scholarly journals Pebble drift and planetesimal formation in protoplanetary discs with embedded planets

2020 ◽  
Vol 635 ◽  
pp. A110 ◽  
Author(s):  
Linn E. J. Eriksson ◽  
Anders Johansen ◽  
Beibei Liu
Keyword(s):  
Spatial Distribution ◽  
Time Scale ◽  
Particle Sizes ◽  
Dust Trapping ◽  
Nominal Model ◽  
Streaming Instability ◽  
Dust Evolution ◽  
Strong Depletion ◽  
Protoplanetary Discs

Nearly axisymmetric gaps and rings are commonly observed in protoplanetary discs. The leading theory regarding the origin of these patterns is that they are due to dust trapping at the edges of gas gaps induced by the gravitational torques from embedded planets. If the concentration of solids at the gap edges becomes high enough, it could potentially result in planetesimal formation by the streaming instability. We tested this hypothesis by performing global 1D simulations of dust evolution and planetesimal formation in a protoplanetary disc that is perturbed by multiple planets. We explore different combinations of particle sizes, disc parameters, and planetary masses, and we find that planetesimals form in all of these cases. We also compare the spatial distribution of pebbles from our simulations with protoplanetary disc observations. Planets larger than one pebble isolation mass catch drifting pebbles efficiently at the edge of their gas gaps, and depending on the efficiency of planetesimal formation at the gap edges, the protoplanetary disc transforms within a few 100 000 yr to either a transition disc with a large inner hole devoid of dust or to a disc with narrow bright rings. For simulations with planetary masses lower than the pebble isolation mass, the outcome is a disc with a series of weak ring patterns but there is no strong depletion between the rings. By lowering the pebble size artificially to a 100 micrometer-sized “silt”, we find that regions between planets get depleted of their pebble mass on a longer time-scale of up to 0.5 million years. These simulations also produce fewer planetesimals than in the nominal model with millimeter-sized particles and always have at least two rings of pebbles that are still visible after 1 Myr.

scholarly journals The efficiency of dust trapping in ringed protoplanetary discs

2020 ◽  
Vol 495 (1) ◽  
pp. 173-181 ◽  
Author(s):  
Giovanni P Rosotti ◽  
Richard Teague ◽  
Cornelis Dullemond ◽  
Richard A Booth ◽  
Cathie J Clarke
Keyword(s):  
Pressure Profile ◽  
Gas Pressure ◽  
Continuum Emission ◽  
Dust Trapping ◽  
Local Maxima ◽  
Grain Sizes ◽  
Dust Distribution ◽  
Dust Grain ◽  
Degree Of Coupling ◽  
Protoplanetary Discs

ABSTRACT When imaged at high resolution, many protoplanetary discs show gaps and rings in their dust sub-mm continuum emission profile. These structures are widely considered to originate from local maxima in the gas pressure profile. The properties of the underlying gas structures are however unknown. In this paper, we present a method to measure the dust–gas coupling α/St and the width of the gas pressure bumps affecting the dust distribution, applying high-precision techniques to extract the gas rotation curve from emission line data cubes. As a proof of concept, we then apply the method to two discs with prominent substructure, HD 163296 and AS 209. We find that in all cases the gas structures are larger than in the dust, confirming that the rings are pressure traps. Although the grains are sufficiently decoupled from the gas to be radially concentrated, we find that the degree of coupling of the dust is relatively good (α/St ∼ 0.1). We can therefore reject scenarios in which the disc turbulence is very low and the dust has grown significantly. If we further assume that the dust grain sizes are set by turbulent fragmentation, we find high values of the α turbulent parameter (α ∼ 10−2). Alternatively, solutions with smaller turbulence are still compatible with our analysis if another process is limiting grain growth. For HD 163296, recent measurements of the disc mass suggest that this is the case if the grain size is 1 mm. Future constraints on the dust spectral indices will help to discriminate between the two alternatives.

scholarly journals Streaming instability of multiple particle species in protoplanetary disks

2018 ◽  
Vol 618 ◽  
pp. A75 ◽  
Author(s):  
Noemi Schaffer ◽  
Chao-Chin Yang ◽  
Anders Johansen
Keyword(s):  
Particle Size ◽  
Size Distribution ◽  
Protoplanetary Disks ◽  
Single Species ◽  
Stokes Number ◽  
Dust Particles ◽  
Particle Sizes ◽  
Particle Species ◽  
Streaming Instability ◽  
Large Particles

