Gas Drag Research Articles

The Evolution of Binaries in a Gaseous Medium: Three-dimensional Simulations of Binary Bondi–Hoyle–Lyttleton Accretion

Abstract Binary stars are common. While only those with small separations may exchange gas with one another, even the widest binaries interact with their gaseous surroundings. Drag forces and accretion rates dictate how these systems are transformed by these interactions. We perform three-dimensional hydrodynamic simulations of Bondi–Hoyle–Lyttleton flows, in which a binary moves supersonically relative to a homogeneous medium, using the adaptive mesh refinement code FLASH. We simulate a range of values of the initial semimajor axis of the orbit relative to the gravitational focusing impact parameter of the pair. When the binary separation is less than the gravitational focusing impact parameter, the pair orbits within a shared bow shock. When the pair is wider, each object has an individual bow shock structure. The long-term evolution of the binary is determined by the timescales for accretion, slowing of the center of mass, and orbital inspiraling. We find a clear hierarchy of these timescales; a binary’s center-of-mass motion is slowed over a shorter timescale than the pair inspirals or accretes. In contrast to previous analytic predictions, which assume an unperturbed background medium, we find that the timescale for orbital inspiraling is proportional to the semimajor axis to the 0.19 ± 0.01 power. This positive scaling indicates that gaseous drag forces can drive binaries either to coalescence or to the critical separation at which gravitational radiation dominates their further evolution. We discuss the implications of our results for binaries embedded in the interstellar medium, active galactic nuclei disks, and common envelope phases.

A Staggered Semi-analytic Method for Simulating Dust Grains Subject to Gas Drag

Abstract Numerical simulations of dust–gas dynamics are one of the fundamental tools in astrophysical research, such as the study of star and planet formation. It is common to find tightly coupled dust and gas in astrophysical systems, which demands that any practical integration method be able to take time steps, , much longer than the stopping time, , due to drag. A number of methods have been developed to ensure stability in this stiff regime, but there remains large room for improvement in terms of accuracy. In this paper, we describe an easy-to-implement method, the “staggered semi-analytic method” (SSA), and conduct numerical tests to compare it to other implicit and semi-analytic methods, including the second-order implicit method and the Verlet method. SSA makes use of a staggered step to better approximate the terminal velocity in the stiff regime. In applications to protoplanetary disks, this not only leads to orders of magnitude higher accuracy than the other methods, but also provides greater stability, making it possible to take time steps 100 times larger in some situations. SSA is also second-order accurate and symplectic when . More generally, the robustness of SSA makes it applicable to linear dust–gas drag in virtually any context.

Spatially resolved spectroscopy of the debris disk HD 32297

Context. Spectro-photometry of debris disks in total intensity and polarimetry can provide new insight into the properties of the dust grains therein (size distribution and optical properties). Aims. We aim to constrain the morphology of the highly inclined debris disk HD 32297. We also intend to obtain spectroscopic and polarimetric measurements to retrieve information on the particle size distribution within the disk for certain grain compositions. Methods. We observed HD 32297 with SPHERE in Y, J, and H bands in total intensity and in J band in polarimetry. The observations are compared to synthetic models of debris disks and we developed methods to extract the photometry in total intensity overcoming the data-reduction artifacts, namely the self-subtraction. The spectro-photometric measurements averaged along the disk mid-plane are then compared to model spectra of various grain compositions. Results. These new images reveal the very inner part of the system as close as 0.15″. The disk image is mostly dominated by the forward scattering making one side (half-ellipse) of the disk more visible, but observations in total intensity are deep enough to also detect the back side for the very first time. The images as well as the surface brightness profiles of the disk rule out the presence of a gap as previously proposed. We do not detect any significant asymmetry between the northeast and southwest sides of the disk. The spectral reflectance features a “gray to blue” color which is interpreted as the presence of grains far below the blowout size. Conclusions. The presence of sub-micron grains in the disk is suspected to be the result of gas drag and/or “avalanche mechanisms”. The blue color of the disk could be further investigated with additional total intensity and polarimetric observations in K and H bands respectively to confirm the spectral slope and the fraction of polarization.

