arXiv subjects

Sebastiaan Krijt

Publications and source records attributed to Sebastiaan Krijt.

At least 55 records · Page 3Linked to original sources

arXiv2021

System-level fractionation of carbon from disk and planetesimal processing

Finding and characterizing extrasolar Earth analogs will rely on interpretation of the planetary system's environmental context. The total budget and fractionation between C-H-O species sensitively affect the climatic and geodynamic state of terrestrial worlds, but their main delivery channels are poorly constrained. We connect numerical models of volatile chemistry and pebble coagulation in the circumstellar disk with the internal compositional evolution of planetesimals during the primary accretion phase. Our simulations demonstrate that disk chemistry and degassing from planetesimals operate on comparable timescales and can fractionate the relative abundances of major water and carbon carriers by orders of magnitude. As a result, individual planetary systems with significant planetesimal processing display increased correlation in the volatile budget of planetary building blocks relative to no internal heating. Planetesimal processing in a subset of systems increases the variance of volatile contents across planetary systems. Our simulations thus suggest that exoplanetary atmospheric compositions may provide constraints on $when$ a specific planet formed.

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arXiv2021

On the stickiness of CO$_{2}$ and H$_{2}$O ice particles

Laboratory experiments revealed that CO$_{2}$ ice particles stick less efficiently than H$_{2}$O ice particles, and there is an order of magnitude difference in the threshold velocity for sticking. However, the surface energies and elastic moduli of CO$_{2}$ and H$_{2}$O ices are comparable, and the reason why CO$_{2}$ ice particles were poorly sticky compared to H$_{2}$O ice particles was unclear. Here we investigate the effects of viscoelastic dissipation on the threshold velocity for sticking of ice particles using the viscoelastic contact model derived by Krijt et al. We find that the threshold velocity for sticking of CO$_{2}$ ice particles reported in experimental studies is comparable to that predicted for perfectly elastic spheres. In contrast, the threshold velocity for sticking of H$_{2}$O ice particles is an order of magnitude higher than that predicted for perfectly elastic spheres. Therefore, we conclude that the large difference in stickiness between CO$_{2}$ and H$_{2}$O ice particles would mainly originate from the difference in the strength of viscoelastic dissipation, which is controlled by the viscoelastic relaxation time.

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arXiv2020

Hints for icy pebble migration feeding an oxygen-rich chemistry in the inner planet-forming region of disks

We present a synergic study of protoplanetary disks to investigate links between inner disk gas molecules and the large-scale migration of solid pebbles. The sample includes 63 disks where two types of measurements are available: i) spatially-resolved disk images revealing the radial distribution of disk pebbles (mm-cm dust grains), from millimeter observations with ALMA or the SMA, and ii) infrared molecular emission spectra as observed with Spitzer. The line flux ratios of H2O with HCN, C2H2, and CO2 all anti-correlate with the dust disk radius R$_{dust}$, expanding previous results found by Najita et al. (2013) for HCN/H2O and the dust disk mass. By normalization with the dependence on accretion luminosity common to all molecules, only the H2O luminosity maintains a detectable anti-correlation with disk radius, suggesting that the strongest underlying relation is between H2O and R$_{dust}$. If R$_{dust}$ is set by large-scale pebble drift, and if molecular luminosities trace the elemental budgets of inner disk warm gas, these results can be naturally explained with scenarios where the inner disk chemistry is fed by sublimation of oxygen-rich icy pebbles migrating inward from the outer disk. Anti-correlations are also detected between all molecular luminosities and the infrared index n$_{13-30}$, which is sensitive to the presence and size of an inner disk dust cavity. Overall, these relations suggest a physical interconnection between dust and gas evolution both locally and across disk scales. We discuss fundamental predictions to test this interpretation and study the interplay between pebble drift, inner disk depletion, and the chemistry of planet-forming material.

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arXiv2020

CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing

The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks decreases by up to 2 orders of magnitude from its ISM value ${\sim}10^{-4}$, even after accounting for freeze-out and photo-dissociation. Previous studies have shown that while local chemical processing of CO and the sequestration of CO ice on solids in the midplane can both contribute, neither of these processes appears capable of consistently reaching the observed depletion factors on the relevant timescale of $1{-}3\mathrm{~Myr}$. In this study, we model these processes simultaneously by including a compact chemical network (centered on carbon and oxygen) to 2D ($r+z$) simulations of the outer ($r>20\mathrm{~au}$) disk regions that include turbulent diffusion, pebble formation, and pebble dynamics. In general, we find that the CO/H$_2$ abundance is a complex function of time and location. Focusing on CO in the warm molecular layer, we find that only the most complete model (with chemistry and pebble evolution included) can reach depletion factors consistent with observations. In the absence of pressure traps, highly-efficient planetesimal formation, or high cosmic ray ionization rates, this model also predicts a resurgence of CO vapor interior to the CO snowline. We show the impact of physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally, connect our predicted pebble ice compositions to those of pristine planetesimals as found in the Cold Classical Kuiper Belt and debris disks.

