scholarly journals Polydisperse streaming instability – III. Dust evolution encourages fast instability

2021 ◽  
Vol 502 (1) ◽  
pp. 1469-1486
Author(s):  
Colin P McNally ◽  
Francesco Lovascio ◽  
Sijme-Jan Paardekooper
Keyword(s):  
Power Law ◽  
Planet Formation ◽  
Full Range ◽  
Density Ratio ◽  
Stopping Times ◽  
Physical Parameters ◽  
Streaming Instability ◽  
Dust Distribution ◽  
Dust Evolution ◽  
Single Size

ABSTRACT Planet formation via core accretion requires the production of kilometre-sized planetesimals from cosmic dust. This process must overcome barriers to simple collisional growth, for which the streaming instability (SI) is often invoked. Dust evolution is still required to create particles large enough to undergo vigorous instability. The SI has been studied primarily with single-size dust, and the role of the full evolved dust distribution is largely unexplored. We survey the polydisperse streaming instability (PSI) with physical parameters corresponding to plausible conditions in protoplanetary discs. We consider a full range of particle stopping times, generalized dust size distributions, and the effect of turbulence. We find that while the PSI grows in many cases more slowly with an interstellar power-law dust distribution than with a single size, reasonable collisional dust evolution, producing an enhancement of the largest dust sizes, produces instability behaviour similar to the monodisperse case. Considering turbulent diffusion, the trend is similar. We conclude that if fast linear growth of PSI is required for planet formation, then dust evolution producing a distribution with peak stopping times on the order of 0.1 orbits and an enhancement of the largest dust significantly above the single power-law distribution produced by a fragmentation cascade is sufficient, along with local enhancement of the dust to gas volume mass density ratio to order unity.

scholarly journals Pebble accretion in self-gravitating protostellar discs

2019 ◽  
Vol 485 (4) ◽  
pp. 4465-4473
Author(s):  
D H Forgan
Keyword(s):  
Time Scales ◽  
Planet Formation ◽  
Density Ratio ◽  
Streaming Instability ◽  
Planetary Cores ◽  
Low Mass ◽  
Spiral Structures ◽  
Protostellar Discs ◽  
Core Accretion ◽  
Solid Bodies

Abstract Pebble accretion has become a popular component to core accretion models of planet formation, and is especially relevant to the formation of compact, resonant terrestrial planetary systems. Pebbles initially form in the inner protoplanetary disc, sweeping outwards in a radially expanding front, potentially forming planetesimals and planetary cores via migration and the streaming instability. This pebble front appears at early times, in what is typically assumed to be a low-mass disc. We argue this picture is in conflict with the reality of young circumstellar discs, which are massive and self-gravitating. We apply standard pebble accretion and streaming instability formulae to self-gravitating protostellar disc models. Fragments will open a gap in the pebble disc, but they will likely fail to open a gap in the gas, and continue rapid inward migration. If this does not strongly perturb the pebble disc, our results show that disc fragments will accrete pebbles efficiently. We find that in general the pebble-to-gas-density ratio fails to exceed 0.01, suggesting that the streaming instability will struggle to operate. It may be possible to activate the instability if 10 cm grains are available, and spiral structures can effectively concentrate them in regions of low gravito-turbulence. If this occurs, lunar mass cores might be assembled on time-scales of a few thousand years, but this is likely to be rare, and is far from proven. In any case, this work highlights the need for study of how self-gravitating protostellar discs define the distribution and properties of solid bodies, for future planet formation by core accretion.

scholarly journals Giant planet formation at the pressure maxima of protoplanetary disks

2020 ◽  
Vol 642 ◽  
pp. A140 ◽  
Author(s):  
Octavio Miguel Guilera ◽  
Zsolt Sándor ◽  
María Paula Ronco ◽  
Julia Venturini ◽  
Marcelo Miguel Miller Bertolami
Keyword(s):  
Planet Formation ◽  
Giant Planet ◽  
Protoplanetary Disks ◽  
Protoplanetary Disk ◽  
Water Ice ◽  
Streaming Instability ◽  
Accretion Model ◽  
Planet Migration ◽  
Pressure Maximum ◽  
Dust Evolution