Droplet Dynamics and Removal from Polymer Electrolyte Fuel Cell Channels Using Acoustic Pressure Waves

Liquid water management in polymer-electrolyte-membrane (PEM) fuel cell reactant channels is critical to system performance, stability, and lifespan. A balance between cathode catalyst layer water generation and removal must be maintained to ensure proper membrane hydration while preventing flooding. Liquid water transport through the gas-diffusion-layer (GDL) results in droplet formation in reactant channels that may be removed via flow gas drag forces. This work investigates the effect that acoustic wave transmission in reactant flow gases has on liquid water droplets growing on the GDL surface. While recent and past work has shown this method may be useful toward preventing liquid buildup in reactant channels, fundamental analysis of the droplet behavior is lacking [1-3]. In this study experiments were performed using a transparent ex-situ flow channel of 3.5 (h) x 3.7 (w) mm cross section as shown in Figure 1a. Water droplet formation was controlled using a digital syringe pump injected through a port on the backside of the GDL. Air and Hydrogen were used to simulate both cathode and anode flow conditions. A speaker was connected in series with the feed gas and flow channel whereby acoustic signals were generated and passed through the reactant channel. Transducers measured pressure upstream and downstream of the droplet while a high speed camera captured images of the droplets motion from the side (Figure 1b). Early results show that pressure waves effect droplets pinning behavior on the GDL surface. Previous work has shown that a maximum contact angle hysteresis exists for droplet removal from GDLs. While gas flow causes droplet contact angle hysteresis, this work shows that oscillations caused by the acoustic waves may shift the droplet over the max hysteresis tipping point enabling removal.

Dust survival rates in clumps passing through the Cas A reverse shock – I. Results for a range of clump densities

ABSTRACT The reverse shock in the ejecta of core-collapse supernovae is potentially able to destroy newly formed dust material. In order to determine dust survival rates, we have performed a set of hydrodynamic simulations using the grid-based code astrobear in order to model a shock wave interacting with clumpy supernova ejecta. Dust motions and destruction rates were computed using our newly developed external, post-processing code paperboats, which includes gas drag, grain charging, sputtering, and grain–grain collisions. We have determined dust destruction rates for the oxygen-rich supernova remnant Cassiopeia A as a function of initial grain sizes and clump gas density. We found that up to $30\,\mathrm{{{\ \rm per\ cent}}}$ of the carbon dust mass is able to survive the passage of the reverse shock if the initial grain size distribution is narrow with radii around ∼10–50 nm for high gas densities, or with radii around $\sim 0.5\!-\!1.5\,\mathrm{\mu m}$ for low and medium gas densities. Silicate grains with initial radii around 10–30 nm show survival rates of up to $40\,\mathrm{{{\ \rm per\ cent}}}$ for medium- and high-density contrasts, while silicate material with micron-sized distributions is mostly destroyed. For both materials, the surviving dust mass is rearranged into a new size distribution that can be approximated by two components: a power-law distribution of small grains and a lognormal distribution of grains having the same size range as the initial distribution. Our results show that grain–grain collisions and sputtering are synergistic and that grain–grain collisions can play a crucial role in determining the surviving dust budget in supernova remnants.