The radial drift and diffusion of dust particles in protoplanetary disks affect both the opacity and temperature of such disks, as well as the location and timing of planetesimal formation. In this paper, we present results of numerical simulations of particle-gas dynamics in protoplanetary disks that include dust grains with various size distributions. We have considered three scenarios in terms of particle size ranges, one where the Stokes number τs = 10−1−100, one where τs = 10−4−10−1, and finally one where τs = 10−3−100. Moreover, we considered both discrete and continuous distributions in particle size. In accordance with previous works we find in our multispecies simulations that different particle sizes interact via the gas and as a result their dynamics changes compared to the single-species case. The larger species trigger the streaming instability and create turbulence that drives the diffusion of the solid materials. We measured the radial equilibrium velocity of the system and find that the radial drift velocity of the large particles is reduced in the multispecies simulations and that the small particle species move on average outwards. We also varied the steepness of the size distribution, such that the exponent of the solid number density distribution, dN∕da ∝ a−q, is either q = 3 or q = 4. Overall, we find that the steepness of the size distribution and the discrete versus continuous approach have little impact on the results. The level of diffusion and drift rates are mainly dictated by the range of particle sizes. We measured the scale height of the particles and observe that small grains are stirred up well above the sedimented midplane layer where the large particles reside. Our measured diffusion and drift parameters can be used in coagulation models for planet formation as well as to understand relative mixing of the components of primitive meteorites (matrix, chondrules and CAIs) prior to inclusion in their parent bodies.

scholarly journals Rapid grain growth in post-AGB disc systems from far-infrared and sub-millimetre photometry

2020 ◽  
Vol 494 (2) ◽  
pp. 2925-2936
Author(s):  
P Scicluna ◽  
F Kemper ◽  
A Trejo ◽  
J P Marshall ◽  
S Ertel ◽  
...  
Keyword(s):  
Grain Growth ◽  
Time Scales ◽  
Far Infrared ◽  
Asymptotic Giant Branch ◽  
Growth Time ◽  
Agb Stars ◽  
Keplerian Orbits ◽  
Dust Evolution ◽  
Astronomical Dust ◽  
Protoplanetary Discs

ABSTRACT The time-scales on which astronomical dust grows remain poorly understood, with important consequences for our understanding of processes like circumstellar disc evolution and planet formation. A number of post-asymptotic giant branch (AGB) stars are found to host optically thick, dust- and gas-rich circumstellar discs in Keplerian orbits. These discs exhibit evidence of dust evolution, similar to protoplanetary discs; however, since post-AGB discs have substantially shorter lifetimes than protoplanetary discs, they may provide new insights on the grain-growth process. We examine a sample of post-AGB stars with discs to determine the far-infrared and sub-millimetre spectral index by homogeneously fitting a sample of data from Herschel, the Submillimeter Array (SMA), and the literature. We find that grain growth to at least hundreds of micrometres is ubiquitous in these systems, and that the distribution of spectral indices is more similar to that of protoplanetary discs than debris discs. No correlation is found with the mid-infrared colours of the discs, implying that grain growth occurs independently of the disc structure in post-AGB discs. We infer that grain growth to ∼millimetre sizes must occur on time-scales <<105 yr, perhaps by orders of magnitude, as the lifetimes of these discs are expected to be ≲105 yr and all objects have converged to the same state. This growth time-scale is short compared to the results of models for protoplanetary discs including fragmentation and may provide new constraints on the physics of grain growth.

scholarly journals Spatial Drought Characterization for Seyhan River Basin in the Mediterranean Region of Turkey

Water ◽  
2019 ◽  
Vol 11 (7) ◽  
pp. 1331 ◽  
Author(s):  
Yonca Cavus ◽  
Hafzullah Aksoy
Keyword(s):  
Spatial Distribution ◽  
River Basin ◽  
Frequency Analysis ◽  
Time Scale ◽  
Mediterranean Region ◽  
Eastern Mediterranean ◽  
Eastern Mediterranean Region ◽  
Natural Phenomenon ◽  
Total Probability ◽  
Return Periods

