arXiv subjects

Chris W. Ormel

Publications and source records attributed to Chris W. Ormel.

At least 37 records · Page 2Linked to original sources

arXiv2020

The ARCiS framework for Exoplanet Atmospheres: Modelling Philosophy and Retrieval

Aims: ARCiS, a novel code for the analysis of exoplanet transmission and emission spectra is presented. The aim of the modelling framework is to provide a tool able to link observations to physical models of exoplanet atmospheres. Methods: The modelling philosophy chosen in this paper is to use physical and chemical models to constrain certain parameters while keeping free the parts where our physical understanding is still more limited. This approach, in between full physical modelling and full parameterisation, allows us to use the processes we understand well and parameterise those less understood. A Bayesian retrieval framework is implemented and applied to the transit spectra of a set of 10 hot Jupiters. The code contains chemistry and cloud formation and has the option for self consistent temperature structure computations. Results: The code presented is fast and flexible enough to be used for retrieval and for target list simulations for e.g. JWST or the ESA Ariel missions. We present results for the retrieval of elemental abundance ratios using the physical retrieval framework and compare this to results obtained using a parameterised retrieval setup. Conclusions: We conclude that for most of the targets considered the current dataset is not constraining enough to reliably pin down the elemental abundance ratios. We find no significant correlations between different physical parameters. We confirm that planets in our sample with a strong slope in the optical transmission spectrum are the planets where we find cloud formation to be most active. Finally, we conclude that with ARCiS we have a computationally efficient tool to analyse exoplanet observations in the context of physical and chemical models.

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arXiv2019

The Galilean Satellites Formed Slowly from Pebbles

It is generally accepted that the four major (Galilean) satellites formed out of the gas disk that accompanied Jupiter's formation. However, understanding the specifics of the formation process is challenging as both small particles (pebbles) as well as the satellites are subject to fast migration processes. Here, we hypothesize a new scenario for the origin of the Galilean system, based on the capture of several planetesimal seeds and subsequent slow accretion of pebbles. To halt migration, we invoke an inner disk truncation radius, and other parameters are tuned for the model to match physical, dynamical, compositional, and structural constraints. In our scenario it is natural that Ganymede's mass is determined by pebble isolation. Our slow-pebble-accretion scenario then reproduces the following characteristics: (1) the mass of all the Galilean satellites; (2) the orbits of Io, Europa, and Ganymede captured in mutual 2:1 mean motion resonances; (3) the ice mass fractions of all the Galilean satellites; (4) the unique ice-rock partially differentiated Callisto and the complete differentiation of the other satellites. Our scenario is unique to simultaneously reproduce these disparate properties.

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arXiv2019

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 \texttt{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 ($\sim$50% solids-to-planets conversion efficiency). [...] 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).

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arXiv2019

Growth after the streaming instability: from planetesimal accretion to pebble accretion

Streaming instability is a key mechanism in planet formation, clustering pebbles into planetesimals. 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 by feeding from other planetesimals in the birth ring as well as by accreting inwardly drifting pebbles from the outer disk. To investigate the growth of planetesimals at a single location by the streaming instability, we test the conditions under which super-Earths are able to form within the lifetime of the gaseous disk. We modify the \texttt{Mercury} N-body code to trace the growth and dynamical evolution of a swarm of planetesimals at the ice line for a solar-mass star. Three distributions of planetesimal sizes are investigated: (i) a mono-dispersed population of 400 km radius planetesimals, (ii) a poly-dispersed populations 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. The mono-disperse population of 400 km size planetesimals cannot form $\gtrsim$ Earth mass protoplanets. Their velocity dispersions are quickly excited, which suppresses both planetesimal and pebble accretion. Planets can form from the poly-dispersed and bimodal distributions. In these circumstances, the two-component nature damps the random velocity of the large embryo by small planetesimals' dynamical friction, allowing the embryo to accrete pebbles efficiently when it approaches $10^{-2}$ Earth mass. We find that super-Earth planets are preferred to form when the pebble mass flux is higher, the disk turbulence is lower, or the Stokes number of the pebbles is higher.