Spinning up planetary bodies by pebble accretion

Most major planetary bodies in the Solar System rotate in the same direction as their orbital motion: their spin is prograde. Theoretical studies to explain the direction as well as the magnitude of the spin vector have had mixed success. When the accreting building blocks are ∼ km-size planetesimals – as predicted by the classical model – the accretion process is so symmetric that it cancels out prograde with retrograde spin contributions, rendering the net spin minute. For this reason, the currently-favored model for the origin of planetary rotation is the giant impact model, in which a single collision suffices to deliver a spin, which magnitude is close to the breakup rotation rate. However, the giant impact model does not naturally explain the preference for prograde spin. Similarly, an increasing number of spin vector measurement of asteroids also shows that the spin vector of large (primordial) asteroids is not isotropic. Here, we re-assess the viability of smaller particles to bestow planetary bodies with a net spin, focusing on the pebble accretion model in which gas drag and gravity join forces to accrete small particles at a large cross section. Similar to the classical calculation for planetesimals, we integrate the pebble equation of motion and measure the angular momentum transfer at impact. We consider a variety of disk conditions and pebble properties and conduct our calculations in the limits of 2D (planar) and 3D (homogeneous) pebble distributions. We find that in certain regions of the parameter space, the angular momentum transfer is significant, much larger than with planetesimals and on par with or exceeding the current spin of planetary bodies. We link this large net spin delivery to the appearance of asymmetries during the accretion process of pebbles. For example, prograde contribution may dominate (in certain regions of the parameter space) because they originate from trajectories that are preferentially captured. For simplicity, our calculations have ignored certain important effects (e.g., collisions, the back-reaction on the gas, and formation of atmospheres) and do not address how the eventual distribution of spin vectors is obtained for which collisions and post-formation processes must have played a role to explain the scatter. Irrespective of these issues, pebble accretion is a viable mechanism to not only grow planetary bodies, but also to impart them with a significant spin.

CHONDRITES AND THEIR COMPONENTS: RECORDS OF EARLY SOLAR SYSTEM PROCESSES.

AbstractChondrites consist of three major components: refractory inclusions (Ca,Al‐rich inclusions [CAIs] and amoeboid olivine aggregates), chondrules, and matrix. Here, I summarize recent results on the mineralogy, petrology, oxygen, and aluminum‐magnesium isotope systematics of the chondritic components (mainly CAIs in carbonaceous chondrites) and their significance for understanding processes in the protoplanetary disk (PPD) and on chondrite parent asteroids. CAIs are the oldest solids originated in the solar system: their U‐corrected Pb‐Pb absolute age of 4567.3 ± 0.16 Ma is considered to represent time 0 of its evolution. CAIs formed by evaporation, condensation, and aggregation in a gas of approximately solar composition in a hot (ambient temperature >1300 K) disk region exposed to irradiation by solar energetic particles, probably near the protoSun; subsequently, some CAIs were melted in and outside their formation region during transient heating events of still unknown nature. In unmetamorphosed, type 2–3.0 chondrites, CAIs show large variations in the initial 26Al/27Al ratios, from <5 × 10–6 to ~5.25 × 10–5. These variations and the inferred low initial abundance of 60Fe in the PPD suggest late injection of 26Al by a wind from a nearby Wolf–Rayet star into the protosolar molecular cloud core prior to or during its collapse. Although there are multiple generations of CAIs characterized by distinct mineralogies, textures, and isotopic (O, Mg, Ca, Ti, Mo, etc.) compositions, the 26Al heterogeneity in the CAI‐forming region(s) precludes determining the duration of CAIs formation using 26Al‐26Mg systematics. The existence of multiple generations of CAIs and the observed differences in CAI abundances in carbonaceous and noncarbonaceous chondrites may indicate that CAIs were episodically formed and ejected by a disk wind from near the Sun to the outer solar system and then spiraled inward due to gas drag. In type 2–3.0 chondrites, most CAIs surrounded by Wark–Lovering rims have uniform Δ17O (= δ17O−0.52 × δ18O) of ~ −24‰; however, there is a large range of Δ17O (from ~−40 to ~ −5‰) among them, suggesting the coexistence of 16O‐rich (low Δ17O) and 16O‐poor (high Δ17O) gaseous reservoirs at the earliest stages of the PPD evolution. The observed variations in Δ17O of CAIs may be explained if three major O‐bearing species in the solar system (CO, H2O, and silicate dust) had different O‐isotope compositions, with H2O and possibly silicate dust being 16O‐depleted relative to both the Genesis solar wind Δ17O of −28.4 ± 3.6‰ and even more 16O‐enriched CO. Oxygen isotopic compositions of CO and H2O could have resulted from CO self‐shielding in the protosolar molecular cloud (PMC) and the outer PPD. The nature of 16O‐depleted dust at the earliest stages of PPD evolution remains unclear: it could have either been inherited from the PMC or the initially 16O‐rich (solar‐like) MC dust experienced O‐isotope exchange during thermal processing in the PPD. To understand the chemical and isotopic composition of the protosolar MC material and the degree of its thermal processing in PPD, samples of the primordial silicates and ices, which may have survived in the outer solar system, are required. In metamorphosed CO3 and CV3 chondrites, most CAIs exhibit O‐isotope heterogeneity that often appears to be mineralogically controlled: anorthite, melilite, grossite, krotite, perovskite, and Zr‐ and Sc‐rich oxides and silicates are 16O‐depleted relative to corundum, hibonite, spinel, Al,Ti‐diopside, forsterite, and enstatite. In texturally fine‐grained CAIs with grain sizes of ~10–20 μm, this O‐isotope heterogeneity is most likely due to O‐isotope exchange with 16O‐poor (Δ17O ~0‰) aqueous fluids on the CO and CV chondrite parent asteroids. In CO3.1 and CV3.1 chondrites, this process did not affect Al‐Mg isotope systematics of CAIs. In some coarse‐grained igneous CV CAIs, O‐isotope heterogeneity of anorthite, melilite, and igneously zoned Al,Ti‐diopside appears to be consistent with their crystallization from melts of isotopically evolving O‐isotope compositions. These CAIs could have recorded O‐isotope exchange during incomplete melting in nebular gaseous reservoir(s) with different O‐isotope compositions and during aqueous fluid–rock interaction on the CV asteroid.