Drought is a natural phenomenon that has great impacts on the economy, society and environment. Therefore, the determination, monitoring and characterization of droughts are of great significance in water resources planning and management. The purpose of this study is to investigate the spatial drought characterizations of Seyhan River basin in the Eastern Mediterranean region of Turkey. The standardized precipitation index (SPI) was calculated from monthly precipitation data at 12-month time scale for 19 meteorological stations scattered over the river basin. Drought with the largest severity in each year is defined as the critical drought of the year. Frequency analysis was applied on the critical drought to determine the best-fit probability distribution function by utilizing the total probability theorem. The sole frequency analysis is insufficient in drought studies unless it is numerically related to other factors such as the severity, duration and intensity. Also, SPI is a technical tool and thus difficult to understand at first glance by end-users and decision-makers. Precipitation deficit defined as the difference between precipitation threshold at SPI = 0 and critical precipitation is therefore more preferable due to its usefulness and for being physically more meaningful to the users. Precipitation deficit is calculated and mapped for 1-, 3-, 6- and 12-month drought durations and 2-, 5-, 10-, 25-, 50- and 100-year return periods at 12-month time scale from the frequency analysis of the critical drought severity. The inverse distance weighted (IDW) interpolation technique is used for the spatial distribution of precipitation deficit over the Seyhan River basin. The spatial and temporal characteristics of drought suggest that the Seyhan River Basin in the Eastern Mediterranean region of Turkey experiences quite mild and severe droughts in terms of precipitation deficit. The spatial distribution would alter greatly with increasing return period and drought duration. While the coastal part of the basin is vulnerable to droughts at all return periods and drought durations, the northern part of the basin would be expected to be less affected by the drought. Another result reached in this study is that it could be common for one point in the basin to suffer dry conditions, whilst surrounding points in the same basin experience normal or even humid conditions. This reinforces the importance of spatial analysis over the basin under investigation instead of the point-scale temporal analysis made in each of the meteorological stations. With the use of spatial mapping of drought, it is expected that the destructive and irreversible effects of hydrological droughts can be realized in a more physical sense.

scholarly journals Streaming instability in the quasi-global protoplanetary discs

2013 ◽  
Vol 434 (2) ◽  
pp. 1460-1468 ◽  
Author(s):  
K. Kowalik ◽  
M. Hanasz ◽  
D. Wóltański ◽  
A. Gawryszczak
Keyword(s):  
Streaming Instability ◽  
Protoplanetary Discs

scholarly journals Dust settling instability in protoplanetary discs

2020 ◽  
Vol 497 (3) ◽  
pp. 2715-2729 ◽  
Author(s):  
Leonardo Krapp ◽  
Andrew N Youdin ◽  
Kaitlin M Kratter ◽  
Pablo Benítez-Llambay
Keyword(s):  
Scale Height ◽  
Weak Turbulence ◽  
Size Distributions ◽  
Linear Regime ◽  
Particle Size Distributions ◽  
Streaming Instability ◽  
Non Linear ◽  
Background Turbulence ◽  
Sudden Decrease ◽  
Protoplanetary Discs

ABSTRACT The streaming instability (SI) has been extensively studied in the linear and non-linear regimes as a mechanism to concentrate solids and trigger planetesimal formation in the mid-plane of protoplanetary discs. A related dust settling instability (DSI) applies to particles while settling towards the mid-plane. The DSI has previously been studied in the linear regime, with predictions that it could trigger particle clumping away from the mid-plane. This work presents a range of linear calculations and non-linear simulations, performed with fargo3d, to assess conditions for DSI growth. We expand on previous linear analyses by including particle size distributions and performing a detailed study of the amount of background turbulence needed to stabilize the DSI. When including binned size distributions, the DSI often produces converged growth rates with fewer bins than the standard SI. With background turbulence, we find that the most favourable conditions for DSI growth are weak turbulence, characterized by α ≲ 10−6 with intermediate-sized grains that settle from one gas scale height. These conditions could arise during a sudden decrease in disc turbulence following an accretion outburst. Ignoring background turbulence, we performed a parameter survey of local 2D DSI simulations. Particle clumping was either weak or occurred slower than particles settle. Clumping was reduced by a factor of 2 in a comparison 3D simulation. Overall, our results strongly disfavour the hypothesis that the DSI significantly promotes planetesimal formation. Non-linear simulations of the DSI with different numerical methods could support or challenge these findings.

scholarly journals Gas versus dust sizes of protoplanetary discs: effects of dust evolution

2019 ◽  
Vol 629 ◽  
pp. A79 ◽  
Author(s):  
L. Trapman ◽  
S. Facchini ◽  
M. R. Hogerheijde ◽  
E. F. van Dishoeck ◽  
S. Bruderer
Keyword(s):  
Grain Growth ◽  
Optical Depth ◽  
Outer Radius ◽  
Size Difference ◽  
Clear Sign ◽  
Thermochemical Model ◽  
High Fraction ◽  
Space Dust ◽  
Dust Evolution ◽  
Protoplanetary Discs