Context. Recent high-resolution observations of protoplanetary disks have revealed ring-like structures that can be associated to pressure maxima. Pressure maxima are known to be dust collectors and planet migration traps. The great majority of planet formation studies are based either on the pebble accretion model or on the planetesimal accretion model. However, recent studies proposed hybrid accretion of pebbles and planetesimals as a possible formation mechanism for Jupiter. Aims. We aim to study the full process of planet formation consisting of dust evolution, planetesimal formation, and planet growth at a pressure maximum in a protoplanetary disk. Methods. We compute, through numerical simulations, the gas and dust evolution in a protoplanetary disk, including dust growth, fragmentation, radial drift, and particle accumulation at a pressure maximum. The pressure maximum appears due to an assumed viscosity transition at the water ice line. We also consider the formation of planetesimals by streaming instability and the formation of a moon-size embryo that grows into a giant planet by the hybrid accretion of pebbles and planetesimals, all within the pressure maximum. Results. We find that the pressure maximum is an efficient collector of dust drifting inwards. The condition of planetesimal formation by streaming instability is fulfilled due to the large amount of dust accumulated at the pressure bump. Subsequently, a massive core is quickly formed (in ~104 yr) by the accretion of pebbles. After the pebble isolation mass is reached, the growth of the core slowly continues by the accretion of planetesimals. The energy released by planetesimal accretion delays the onset of runaway gas accretion, allowing a gas giant to form after ~1 Myr of disk evolution. The pressure maximum also acts as a migration trap. Conclusions. Pressure maxima generated by a viscosity transition at the water ice line are preferential locations for dust traps, planetesimal formation by streaming instability, and planet migration traps. All these conditions allow the fast formation of a giant planet by the hybrid accretion of pebbles and planetesimals.

scholarly journals Planet formation in action: resolved gas and dust images of a transitional disk and its cavity

2013 ◽  
Vol 8 (S299) ◽  
pp. 90-93
Author(s):  
Nienke van der Marel ◽  
Ewine F. van Dishoeck ◽  
Simon Bruderer ◽  
Til Birnstiel ◽  
Paola Pinilla ◽  
...  
Keyword(s):  
Planet Formation ◽  
Early Stage ◽  
Continuum Emission ◽  
Dust Trapping ◽  
Spatially Resolved ◽  
First Results ◽  
Disk Evolution ◽  
Gas Cavity ◽  
Dust Distribution ◽  
The Continuum

AbstractPlanet formation and clearing of protoplanetary disks is one of the long standing problems in disk evolution theory. The best test of clearing scenarios is observing systems that are most likely to be actively forming planets: the transitional disks with large inner dust cavities. We present the first results of our ALMA (Atacama Large Millimeter/submillimeter Array) Cycle 0 program using Band 9, imaging the Herbig Ae star Oph IRS 48 in CO 6−5 and the submillimeter continuum in the extended configuration. The resulting ~0.2″ spatial resolution completely resolves the cavity of this disk in the gas and the dust. The gas cavity of IRS 48 is half as large as the dust cavity, ruling out grain growth and photoevaporation as the primary cause of the truncation. On the other hand, the continuum emission reveals an unexpected large azimuthal asymmetry and steep edges in the dust distribution along the ring, suggestive of dust trapping. We will discuss the implications of the combined gas and dust distribution for planet formation at a very early stage. This is one of the first transition disks with spatially resolved gas inside the cavity, demonstrating the superb capabilities of the Band 9 receivers.

scholarly journals Dust Distribution in IRAS Seyfert Galaxies

1994 ◽  
Vol 159 ◽  
pp. 355-355
Author(s):  
M. G. Pastoriza ◽  
Charles Bonatto ◽  
Eduardo Bica ◽  
T. Storchi-Bergmann
Keyword(s):  
Emission Line ◽  
Power Law ◽  
Dust Emission ◽  
Seyfert Galaxies ◽  
Emission Model ◽  
Line Region ◽  
Population Type ◽  
The Galaxy ◽  
Dilution Effects ◽  
Dust Distribution