Pebble-driven planet formation for TRAPPIST-1 and other compact systems

Recently, seven Earth-sized planets were discovered around the M-dwarf star TRAPPIST-1. Thanks to transit-timing variations, the masses and therefore the bulk densities of the planets have been constrained, suggesting that all TRAPPIST-1 planets are consistent with water mass fractions on the order of 10%. These water fractions, as well as the similar planet masses within the system, constitute strong constraints on the origins of the TRAPPIST-1 system. In a previous work, we outlined a pebble-driven formation scenario. In this paper we investigate this formation scenario in more detail. We used a Lagrangian smooth-particle method to model the growth and drift of pebbles and the conversion of pebbles to planetesimals through the streaming instability. We used the N-body code MERCURY to follow the composition of planetesimals as they grow into protoplanets by merging and accreting pebbles. This code is adapted to account for pebble accretion, type-I migration, and gas drag. In this way, we modelled the entire planet formation process (pertaining to planet masses and compositions, not dynamical configuration). We find that planetesimals form in a single, early phase of streaming instability. The initially narrow annulus of planetesimals outside the snowline quickly broadens due to scattering. Our simulation results confirm that this formation pathway indeed leads to similarly-sized planets and is highly efficient in turning pebbles into planets. Our results suggest that the innermost planets in the TRAPPIST-1 system grew mostly by planetesimal accretion at an early time, whereas the outermost planets were initially scattered outwards and grew mostly by pebble accretion. The water content of planets resulting from our simulations is on the order of 10%, and our results predict a “V-shaped” trend in the planet water fraction with orbital distance: from relatively high (innermost planets) to relatively low (intermediate planets) to relatively high (outermost planets).