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arXiv2019

The ARCiS framework for Exoplanet Atmospheres: The Cloud Transport Model

Understanding of clouds is instrumental in interpreting current and future spectroscopic observations of exoplanets. Modelling clouds consistently is complex, since it involves many facets of chemistry, nucleation theory, condensation physics, coagulation, and particle transport. We develop a simple physical model for cloud formation and transport, efficient and versatile enough that it can be used in modular fashion for parameter optimization searches of exoplanet atmosphere spectra. The transport equations are formulated in 1D, accounting for sedimentation and diffusion. The grain size is obtained through a moment method. For simplicity, only one cloud species is considered and the nucleation rate is parametrized. From the resulting physical profiles we simulate transmission spectra covering the visual to mid-IR wavelength range. We apply our models towards KCl clouds in the atmosphere of GJ1214 b and towards MgSiO3 clouds of a canonical hot-Jupiter. We find that larger cloud diffusivity $K_{zz}$ increases the thickness of the cloud, pushing the $τ=1$ surface to a lower pressure layer higher in the atmosphere. A larger nucleation rate also increases the cloud thickness while it suppresses the grain size. Coagulation is most important at high nuclei injection rates ($\dotΣ_n$) and low $K_{zz}$. We find that the investigated combinations of $K_{zz}$ and $\dotΣ_n$ greatly affect the transmission spectra in terms of the slope at near-IR wavelength (a proxy for grain size), the molecular features seen at ~1\micr (which disappear for thick clouds, high in the atmosphere), and the 10\micr silicate feature, which becomes prominent for small grains high in the atmosphere. The result of our hybrid approach -- aimed to provide a good balance between physical consistency and computational efficiency -- is ideal towards interpreting (future) spectroscopic observations of exoplanets.

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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

Pebble dynamics and accretion onto rocky planets. I. Adiabatic and convective models

We present nested-grid, high-resolution hydrodynamic simulations of gas and particle dynamics in the vicinity of Mars- to Earth-mass planetary embryos. The simulations extend from the surface of the embryos to a few vertical disk scale heights, with \rev{a spatial} dynamic range \rev{of} $\sim\! 1.4\times 10^5$. Our results confirm that "pebble"-sized particles are readily accreted, with accretion rates continuing to increase up to metre-size "boulders" for a 10\% MMSN surface density model. The gas mass flux in and out of the Hill sphere is consistent with the Hill rate, $ΣΩR_\mathrm{H}^2 = 4\, 10^{-3}$ M$_\oplus$ yr$^{-1}$. While smaller size particles mainly track the gas, a net accretion rate of $\approx 2\,10^{-5}$ M$_\oplus$ yr$^{-1}$ is reached for 0.3--1 cm particles, even though a significant fraction leaves the Hill sphere again. Effectively all pebble-sized particles that cross the Bondi sphere are accreted. The resolution of these simulations is sufficient to resolve accretion-driven convection. Convection driven by a nominal accretion rate of $10^{-6}$ M$_\oplus$ yr$^{-1}$ does not significantly alter the pebble accretion rate. We find that, due to cancellation effects, accretion rates of pebble-sized particles are nearly independent of disk surface density. As a result, we can estimate accurate growth times for specified particle sizes. For 0.3--1 cm size particles, the growth time from a small seed is $\sim$0.15 million years for an Earth mass planet at 1 AU and $\sim$0.1 million years for a Mars mass planet at 1.5 AU.

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arXiv2018

Catching drifting pebbles I. Enhanced pebble accretion efficiencies for eccentric planets

Coagulation theory predicts that micron-sized dust grains grow into pebbles which drift inward towards the star, when they reach sizes of mm-cm. When they cross the orbit of a planet, a fraction of these drifting pebbles will be accreted. In the pebble accretion mechanism, the combined effects of the planet's gravitational attraction and gas drag greatly increase the accretion rate. We calculate the pebble accretion efficiency $\varepsilon_{2D}$ -- the probability a pebble is accreted by the planet -- in the 2D limit (pebbles reside in the midplane). In particular, we investigate the dependence of $\varepsilon_{2D}$ on the planet eccentricity and its implications for planet formation models. We conduct N-body simulations to calculate the pebble accretion efficiency in both the local frame and the global frame. With the global method we investigate the pebble accretion efficiency when the planet is on an eccentric orbit. We find that the local and the global methods generally give consistent results. The efficiency increases with the planet's eccentricity once the relative velocity between the pebble and the planet is determined by the planet's eccentric velocity. At large eccentricities, however, the relative velocity becomes too large for pebble accretion. The efficiency then drops significantly and the accretion enters the ballistic regime. We present general expressions for $\varepsilon_{2D}$. Applying the obtained formula to the formation of a secondary planet, in resonance with an already-formed giant planet, we find that the embryo grows quickly due to its larger eccentricity. The maximum $\varepsilon_{2D}$ for a planet on an eccentric orbit is several times higher than for a planet on a circular orbit, but this increase gives the planet an important headstart and boosts its following mass growth.