Observational evidences of dust in the nuclear region of AGNs are substantial (Rudy 1984, ApJ, 284, 33; Jones et al. 1984, PASP, 96, 692). The ionization cones observed in several Seyfert galaxies has been interpreted as shadowing effects by a dust obscuring torus which hides the broad emission line region (BLR) and the central source (Wilson 1992; Storchi-Bergmann, Mulchaey and Wilson 1992, ApJ 395, L73). A large sample of optical and far-IR data for IRAS Seyfert galaxies has been analysed together with dust emission models (Bonatto and Pastoriza 1993), where it has been concluded that the same dust emission model can be applied to both Seyfert types. In order to further study the effects of dust in the spectra of active galactic nuclei, we have obtained spectrophotometry of 21 IRAS Seyfert galaxies in the range 3500–7200 Å and analyse them in conjuction with their IRAS fluxes. The stellar population type is derived from comparisons with normal galaxy templates using dilution effects in the K CaII line as discriminator. For 55% of the sample the population is of late type. For the rest, blue continua due to recent star formation and/or power-law may amount up to 30% at 4000Å. We conclude that the bulge stellar populations of IRAS Seyfert galaxies are similar to those of normal spirals, except that they are more reddened by E(B-V)i ∼ 0.20. Population-subtracted emission line ratios indicate on average stronger reddening for the narrow-line region (E(B-V)l ∼ 0.8. From photoionization models a power-law index for the ionizing continuum α=1.5, and a metallicity larger than solar are obtained. The most luminous IRAS galaxy of the sample (IRAS555) is discuss in detail: in order to be compatible with the observed IRAS fluxes and the optical stellar continuum, the ionizing continuum must be reddened by AV > 10 magnitudes. Consequently a dust structure in this galaxy appears to be increasingly affecting stars and gas towards the galaxy center.

scholarly journals Estimating Finite Source Effects in Microlensing Events due to Free-Floating Planets with the Euclid Survey

2015 ◽  
Vol 2015 ◽  
pp. 1-8 ◽  
Author(s):  
Lindita Hamolli ◽  
Mimoza Hafizi ◽  
Francesco De Paolis ◽  
Achille A. Nucita
Keyword(s):  
Spatial Distribution ◽  
Power Law ◽  
Mass Function ◽  
Physical Parameters ◽  
Galactic Bulge ◽  
Source Effect ◽  
Source Effects ◽  
Function Index ◽  
Finite Source ◽  
Einstein Radius

In recent years free-floating planets (FFPs) have drawn a great interest among astrophysicists. Gravitational microlensing is a unique and exclusive method for their investigation which may allow obtaining precious information about their mass and spatial distribution. The planned Euclid space-based observatory will be able to detect a substantial number of microlensing events caused by FFPs towards the Galactic bulge. Making use of a synthetic population algorithm, we investigate the possibility of detecting finite source effects in simulated microlensing events due to FFPs. We find a significant efficiency for finite source effect detection that turns out to be between 20% and 40% for a FFP power law mass function index in the range [0.9, 1.6]. For many of such events it will also be possible to measure the angular Einstein radius and therefore constrain the lens physical parameters. These kinds of observations will also offer a unique possibility to investigate the photosphere and atmosphere of Galactic bulge stars.