Vortex instabilities triggered by low-mass planets in pebble-rich, inviscid protoplanetary discs

Abstract In the innermost regions of protoplanerary discs, the solid-to-gas ratio can be increased considerably by a number of processes, including photoevaporative and particle drift. Magnetohydrodynamic disc models also suggest the existence of a dead zone at R ≲ 10 au, where the regions close to the mid-plane remain laminar. In this context, we use two-fluid hydrodynamical simulations to study the interaction between a low-mass planet (∼1.7 M⊕) on a fixed orbit and an inviscid pebble-rich disc with solid-to-gas ratio ϵ ≥ 0.5. For pebbles with Stokes numbers St = 0.1, 0.5, multiple dusty vortices are formed through the Rossby wave instability at the planet separatrix. Effects due to gas drag then lead to a strong enhancement in the solid-to-gas ratio, which can increase by a factor of ∼103 for marginally coupled particles with St = 0.5. As in streaming instabilities, pebble clumps reorganize into filaments that may plausibly collapse to form planetesimals. When the planet is allowed to migrate in an Minimum Mass Solar Nebula (MMSN) disc, the vortex instability is delayed due to migration but sets in once inward migration stops due a strong positive pebble torque. Again, particle filaments evolving in a gap are formed in the disc while the planet undergoes an episode of outward migration. Our results suggest that vortex instabilities triggered by low-mass planets could play an important role in forming planetesimals in pebble-rich, inviscid discs, and may significantly modify the migration of low-mass planets. They also imply that planetary dust gaps may not necessarily contain planets if these migrated away.

Radiation-pressure-driven dust transport to galaxy haloes at z ∼ 10

The origin of dust in galaxy halos or in the circum-galactic medium (CGM) is still a mystery. We investigate if the radiation pressure in high-redshift ($z\sim 10$) galaxies can efficiently transport dust to halos. To clarify the first dust enrichment of galaxy halos in the early Universe, we solve the motion of a dust grain considering radiation pressure, gas drag, and gravity in the vertical direction of the galactic disc. Radiation pressure is estimated in a consistent manner with the stellar spectra and dust extinction. As a consequence, we find that dust grains with radii $a\sim 0.1~\mu$m successfully escape from the galactic disc if the ongoing star formation episode converts more than 15 per cent of the baryon content into stars and lasts $\gtrsim 30$ Myr, while larger and smaller grains are trapped in the disc because of gravity and gas drag, respectively. We also show that grain charge significantly enhances gas drag at a few--10 scale heights of the galactic disc, where the grain velocities are suppressed to $\sim 1$ km s$^{-1}$. There is an optimum dust-to-gas ratio ($\sim 10^{-3}$) in the galactic disc and an optimum virial mass $\sim 10^{10}$--$10^{11}$ M$_{\odot}$ for the transport of $a\sim 0.1~\mu$m grains to the halo. We conclude that early dust enrichment of galaxy halos at $z\gtrsim 10$ is important for the origin of dust in the CGM.

Growth after the streaming instability

Context. Streaming instability is a key mechanism in planet formation, clustering pebbles into planetesimals with the help of self-gravity. It is triggered at a particular disk location where the local volume density of solids exceeds that of the gas. After their formation, planetesimals can grow into protoplanets by feeding from other planetesimals in the birth ring as well as by accreting inwardly drifting pebbles from the outer disk. Aims. We aim to investigate the growth of planetesimals into protoplanets at a single location through streaming instability. For a solar-mass star, we test the conditions under which super-Earths are able to form within the lifetime of the gaseous disk. Methods. We modified the Mercury N-body code to trace the growth and dynamical evolution of a swarm of planetesimals at a distance of 2.7 AU from the star. The code simulates gravitational interactions and collisions among planetesimals, gas drag, type I torque, and pebble accretion. Three distributions of planetesimal sizes were investigated: (i) a mono-dispersed population of 400 km radius planetesimals, (ii) a poly-dispersed population of planetesimals from 200 km up to 1000 km, (iii) a bimodal distribution with a single runaway body and a swarm of smaller, 100 km size planetesimals. Results. The mono-dispersed population of 400 km size planetesimals cannot form protoplanets of a mass greater than that of the Earth. Their eccentricities and inclinations are quickly excited, which suppresses both planetesimal accretion and pebble accretion. Planets can form from the poly-dispersed and bimodal distributions. In these circumstances, it is the two-component nature that damps the random velocity of the large embryo through the dynamical friction of small planetesimals, allowing the embryo to accrete pebbles efficiently when it approaches 10−2 M⊕. Accounting for migration, close-in super-Earth planets form. Super-Earth planets are likely to form when the pebble mass flux is higher, the disk turbulence is lower, or the Stokes number of the pebbles is higher. Conclusions. For the single site planetesimal formation scenario, a two-component mass distribution with a large embryo and small planetesimals promotes planet growth, first by planetesimal accretion and then by pebble accretion of the most massive protoplanet. Planetesimal formation at single locations such as ice lines naturally leads to super-Earth planets by the combined mechanisms of planetesimal accretion and pebble accretion.