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arXiv2019

Tracking Dust Grains During Transport and Growth in Protoplanetary Disks

Protoplanetary disks are dynamic objects, within which dust grains and gas are expected to be redistributed over large distances. Evidence for this redistribution is seen both in other protoplanetary disks and in our own Solar System, with high-temperature materials thought to originate close to the central star found in the cold, outer regions of the disks. While models have shown this redistribution is possible through a variety of mechanisms, these models have generally ignored the possible growth of solids via grain-grain collisions that would occur during transit. Here we investigate the interplay of coagulation and radial and vertical transport of solids in protoplanetary disks, considering cases where growth is limited by bouncing or by fragmentation. We find that in all cases, growth effectively limits the ability for materials to be carried outward or preserved at large distances from the star. This is due to solids being incorporated into large aggregates which drift inwards rapidly under the effects of gas drag. We discuss the implications for mixing in protoplanetary disks, and how the preservation of high temperature materials in outer disks may require structures or outward flow patterns to avoid them being lost via radial drift.

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arXiv2019

Systematic Variations of CO Gas Abundance with Radius in Gas-rich Protoplanetary Disks

CO is the most widely used gas tracer of protoplanetary disks. Its abundance is usually assumed to be an interstellar ratio throughout the warm molecular layer of the disk. But recent observations of low CO gas abundance in many protoplanetary disks challenge our understanding of physical and chemical evolutions in disks. Here we investigate the CO abundance structures in four well-studied disks and compare their structures with predictions of chemical processing of CO and transport of CO ice-coated dust grains in disks. We use spatially resolved CO isotopologue line observations and detailed thermo-chemical models to derive CO abundance structures. We find that the CO abundance varies with radius by an order of magnitude in these disks. We show that although chemical processes can efficiently reduce the total column of CO gas within 1 Myr under an ISM level of cosmic-ray ionization rate, the depletion mostly occurs at the deep region of a disk. Without sufficient vertical mixing, the surface layer is not depleted enough to reproduce weak CO emissions observed. The radial profiles of CO depletion in three disks are qualitatively consistent with predictions of pebble formation, settling, and drifting in disks. But the dust evolution alone cannot fully explain the high depletion observed in some disks. These results suggest that dust evolution may play a significant role in transporting volatile materials and a coupled chemical-dynamical study is necessary to understand what raw materials are available for planet formation at different distances from the central star.

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arXiv2019

Dust Transport and Processing in Centrifugally Driven Protoplanetary Disk Winds

There is evidence that protoplanetary disks--including the protosolar one--contain crystalline dust grains on spatial scales where the dust temperature is lower than the threshold value for their formation through thermal annealing of amorphous interstellar silicates. We interpret these observations in terms of an extended, magnetocentrifugally driven disk wind that transports grains from the inner disk--where they are thermally processed by the stellar radiation after being uplifted from the disk surfaces--to the outer disk regions. For any disk radius $r$ there is a maximum grain size $a_\mathrm{max}(r)$ that can be uplifted from that location: grains of size $a$$\ll$$a_\mathrm{max}$ are carried away by the wind, whereas those with $a$$\lesssim$$a_\mathrm{max}$ reenter the disk at larger radii. A significant portion of the reentering grains converge to--and subsequently accumulate in--a narrow region just beyond $r_\mathrm{max}(a)$, the maximum radius from which grains of size $a$ can be uplifted. We show that this model can account for the inferred crystallinity fractions in classical T Tauri and Herbig Ae disks and for their indicated near constancy after being established early in the disk evolution. It is also consistent with the reported radial gradients in the mean grain size, crystallinity, and crystal composition. In addition, this model yields the properties of the grains that remain embedded in the outflows from protoplanetary disks and naturally explains the inferred persistence of small grains in the surface layers of these disks.