Multiple Analytic Solutions of Heat and Mass Transfer of Magnetohydrodynamic Slip Flow for Two Types of Viscoelastic Fluids Over a Stretching Surface

2012 ◽  
Vol 134 (7) ◽  
Author(s):  
Mustafa Turkyilmazoglu
Keyword(s):  
Viscoelastic Fluid ◽  
Power Law ◽  
Multiple Solutions ◽  
Slip Flow ◽  
Analytic Solutions ◽  
Physical Parameters ◽  
Main Concern ◽  
Stretching Surface ◽  
Boundary Layer Equations ◽  
Nusselt Numbers

This paper focuses on the magnetohydrodynamic (MHD) slip flow of an electrically conducting, viscoelastic fluid past a stretching surface. The main concern is to analytically investigate the structure of the solutions and determine the thresholds beyond which multiple solutions exist or the physically pure exponential type solution ceases to exist. In the case of the presence of multiple solutions, closed-form formulae for the boundary layer equations of the flow are presented for two classes of viscoelastic fluid, namely, the second-grade and Walter’s liquid B fluids. Heat transfer analyzes are also carried out for two general types of boundary heating processes, either by a prescribed quadratic power law surface temperature or by a prescribed quadratic power law surface heat flux. The flow field is affected by the presence of several physical parameters, whose influences on the unique/multiple solutions of velocity and temperature profiles, and Nusselt numbers are examined and discussed.

Effect of Melting on Mixed Convection Heat and Mass Transfer in a Non-Newtonian Fluid Saturated Non-Darcy Porous Medium

2012 ◽  
Vol 134 (4) ◽  
Author(s):  
R. R. Kairi ◽  
P. V. S. N. Murthy
Keyword(s):  
Mass Transfer ◽  
Porous Medium ◽  
Mixed Convection ◽  
Heat And Mass Transfer ◽  
Newtonian Fluid ◽  
Power Law ◽  
Soret Effect ◽  
Physical Parameters ◽  
Convection Heat ◽  
Wide Range

In this paper, we investigate the influence of melting on mixed convection heat and mass transfer from vertical flat plate in a non-Newtonian fluid-saturated non-Darcy porous medium including the prominent Soret effect. The wall and the ambient medium are maintained at constant but different levels of temperature and concentration such that the heat and mass transfer occurs from the wall to the medium. The Ostwald–de Waele power law model is used to characterize the non-Newtonian fluid behavior. A similarity solution for the transformed governing equations is obtained. The numerical computation is carried out for various values of the nondimensional physical parameters. The variation of temperature, concentration, and heat and mass transfer coefficients with the power law index, mixed convection parameter, inertia parameter, melting parameter, Soret number, buoyancy ratio, and Lewis number is discussed for a wide range of values of these parameters.

scholarly journals Disk Masses and Dust Evolution of Protoplanetary Disks around Brown Dwarfs

2021 ◽  
Vol 921 (2) ◽  
pp. 182
Author(s):  
Anneliese M. Rilinger ◽  
Catherine C. Espaillat
Keyword(s):  
Planet Formation ◽  
Spectral Energy ◽  
Star Forming ◽  
Star Forming Regions ◽  
Disk Mass ◽  
T Tauri ◽  
Multiple Regions ◽  
Disk Evolution ◽  
Dust Grain ◽  
Dust Evolution

Abstract We present the largest sample of brown dwarf (BD) protoplanetary disk spectral energy distributions modeled to date. We compile 49 objects with ALMA observations from four star-forming regions: ρ Ophiuchus, Taurus, Lupus, and Upper Scorpius. Studying multiple regions with various ages enables us to probe disk evolution over time. Specifically, from our models, we obtain values for dust grain sizes, dust settling, and disk mass; we compare how each of these parameters vary between the regions. We find that disk mass decreases with age. We also find evidence of disk evolution (i.e., grain growth and significant dust settling) in all four regions, indicating that planet formation and disk evolution may begin to occur at earlier stages. We generally find that these disks contain too little mass to form planetary companions, though we cannot rule out that planet formation may have already occurred. Finally, we examine the disk mass–host mass relationship and find that BD disks are largely consistent with previously determined relationships for disks around T Tauri stars.