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arXiv2018

Catching drifting pebbles II. A stochastic equation of motions for pebbles

Turbulence plays a key role in the transport of pebble-sized particles. It also affects the ability of pebbles to be accreted by protoplanets, because it stirs pebbles out of the disk midplane. In addition, turbulence can suppress pebble accretion once the relative velocities become too large for the settling mechanism to be viable. Following Paper I, we aim to quantify these effects by calculating the pebble accretion efficiency $\varepsilon$ using three-body simulations. To model the effect of turbulence on the pebbles, we derive a stochastic equation of motion (SEOM) applicable to stratified disk configurations. In the strong coupling limit (ignoring particle inertia) the limiting form of this equation agrees with previous works. We conduct a parameter study and calculate $\varepsilon$ in 3D, varying pebble and gas (turbulence) properties and accounting for the planet inclination. We find that strong turbulence suppresses pebble accretion through turbulent diffusion, agreeing within factors of order unity with previous works. Another reduction of $\varepsilon$ occurs when the turbulent rms motions are large and the settling mechanism fails. Efficiency-wise, the outer disk regions are more affected by turbulence than the inner regions. At the location of the H$_2$O iceline, planets around low-mass stars achieve much higher efficiencies. Including the results from Paper I, we present a framework to obtain $\varepsilon$ under general circumstances.

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arXiv2017

What pebbles are made of: Interpretation of the V883 Ori disk

Recently, an Atacama Large Millimeter/submillimeter Array (ALMA) observation of the water snow line in the protoplanetary disk around the FU Orionis star V883 Ori was reported. The radial variation of the spectral index at mm-wavelengths around the snow line was interpreted as being due to a pileup of particles interior to the snow line. However, radial transport of solids in the outer disk operates on timescales much longer than the typical timescale of an FU Ori outburst ($10^{1}$--$10^{2}$ yr). Consequently, a steady-state pileup is unlikely. We argue that it is only necessary to consider water evaporation and re-coagulation of silicates to explain the recent ALMA observation of V883 Ori because these processes are short enough to have had their impact since the outburst. Our model requires the inner disk to have already been optically thick before the outburst, and our results suggest that the carbon content of pebbles is low.

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arXiv2017

Dynamical rearrangement of super-Earths during disk dispersal II. Assessment of the magnetospheric rebound model for planet formation scenarios

Context.The Kepler mission has provided a large sample to statistically analyze the orbital properties of the super-Earth planets. We hypothesize that these planets formed early and consider the problem of matching planet formation theory to the current observations. Two scenarios, disk migration and in-situ formation, have been proposed to explain their origin. In the migration scenario planets migrate inward due to planet-disk interaction, whereas in the in-situ scenario planets assemble locally. Therefore, planets formed by migration are expected to end up in resonances, whereas those formed in-situ are expected to stay in short period ratios and in non-resonant orbits. Both predictions are at odds with observations. Aims. We investigate whether a preferred formation scenario can be identified through a comparison between the magnetospheric rebound model and the Kepler data. Methods. We conduct N-body simulations of two-planet systems during the disk dispersal phase, and make a statistical comparison between the simulations and the Kepler observations. Results. Comparing the two scenarios, we find that magnetospheric rebound tends to erase the difference in the orbital configuration that was initially presented. After disk dispersal, not all planets are in resonance in the migration scenario, whereas planets do not remain in compact configurations in the in-situ scenario. In both scenarios, the orbits of planets increase with the cavity expansion, and their period ratios have a wider distribution. Conclusions. From a statistical perspective, the magnetospheric rebound model reproduces several observed properties of Kepler planets, such as the significant number of planets are not in resonances and planet pairs can end up at large period ratios. The disparity in orbital configuration between the two formation scenarios is substantially reduced after disk dispersal.

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arXiv2017

Hydrodynamics of embedded planets' first atmospheres - III. The role of radiation transport for super-Earth planets