High-drag Interstellar Objects and Galactic Dynamical Streams

Abstract The nature of 1I/’Oumuamua (henceforth, 1I), the first interstellar object (ISO) known to pass through the solar system, remains mysterious. Feng & Jones noted that the incoming 1I velocity vector “at infinity” ( ) is close to the motion of the Pleiades dynamical stream (or Local Association), and suggested that 1I is a young object ejected from a star in that stream. Micheli et al. subsequently detected nongravitational acceleration in the 1I trajectory; this acceleration would not be unusual in an active comet, but 1I observations failed to reveal any signs of activity. Bialy & Loeb hypothesized that the anomalous 1I acceleration was instead due to radiation pressure, which would require an extremely low mass-to-area ratio (or area density). Here I show that a low area density can also explain the very close kinematic association of 1I and the Pleiades stream, as it renders 1I subject to drag capture by interstellar gas clouds. This supports the radiation pressure hypothesis and suggests that there is a significant population of low area density ISOs in the Galaxy, leading, through gas drag, to enhanced ISO concentrations in the galactic dynamical streams. Any ISO entrained in a dynamical stream will have a predictable incoming targeted deep surveys using this information should be able to find dynamical stream objects from months to as much as a year before their perihelion, providing the lead time needed for fast-response missions for the future in situ exploration of such objects.

Shock-generating Planetesimals Perturbed by a Giant Planet in a Gas Disk

Abstract We examined the excitations of planetesimals caused by the resonances of a giant planet in a protoplanetary gas disk. The highly excited planetesimals generate bow shocks, the mechanism of which results in chondrule formation, crystallization of silicate dust, and evaporation of icy planetesimals. The planetesimals beyond 2:1 resonance migrate owing to the gas drag and obtain the maximum eccentricity around 3:1 resonance, which is located at approximately half the planetary distance. The eccentricity depends on the parameters of the planetesimals and the Jovian planet, such as size and location, and the gas density of the disk. The maximum relative velocity of a 100 km sized planetesimal with respect to the gas disk reaches up to ∼12 km s−1 in the case of Jupiter owing to secular resonance, which occurs because of the disk’s gravity. We find that if a Jovian-mass planet is located within 10 au, the planetesimals larger than 100 km gain sufficient velocity to cause the melting of chondrule precursors and crystallization of the silicate. The maximum velocity is higher for large planetesimals and eccentric planets. Planetesimals are trapped temporarily in the resonances and continue to have high speed over ≳1 Myr after the formation of a Jovian planet. This duration fits into the timescale of chondrule formation suggested by the isotopic data. The evaporation of icy planetesimals occurs when a Jovian planet is located within 15 au. This mechanism can be a new indicator of planet formation in exosystems if some molecules ejected from icy planetesimals are detected.

Fuel Pellet Alignment in Heavy-Ion Inertial Fusion Reactor

In inertial confinement fusion, the scientific issues include the generation and transport of driver energy, the pellet design, the uniform target implosion physics, the realistic nuclear fusion reactor design, etc. In this paper, we present a pellet injection into a power reactor in heavy ion inertial fusion. We employ a magnetic correction method to reduce the pellet alignment error in heavy ion inertial fusion reactor chamber, including the gravity, the reactor gas drag force and the injection errors. We found that the magnetic correction device proposed in this paper is effective to construct a robust pellet injection system with a sufficiently small pellet alignment error.