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arXiv2019

Planet formation: The case for large efforts on the computational side

Modern astronomy has finally been able to observe protoplanetary disks in reasonable resolution and detail, unveiling the processes happening during planet formation. These observed processes are understood under the framework of disk-planet interaction, a process studied analytically and modeled numerically for over 40 years. Long a theoreticians' game, the wealth of observational data has been allowing for increasingly stringent tests of the theoretical models. Modeling efforts are crucial to support the interpretation of direct imaging analyses, not just for potential detections but also to put meaningful upper limits on mass accretion rates and other physical quantities in current and future large-scale surveys. This white paper addresses the questions of what efforts on the computational side are required in the next decade to advance our theoretical understanding, explain the observational data, and guide new observations. We identified the nature of accretion, ab initio planet formation, early evolution, and circumplanetary disks as major fields of interest in computational planet formation. We recommend that modelers relax the approximations of alpha-viscosity and isothermal equations of state, on the grounds that these models use flawed assumptions, even if they give good visual qualitative agreement with observations. We similarly recommend that population synthesis move away from 1D hydrodynamics. The computational resources to reach these goals should be developed during the next decade, through improvements in algorithms and the hardware for hybrid CPU/GPU clusters. Coupled with high angular resolution and great line sensitivity in ground based interferometers, ELTs and JWST, these advances in computational efforts should allow for large strides in the field in the next decade.

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arXiv2018

The Efficiency of Noble Gas Trapping in Astrophysical Environments

Amorphous ice has long been invoked as a means for trapping extreme volatiles into solids, explaining the abundances of these species in comets and planetary atmospheres. Experiments have shown that such trapping is possible and have been used to estimate the abundances of each species in primitive ices after they formed. However, these experiments have been carried out at deposition rates which exceed those expected in a molecular cloud or solar nebula by many orders of magnitude. Here we develop a numerical model which reproduces the experimental results and apply it to those conditions expected in molecular clouds and protoplanetary disks. We find that two regimes of ice trapping exist: `burial trapping' where the ratio of trapped species to water in the ice reflects that same ratio in the gas and `equilibrium trapping' where the ratio in the ice depends only on the partial pressure of the trapped species in the gas. The boundary between these two regimes is set by both the temperature and rate of ice deposition. Such effects must be accounted for when determining the source of trapped volatiles during planet formation.

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arXiv2018

A Lagrangian Model for Dust Evolution in Protoplanetary Disks: Formation of Wet and Dry Planetesimals at Different Stellar Masses

We introduce a new Lagrangian smooth-particle method to model the growth and drift of pebbles in protoplanetary disks. The Lagrangian nature of the model makes it especially suited to follow characteristics of individual (groups of) particles, such as their composition. In this work we focus on the water content of solid particles. Planetesimal formation via streaming instability is taken into account, partly based on previous results on streaming instability outside the water snowline that were presented in Schoonenberg & Ormel (2017). We validate our model by reproducing earlier results from the literature and apply our model to steady-state viscous gas disks (with constant gas accretion rate) around stars with different masses. We also present various other models where we explore the effects of pebble accretion, the fragmentation velocity threshold, the global metallicity of the disk, and a time-dependent gas accretion rate. We find that planetesimals preferentially form in a local annulus outside the water snowline, at early times in the lifetime of the disk ($\lesssim$$10^{5} \: \rm{yr}$), when the pebble mass fluxes are high enough to trigger the streaming instability. During this first phase in the planet formation process, the snowline location hardly changes due to slow viscous evolution, and we conclude that assuming a constant gas accretion rate is justified in this first stage. The efficiency of converting the solids reservoir of the disk to planetesimals depends on the location of the water snowline. Cooler disks with a closer-in water snowline are more efficient at producing planetesimals than hotter disks where the water snowline is located further away from the star. Therefore, low-mass stars tend to form planetesimals more efficiently, but any correlation may be overshadowed by the spread in disk properties.