Context. The extent of the gas in protoplanetary discs is observed to be universally larger than the extent of the dust. This is often attributed to radial drift and grain growth of the millimetre grains, but line optical depth produces a similar observational signature. Aims. We investigate in which parts of the disc structure parameter space dust evolution and line optical depth are the dominant drivers of the observed gas and dust size difference. Methods. Using the thermochemical model DALI with dust evolution included we ran a grid of models aimed at reproducing the observed gas and dust size dichotomy. Results. The relation between Rdust and dust evolution is non-monotonic and depends on the disc structure. The quantity Rgas is directly related to the radius where the CO column density drops below 1015 cm−2 and CO becomes photodissociated; Rgas is not affected by dust evolution but scales with the total CO content of the disc. While these cases are rare in current observations, Rgas/Rdust > 4 is a clear sign of dust evolution and radial drift in discs. For discs with a smaller Rgas/Rdust, identifying dust evolution from Rgas/Rdust requires modelling the disc structure including the total CO content. To minimize the uncertainties due to observational factors requires FWHMbeam < 1× the characteristic radius and a peak S/N > 10 on the 12CO emission moment zero map. For the dust outer radius to enclose most of the disc mass, it should be defined using a high fraction (90–95%) of the total flux. For the gas, any radius enclosing >60% of the 12CO flux contains most of the disc mass. Conclusions. To distinguish radial drift and grain growth from line optical depth effects based on size ratios requires discs to be observed at high enough angular resolution and the disc structure should to be modelled to account for the total CO content of the disc.

scholarly journals Migrating low-mass planets in inviscid dusty protoplanetary discs

2020 ◽  
Vol 497 (2) ◽  
pp. 2425-2441
Author(s):  
He-Feng Hsieh ◽  
Min-Kai Lin
Keyword(s):  
Central Star ◽  
Small Scale ◽  
Dust Trapping ◽  
Drag Forces ◽  
Planetary Cores ◽  
Planet Migration ◽  
Low Mass ◽  
Future Work ◽  
Gas Drag ◽  
Protoplanetary Discs

ABSTRACT Disc-driven planet migration is integral to the formation of planetary systems. In standard, gas-dominated protoplanetary discs, low-mass planets or planetary cores undergo rapid inwards migration and are lost to the central star. However, several recent studies indicate that the solid component in protoplanetary discs can have a significant dynamical effect on disc–planet interaction, especially when the solid-to-gas mass ratio approaches unity or larger and the dust-on-gas drag forces become significant. As there are several ways to raise the solid abundance in protoplanetary discs, for example through disc winds and dust trapping in pressure bumps, it is important to understand how planets migrate through a dusty environment. To this end, we study planet migration in dust-rich discs via a systematic set of high-resolution, two-dimensional numerical simulations. We show that the inwards migration of low-mass planets can be slowed down by dusty dynamical corotation torques. We also identify a new regime of stochastic migration applicable to discs with dust-to-gas mass ratios of ≳0.3 and particle Stokes numbers ≳0.03. In these cases, disc–planet interaction leads to the continuous development of small-scale, intense dust vortices that scatter the planet, which can potentially halt or even reverse the inwards planet migration. We briefly discuss the observational implications of our results and highlight directions for future work.

scholarly journals Ice Condensation as a Planet Formation Mechanism

2013 ◽  
Vol 8 (S299) ◽  
pp. 382-383
Author(s):  
Katrin Ros
Keyword(s):  
Formation Mechanism ◽  
Time Scale ◽  
Planet Formation ◽  
Particle Growth ◽  
Dynamical Behaviour ◽  
Water Ice ◽  
Ice Particles ◽  
Size Evolution ◽  
Protoplanetary Discs

AbstractParticles in protoplanetary discs grow rapidly to millimetre-sizes via coagulation, but further growth to centimetre-sized pebbles is not yet completely understood. We investigate particle growth by ice condensation in a model where we take the dynamical behaviour of vapour and ice particles into account, as well as the size evolution due to condensation and sublimation. Our results show that efficient growth from dust to pebbles is possible close to the water ice line at ~3 AU, with particles growing from millimetres to decimetres on a time scale of 10000 yr.

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