The formation of rings and gaps in wind-launching non-ideal MHD discs: three-dimensional simulations

Previous axisymmetric investigations in two dimensions (2D) have shown that rings and gaps develop naturally in non-ideal magnetohydrodynamic (MHD) disk-wind systems, especially in the presence of ambipolar diffusion (AD). Here we extend these 2D simulations to three dimensions (3D) and find that rings and gaps are still formed in the presence of a moderately strong ambipolar diffusion. The rings and gaps form from the same basic mechanism that was identified in the 2D simulations, namely, the redistribution of the poloidal magnetic flux relative to the disk material as a result of the reconnection of a sharply pinched poloidal magnetic field lines. Thus, the less dense gaps are more strongly magnetized with a large poloidal magnetic field compared to the less magnetized (dense) rings. The rings and gaps start out rather smoothly in 3D simulations that have axisymmetric initial conditions. Non-axisymmetric variations arise spontaneously at later times, but they do not grow to such an extent as to disrupt the rings and gaps. These disk substructures persist to the end of the simulations, lasting up to 3000 orbital periods at the inner edge of the simulated disk. The longevity of the perturbed yet still coherent rings make them attractive sites for trapping large grains that would otherwise be lost to rapid radial migration due to gas drag. As the ambipolar diffusivity decreases, both the disk and the wind become increasingly turbulent, driven by the magnetorotational instability, with tightly-wound spiral arms becoming more prominent in the disk.

Gravitational Instability in the Dust Layer of a Protoplanetary Disk with Interaction between the Layer and the Surrounding Gas

We consider gravitational instability of the dust layer in the midplane of a protoplanetary disk with turbulence and shear stresses between the gas in the disk and that in the dust layer. We solve a linearized system of hydrodynamic equations for perturbations of dust (monodisperse) and gas phases in the incompressible gas approximation. We take into account the gas drag of solid particles (dust aggregates), turbulent diffusion and the velocity dispersion of particles, and the perturbation of the azimuthal velocity of gas in the layer upon the transfer of angular momentum from solid particles to it and from this gas to the surrounding gas in the disk. We obtain and solve the dispersion equation for the layer with the ratio of surface densities of the dust phase and gas being well above unity. The following parameters of gravitational instability in the dust layer are calculated: the critical surface density of solid matter and the Stokes number of particles corresponding to the onset of instability, the wavelength range in which instability occurs, and the rate of its growth as a function of the perturbation wavelength in the circumsolar disk at radial distances of 1 and 10 AU. We show that at 10 AU, the maximum instability growth rate increases due to the transfer of angular momentum of gas in the layer to gas outside it, a new maximum emerges at a longer wavelength, a long-wavelength instability “tail” forms, and the critical surface density initiating instability decreases relative to that determined without the transfer of angular momentum to gas outside the layer. None of these effects are observed at 1 AU, since instability in this region probably develops faster than the transfer of angular momentum to the surrounding gаs of a protoplanetary disk occurs.

Stability of the triangular points in the photogravitational triaxial elliptic RTBP with dissipations

In this work, the author aims to make a substantial extension to the work developed by Abd El-Salam (2015). This model has been amended with photogravitational effects and three different kinds of dissipation forces, namely the simple nebular drag, the Poynting–Robertson drag (in brief PR) and nebular gas (Stokes) drag. It is observed that at certain values of the considered perturbing parameters, the triangular equilibrium points are strongly perturbed, especially when the mass loss of the primary is q=0.5. In this case, the triangular equilibrium points will be found very far from the barycenter. Also, the L4, 5 locations are highly perturbed when the eccentricity becomes high. The change in the stability and instability regions with different considered perturbations is investigated. It seems that if the eccentricity is increased then, the stability regions is enlarged. It is revealed that we have two disjoint stability regions. For the very high eccentricities e ∈ [0.9, 0.908], we have stability region for the whole domain of the mass ratio except in the neighborhood of μ=0.3. Different regions of stability, asymptotic stability and instability are revealed due to some specific values of photogravitational effects.