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arXiv2018

Transport of CO in Protoplanetary Disks: Consequences of Pebble Formation, Settling, and Radial Drift

Current models of (exo)planet formation often rely on a large influx of so-called `pebbles' from the outer disk into the planet formation region. In this paper, we investigate how the formation of pebbles in the cold outer regions of protoplanetary disks and their subsequent migration to the inner disk can alter the gas-phase CO distribution both interior and exterior to the midplane CO snowline. By simulating the resulting CO abundances in the midplane as well as the warm surface layer, we identify observable signatures of large-scale pebble formation and migration that can be used as `smoking guns' for these important processes. Specifically, we find that after $1\mathrm{~Myr}$, the formation and settling of icy pebbles results in the removal of up to $80\%$ of the CO vapor in the warm ($T>22\mathrm{~K}$) disk layers outside the CO snowline, while the radial migration of pebbles results in the generation of a plume of CO vapor interior the snowline, increasing the CO abundance by a factor ${\sim}2{-}6$ depending on the strength of the turbulence and the sizes of the individual pebbles. The absence of this plume of CO vapor in young nearby disks could indicate efficient conversion of CO into a more refractory species, or a reduction in the radial mass flux of pebbles by, for example, disk inhomogeneities or early planetesimal formation.

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arXiv2018

The physics of protoplanetesimal dust agglomerates. X. High-velocity collisions between small and large dust agglomerates as growth barrier

In a protoplanetary disk, dust aggregates in the $μ$m to mm size range possess mean collision velocities of 10 to 60 ms$^{-1}$ with respect to dm- to m-size bodies. We performed laboratory collision experiments to explore this parameter regime and found a size- and velocity-dependent threshold between erosion and growth. By using a local Monte Carlo coagulation calculation and complementary a simple semi-analytical timescale approach, we show that erosion considerably limits particle growth in protoplanetary disks and leads to a steady-state dust-size distribution from $μ$m to dm sized particles.

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arXiv2017

Fast litho-panspermia in the habitable zone of the TRAPPIST-1 system

With several short-period, Earth-mass planets in the habitable zone, the TRAPPIST-1 system potentially allows litho-panspermia to take place on very short timescales. We investigate the efficiency and speed of inter-planetary material transfer resulting from impacts onto the habitable zone planets. By simulating trajectories of impact ejecta from their moment of ejection until (re-)accretion, we find that transport between the habitable zone planets is fastest for ejection velocities around and just above planetary escape velocity. At these ejection velocities, ${\sim}10\%$ of the ejected material reaches another habitable zone planet within $10^2\mathrm{~yr}$, indicating litho-panspermia can be 4 to 5 orders of magnitude faster in TRAPPIST-1 than in the Solar System.

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arXiv2016

Tracing water vapor and ice during dust growth

The processes that govern the evolution of dust and water (in the form of vapor or ice) in protoplanetary disks are intimately connected. We have developed a model that simulates dust coagulation, dust dynamics (settling, turbulent mixing), vapor diffusion, and condensation/sublimation of volatiles onto grains in a vertical column of a protoplanetary disk. We employ the model to study how dust growth and dynamics influence the vertical distribution of water vapor and water ice in the region just outside the radial snowline. Our main finding is that coagulation (boosted by the enhanced stickiness of icy grains) and the ensuing vertical settling of solids results in water vapor being depleted, but not totally removed, from the region above the snowline on a timescale commensurate with the vertical turbulent mixing timescale. Depending on the strength of the turbulence and the temperature, the depletion can reach factors of up to ${\sim}50$ in the disk atmosphere. In our isothermal column, this vapor depletion results in the vertical snowline moving closer to the midplane (by up to 2 gas scale heights) and the gas-phase C/O ratio above the vertical snowline increasing. Our findings illustrate the importance of dynamical effects and the need for understanding coevolutionary dynamics of gas and solids in planet-forming environments.

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arXiv2016

Dust diffusion and settling in the presence of collisions: Trapping (sub)micron grains in the midplane

In protoplanetary disks, the distribution and abundance of small (sub)micron grains are important for a range of physical and chemical processes. For example, they dominate the optical depth at short wavelengths and their surfaces are the sites of many important chemical reactions such as the formation of water. Based on their aerodynamical properties (i.e., their strong dynamical coupling with the surrounding gas) it is often assumed that these small grains are well-mixed with the gas. Our goal is to study the vertical (re)distribution of grains taking into account settling, turbulent diffusion, as well as collisions with other dust grains. Assuming a fragmentation-limited background dust population, we developed a Monte Carlo approach that follows single monomers as they move through a vertical column of gas and become incorporated in different aggregates as they undergo sticking and fragmenting collisions. We find that (sub)micron grains are not necessarily well-mixed vertically, but can become trapped in a thin layer with a scale-height that is significantly smaller than that of the gas. This collisional trapping occurs when the timescale for diffusion is comparable to or longer than the collision timescale in the midplane and its effect is strongest when the most massive particles in the size-distribution show significant settling. Based on simulations and analytical considerations we conclude that for typical dust-to-gas ratios and turbulence levels, the collisional trapping of small grains should be a relatively common phenomenon. The absence of trapping could then indicate a low dust-to-gas ratio, possibly because a large portion of the dust mass has been removed through radial drift or is locked up in planetesimals.