Viscosity Models for Drilling Fluids: Viscosity Parameters and Their Use

2019 ◽  
Author(s):  
Arild Saasen ◽  
Jan David Ytrehus
Keyword(s):  
Shear Rate ◽  
Yield Stress ◽  
Yield Point ◽  
Power Law ◽  
Physical Parameters ◽  
Fluid Viscosity ◽  
Reasonable Accuracy ◽  
Bingham Model ◽  
Viscosity Models ◽  
Power Law Exponent

Abstract The most common viscosity models used in the drilling industry are the Bingham, the Power-Law and the Herschel-Bulkley models. The scope of the present paper is to outline how to select the individual models, and how the models need to be re-formulated to be able to have parameters with a physical meaning. In principle, the Bingham model itself have physical parameters being the yield point and the plastic viscosity. However, the Bingham model very often only very poorly describe the viscosity in complex fluids. This yield stress can be described within a reasonable accuracy by application of the low-shear yield point. A similar problem exists with the Power-Law model resulting from the model’s absence of a yield stress. The compromise model is the Herschel-Bulkley model which contains a yield stress and a power-law term. This model describes the drilling fluid viscosity with reasonable accuracy and includes both the Bingham and Power-Law models as limit formulations. It is not possible to select fluids based on the Herschel-Bulkley traditional parameters alone. The reason is that the Herschel-Bulkley power-law term’s viscosity parameter has a unit dependent on its power-law exponent. In the present approach the fluid is described using a yield stress, a surplus stress at a characteristic shear rate of the fluid flow and finally a power-law exponent making the fluid applicable in the practical shear rate ranges. The surplus stress is no-longer dependent on other parameters. Hence, we have re-arranged the viscosity model to have independent measurable quantities.

scholarly journals Resolving the excess of long GRB’s at low redshift in the Swift era

2020 ◽  
Vol 493 (1) ◽  
pp. 1479-1491 ◽  
Author(s):  
Truong Le ◽  
Cecilia Ratke ◽  
Vedant Mehta
Keyword(s):  
Physical Parameter ◽  
Power Law ◽  
Gamma Ray ◽  
Hubble Constant ◽  
High Energy ◽  
Star Formation History ◽  
Physical Parameters ◽  
Data Set ◽  
Energy Indices ◽  
Parameter Values

ABSTRACT Utilizing more than 100 long gamma-ray bursts (LGRBs) in the Swift-Ryan-2012 sample that includes the observed redshifts and jet angles, Le & Mehta performed a timely study of the rate density of LGRBs with an assumed broken power-law GRB spectrum and obtained a GRB-burst-rate functional form that gives acceptable fits to the pre-Swift and Swift redshift, and jet angle distributions. The results indicated an excess of LGRBs at redshift below z ∼ 2 in the Swift sample. In this work, we are investigating if the excess is caused by the cosmological Hubble constant H0, the gamma-ray energy released ${\cal E}_{*\gamma }$, the low- and high-energy indices (α, β) of the Band function, the minimum and maximum jet angles θj, min and θj, max, or that the excess is due to a bias in the Swift-Ryan-2012 sample. Our analyses indicate that none of the above physical parameters resolved the excess problem, but suggesting that the Swift-Ryan-2012 sample is biased with possible afterglow selection effect. The following model physical parameter values provide the best fit to the Swift-Ryan-2012 and pre-Swift samples: the Hubble constant $H_0 = 72 \, {\rm km s^{-1} Mpc^{-1}}$, the energy released ${\cal E}_{*\gamma }\sim 4.47 \times 10^{51}$ erg, the energy indices α ∼ 0.9 and β ∼ −2.13, the jet angles of θj, max ∼ 0.8 rad, and θj, min ∼ 0.065 and ∼0.04 rad for pre-Swift and Swift, respectively, s ∼ −1.55 the jet angle power-law index, and a GRB formation rate that is similar to the Hopkins & Beacom observed star formation history and as extended by Li. Using the Swift Gamma-Ray Burst Host Galaxy Legacy Survey (SHOALS) Swift-Perley LGRB sample and applying the same physical parameter values as above, however, our model provides consistent results with this data set and indicating no excess of LGRBs at any redshift.

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