The backscattering ratio of comet 67P/Churyumov-Gerasimenko dust coma as seen by OSIRIS onboard Rosetta

Remote sensing observations of dust particles ejected from comets provide important hints on the intimate nature of the materials composing these primitive objects. The measurement of dust coma backscattering ratio, BSR, defined as the ratio of the reflectance at phase angle 0° and 30°, helps tuning theoretical models aimed at solving the inverse scattering problem deriving information on the nature of the ejected particles. The Rosetta/OSIRIS camera sampled the coma phase function of comet 67P, with four series acquired at low phase angles from 2015 January to 2016 May. We also added previously published data to our analysis to increase the temporal resolution of our findings. We measured a BSR in the range ˜ [1.7-3.6], broader than the range found in literature from ground-based observations of other comets. We found that during the post-perihelion phase, the BSR is systematically larger than the classical cometary dust values only for nucleocentric distances smaller than ˜100 km. We explain this trend in terms of a cloud of chunks orbiting the nucleus at distances <100 km ejected during perihelion and slowly collapsing on the nucleus over a few months because of the coma gas drag. This also implies that the threshold particle size for the dust phase function to become similar to the nucleus phase function is between 2.5 mm and 0.1 m, taking into account previous Rosetta findings.

A balanced budget view on forming giant planets by pebble accretion

Pebble accretion refers to the assembly of rocky planet cores from particles whose velocity dispersions are damped by drag from circumstellar disc gas. Accretion cross-sections can approach maximal Hill-sphere scales for particles whose Stokes numbers approach unity. While fast, pebble accretion is also lossy. Gas drag brings pebbles to protocores but also sweeps them past; those particles with the largest accretion cross-sections also have the fastest radial drift speeds and are the most easily drained out of discs. We present a global model of planet formation by pebble accretion that keeps track of the disc's mass budget. Cores, each initialized with a lunar mass, grow from discs whose finite stores of mm-cm sized pebbles drift inward across all radii in viscously accreting gas. For every 1 $M_\oplus$ netted by a core, at least 10 $M_\oplus$ and possibly much more are lost to radial drift. Core growth rates are typically exponentially sensitive to particle Stokes number, turbulent Mach number, and solid surface density. This exponential sensitivity, when combined with disc migration, tends to generate binary outcomes from 0.1-30 AU: either sub-Earth cores remain sub-Earth, or explode into Jupiters, with the latter migrating inward to varying degrees. When Jupiter-breeding cores assemble from mm-cm sized pebbles, they do so in discs where such particles drain out in $\sim$10$^5$ yr or less; such fast-draining discs do not fit mm-wave observations.

Costs of epibionts on Antarctic sea spiders

Nearly all marine animals harbor epibionts, organisms living on their body surfaces. The positive or negative effects that epibionts have on their hosts depend on many factors, including the size and location of the epibionts on their host. The present study examined the effects of epibionts on gas exchange, locomotion, and drag of three species of Antarctic sea spiders (pycnogonids). Sea spiders are a cosmopolitan group of marine arthropods that lack gills and rely instead on the diffusion of oxygen directly across their cuticle. Encrusting epibionts, such as bryozoans and algae, had only minor effects on surface oxygen levels, but they reduced the functional diffusion coefficient of oxygen through the cuticle by about half. Although these effects are significant locally and may be severe in individuals with high coverage by epibionts, the total coverage on most individuals was not high enough to significantly alter oxygen fluxes into the animal. Macroepibionts, such as barnacles, had no effect on host walking speeds, but they increased by two-to-threefold the drag experienced by host sea spiders. This likely increases the energetic costs of walking and increases the chance of being dislodged by high currents. These results suggest that epibionts can impose diverse costs to their hosts but only in subtle ways that depend on total epibiont coverage of the host and rates of water flow.