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arXiv2015

A panoptic model for planetesimal formation and pebble delivery

The journey from dust particle to planetesimal involves physical processes acting on scales ranging from micrometers (the sticking and restructuring of aggregates) to hundreds of astronomical units (the size of the turbulent protoplanetary nebula). Considering these processes simultaneously is essential when studying planetesimal formation. We develop a novel, global, semi-analytical model for the evolution of the mass-dominating dust particles in a turbulent protoplanetary disk that takes into account the evolution of the dust surface density while preserving the essential characteristics of the porous coagulation process. This panoptic model is used to study the growth from submicron to planetesimal sizes in disks around Sun-like stars. For highly porous ices, unaffected by collisional fragmentation and erosion, rapid growth to planetesimal sizes is possible in a zone stretching out to ${\sim}10\mathrm{~AU}$ for massive disks. When porous coagulation is limited by erosive collisions, the formation of planetesimals through direct coagulation is not possible, but the creation of a large population of aggregates with Stokes numbers close to unity might trigger the streaming instability (SI). However, we find that reaching conditions necessary for SI is difficult and limited to dust-rich disks, (very) cold disks, or disks with weak turbulence. Behind the snow-line, porosity-driven aggregation of icy grains results in rapid (${\sim}10^{4}\mathrm{~yr}$) formation of planetesimals. If erosive collisions prevent this, SI might be triggered for specific disk conditions. The numerical approach introduced in this work is ideally suited for studying planetesimal formation and pebble delivery simultaneously and will help build a coherent picture of the start of the planet formation process.

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arXiv2014

Erosion and the limits to planetesimal growth

The coagulation of microscopic dust into planetesimals is the first step towards planet formation. The size and shape of the growing aggregates determine the efficiency of this early growth. It has been proposed that fluffy ice aggregates can grow very efficiently, suffering less from the bouncing and radial drift barriers. While the collision velocity between icy aggregates of similar size is thought to stay below the fragmentation threshold, they may nonetheless lose mass from collisions with much smaller projectiles. We investigate the effect of these erosive collisions on the ability of porous ice aggregates to cross the radial drift barrier. We develop a Monte Carlo code that calculates the evolution of the growing aggregates, while resolving the entire mass distribution at all times. The aggregate's porosity is treated independently of its mass, and is determined by collisions, gas compaction, and eventually self-gravity compaction. For erosion threshold velocities of 20-40 m/s, high-velocity collisions with small projectiles prevent the largest aggregates from growing when they start to drift. In these cases, our local simulations result in a steady-state distribution, with the majority of the dust mass in particles with Stokes numbers close to unity. Only for the highest erosion threshold considered (60 m/s), do porous aggregates manage to cross the radial drift barrier in the inner 10 AU of MMSN-like disks. Erosive collisions are more effective in limiting the growth than fragmentary collisions between similar-size particles. Conceivably, erosion limits the growth before the radial drift barrier, although the robustness of this statement depends on (uncertain) material properties of icy aggregates. If erosion inhibits planetesimal formation through direct sticking, the sea of ${\sim}10^9$ g, highly porous particles appears well-suited for triggering streaming instability.

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arXiv2014

A dearth of small particles in debris disks: An energy-constrained smallest fragment size

A prescription for the fragment size distribution resulting from dust grain collisions is essential when modelling a range of astrophysical systems, such as debris disks and planetary rings. While the slope of the fragment size distribution and the size of the largest fragment are well known, the behaviour of the distribution at the small size end is theoretically and experimentally poorly understood. This leads debris disk codes to generally assume a limit equal to, or below, the radiation blow-out size. We use energy conservation to analytically derive a lower boundary of the fragment size distribution for a range of collider mass ratios. Focussing on collisions between equal-sized bodies, we apply the method to debris disks. For a given collider mass, the size of the smallest fragments is found to depend on collision velocity, material parameters, and the size of the largest fragment. We provide a physically motivated recipe for the calculation of the smallest fragment, which can be easily implemented in codes for modelling collisional systems. For plausible parameters, our results are consistent with the observed predominance of grains much larger than the blow-out size in Fomalhaut's main belt and in the Herschel cold debris disks.

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