phenolic contents.•Browning indices of almond kernels increased with roasting temperature and duration.•Maillard reaction products contribute to antioxidant effects of roasted almond kernels.
Roasting treatment increased levels of unsaturated fatty acids (linoleic, oleic and elaidic acids) as well as saturated fatty acids (palmitic and stearic acids) in almond (Prunus dulcis) kernel oils with temperature (150 or 180°C) and duration (5, 10 or 20min). Nonetheless, higher temperature (200°C) and longer duration (10 or 20min) roasting might result in breakdown of fatty acids especially for unsaturated fatty acids. Phenolic components (total phenols, flavonoids, condensed tannins and phenolic acids) of almond kernels substantially lost in the initial phase; afterward these components gradually increased with roasting temperature and duration. Similar results also observed for their antioxidant activities (scavenging DPPH and ABTS+ radicals and ferric reducing power). The changes of phenolic acid and flavonoid compositions were also determined by HPLC. Maillard reaction products (estimated with non-enzymatic browning index) also increased with roasting temperature and duration; they might also contribute to enhancing the antioxidant attributes.
ABSTRACT
The streaming instability, a promising mechanism to drive planetesimal formation in dusty protoplanetary discs, relies on aerodynamic drag naturally induced by the background radial pressure ...gradient. This gradient should vary in discs, but its effect on the streaming instability has not been sufficiently explored. For this purpose, we use numerical simulations of an unstratified disc to study the non-linear saturation of the streaming instability with mono-disperse dust particles and survey a wide range of gradients for two distinct combinations of the particle stopping time and the dust-to-gas mass ratio. As the gradient increases, we find most kinematic and morphological properties increase but not always in linear proportion. The density distributions of tightly coupled particles are insensitive to the gradient whereas marginally coupled particles tend to concentrate by more than an order of magnitude as the gradient decreases. Moreover, dust–gas vortices for tightly coupled particles shrink as the gradient decreases, and we note higher resolutions are required to trigger the instability in this case. In addition, we find various properties at saturation that depend on the gradient may be observable and may help reconstruct models of observed discs dominated by streaming turbulence. In general, increased dust diffusion from stronger gradients can lower the concentration of dust filaments and can explain the higher solid abundances needed to trigger strong particle clumping and the reduced planetesimal formation efficiency previously found in vertically stratified simulations.
Optical buffering is one of the essential functions in optical packet switching (OPS) network. This letter employs optical code-division multiple-access (OCDMA) signals of spectral-amplitude-coding ...(SAC) in a packet-buffering architecture. In the proposed scheme, the original OCDMA label of the incoming packet is identified and converted to a new label by a code-switching system. As multiple packets overlap in the time domain in a single device, SAC is adopted for packet encoding to avoid multiple access interference (MAI). The concepts of queuing system and Gaussian approximation are employed to model the buffer architecture. Simulations were conducted to demonstrate the tradeoff between packet dropping probability and the signal variations in the label identification process. The buffering performance in terms of bit-error rate (BER) is examined. The results show SAC labels perform well on packet buffering.
We investigate whether the regular Galilean satellites could have formed in the dead zone of a circumplanetary disc. A deadzone is a region of weak turbulence in which the magnetorotational ...instability is suppressed, potentially an ideal environmentfor satellite formation. With the grid-based hydrodynamic codeFAR GO3D, we examine the evolution of a circumplanetary discmodel with a dead zone. Material accumulates in the dead zone of the disc leading to a higher total mass and but a similartemperature profile compared to a fully turbulent disc model. The tidal torque increases the rate of mass transport through thedead zone leading to a steady-state disc with a dead zone that does not undergo accretion outbursts. We explore a range of disc,dead zone, and mass inflow parameters and find that the maximum mass of the disc is around 0.001MJ. Since the total solidmass of such a disc is much lower, we find that there is not sufficient material in the disc forin situformation of the Galileansatellites and that external supplement is required.
The current study quantified the risk of releasing African swine fever virus (ASFV) into Taiwan from pork products illegally carried by international travellers from 157 countries or territories ...through six international airports and three international seaports. The association between various factors and the number of pork products detected by the border control authorities was also examined. The risk was estimated with a stochastic process after modelling the number of undetected illegal pork products, probability of pork product detection at international airports and seaports and probability of ASFV contamination of pork products from various countries. The overall annual probability of ASFV release to Taiwan was estimated to be 1 95% confidence interval (CI): 1–1 under no enhanced mitigation measures. All the median airport‐level risks were higher than .921, and four of them reached 1. The total annual risk was .570 (95% CI: .109−.937) for international seaports. The country or territory level risk was estimated to be 1 for Vietnam, China, Hong Kong, the Philippines and South Korea, .999 (95% CI: .628–1) for Macao and .967 (95% CI: .359–1) for Indonesia. After the total number of travellers was factored in, the number of detected illegal pork products was the highest in January and February, and travellers from Vietnam risk ratio to Japan (RR): 80.45; 95% CI: 58.68–110.3, the Philippines (RR: 37.67; 95% CI: 26.9–52.74) and Cambodia (RR: 28.39; 95% CI: 12.69–63.51) were most likely to bring pork products to Taiwan. Our study indicated a high risk of ASFV introduction through international travellers and also identified the factors associated with the risk. This information can be used as empirical evidence for cost‐effective risk mitigation practices.
ABSTRACT
Spiral arms are observed in numerous protoplanetary discs. These spiral arms can be excited by companions, either on bound or unbound orbits. We simulate a scenario where an unbound ...perturber, i.e. a flyby, excites spiral arms during a periastron passage. We run three-dimensional hydrodynamical simulations of a parabolic flyby encountering a gaseous protoplanetary disc. The perturber mass ranges from $10\, \rm M_J$ to $1\, \rm {\rm M}_{\odot }$. The perturber excites a two-armed spiral structure, with a more prominent spiral feature for higher mass perturbers. The two arms evolve over time, eventually winding up, consistent with previous works. We focus on analysing the pattern speed and pitch angle of these spirals during the whole process. The initial pattern speed of the two arms are close to the angular velocity of the perturber at periastron, and then it decreases over time. The pitch angle also decreases over time as the spiral winds up. The spirals disappear after several local orbital times. An inclined prograde orbit flyby induces similar disc substructures as a coplanar flyby. A solar-mass flyby event causes increased eccentricity growth in the protoplanetary disc, leading to an eccentric disc structure which dampens over time. The spirals’ morphology and the disc eccentricity can be used to search for potential unbound stars or planets around discs where a flyby is suspected. Future disc observations at high resolution and dedicated surveys will help to constrain the frequency of such stellar encounters in nearby star-forming regions.
ABSTRACT
We investigate the formation mechanism for the observed nearly polar aligned (perpendicular to the binary orbital plane) debris ring around the eccentric orbit binary 99 Herculis. An ...initially inclined non-polar debris ring or disc will not remain flat and will not evolve to a polar configuration, due to the effects of differential nodal precession that alter its flat structure. However, a gas disc with embedded well coupled solids around the eccentric binary may evolve to a polar configuration as a result of pressure forces that maintain the disc flatness and as a result of viscous dissipation that allows the disc to increase its tilt. Once the gas disc disperses, the debris disc is in a polar aligned state in which there is little precession. We use three-dimensional hydrodynamical simulations, linear theory, and particle dynamics to study the evolution of a misaligned circumbinary gas disc and explore the effects of the initial disc tilt, mass, and size. We find that for a wide range of parameter space, the polar alignment time-scale is shorter than the lifetime of the gas disc. Using the observed level of alignment of 3° from polar, we place an upper limit on the mass of the gas disc of about $0.014 \, \mathrm{M}_\odot$ at the time of gas dispersal. We conclude that the polar debris disc around 99 Her can be explained as the result of an initially moderately inclined gas disc with embedded solids. Such a disc may provide an environment for the formation of polar planets.
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