The population of close-in super-Earths, with gas mass fractions of up to 10% represents a challenge for planet formation theory: how did they avoid runaway gas accretion and collapsing to hot Jupiters despite their core masses being in the critical range of $M_\mathrm{c} \simeq 10 M_\mathrm{\oplus}$? Previous three-dimensional (3D) hydrodynamical simulations indicate that atmospheres of low-mass planets cannot be considered isolated from the protoplanetary disc, contrary to what is assumed in 1D-evolutionary calculations. This finding is referred to as the recycling hypothesis. In this Paper we investigate the recycling hypothesis for super-Earth planets, accounting for realistic 3D radiation hydrodynamics. Also, we conduct a direct comparison in terms of the evolution of the entropy between 1D and 3D geometries. We clearly see that 3D atmospheres maintain higher entropy: although gas in the atmosphere loses entropy through radiative cooling, the advection of high entropy gas from the disc into the Bondi/Hill sphere slows down Kelvin-Helmholtz contraction, potentially arresting envelope growth at a sub-critical gas mass fraction. Recycling, therefore, operates vigorously, in line with results by previous studies. However, we also identify an "inner core" -- in size $\approx$ 25% of the Bondi radius -- where streamlines are more circular and entropies are much lower than in the outer atmosphere. Future studies at higher resolutions are needed to assess whether this region can become hydrodynamically-isolated on long time-scales.

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arXiv2017

Planetesimal formation near the snowline: in or out?

The formation of planetesimals in protoplanetary disks is not well-understood. Streaming instability is a promising mechanism to directly form planetesimals from pebble-sized particles, provided a high enough solids-to-gas ratio. However, local enhancements of the solids-to-gas ratio are difficult to realize in a smooth disk, which motivates the consideration of special disk locations such as the snowline. In this article we investigate the viability of planetesimal formation by streaming instability near the snowline due to water diffusion and condensation. We adopt a viscous disk model, and numerically solve the transport equations for vapor and solids on a cylindrical, 1D grid. We take into account radial drift of solids, gas accretion on to the central star, and turbulent diffusion. We study the importance of the back-reaction of solids on the gas and of the radial variation of the mean molecular weight of the gas. We find that water diffusion and condensation can locally enhance the ice surface density by a factor 3--5 outside the snowline. Assuming that icy pebbles contain many micron-sized silicate grains that are released during evaporation, the enhancement is increased by another factor $\sim$$2$. In this `many-seeds' model, the solids-to-gas ratio interior to the snowline is enhanced as well, but not as much as just outside the snowline. In the context of a viscous disk, the diffusion-condensation mechanism is most effective for high values of the turbulence parameter $α$ ($10^{-3}$--$10^{-2}$). Therefore, assuming young disks are more vigorously turbulent than older disks, planetesimals near the snowline can form in an early stage of the disk. In highly turbulent disks, tens of Earth masses can be stored in an annulus outside the snowline, which can be identified with recent ALMA observations.

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arXiv2017

Dynamical rearrangement of super-Earths during disk dispersal I. Outline of the magnetospheric rebound model

The Kepler mission has discovered that multiple close-in super-Earth planets are common around solar-type stars, but their period ratios do not show strong pile-ups near mean motion resonances (MMRs). One scenario is that super-Earths form in a gas-rich disk, and they interact gravitationally with the surrounding gas, inducing their orbital migration. Disk migration theory predicts, however, that planets would end up at resonant orbits due to their differential migration speed. Motivated by the discrepancy between observation and theory, we seek for a mechanism that moves planets out of resonances. We examine the orbital evolution of planet pairs near the magnetospheric cavity during the gas disk dispersal phase. Our study determines the conditions under which planets can escape resonances. We perform two-planet N-body simulations, varying the planet masses, stellar magnetic field strengths, disk accretion rates and gas disk depletion timescales. As planets migrate outward with the expanding magnetospheric cavity, their dynamical configurations can be rearranged. Migration of planets is substantial (minor) in a massive (light) disk. When the outer planet is more massive than the inner planet, the period ratio of two planets increases through outward migration. On the other hand, when the inner planet is more massive, the final period ratio tends to remain similar to the initial one. Larger stellar magnetic field strengths result in planets stopping their migration at longer periods. We highlight \textit{magnetospheric rebound} as an important ingredient able to reconcile disk migration theory with observations. Even when planets are trapped into MMR during the early gas-rich stage, subsequent cavity expansion would induce substantial changes to their orbits, moving them out of resonance.

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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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arXiv2015

On the growth of pebble-accreting planetesimals

Pebble accretion is a new mechanism to quickly grow the cores of planets. In pebble accretion, gravity and gas drag conspire to yield large collisional cross sections for small particles in protoplanetary disks. However, before pebble accretion commences, aerodynamical deflection may act to prevent planetesimals from becoming large, because particles tend to follow gas streamlines. We derive the planetesimal radius where pebble accretion is initiated and determine the growth timescales of planetesimals by sweepup of small particles. We obtain the collision efficiency factor as the ratio of the numerically-obtained collisional cross section to the planetesimal surface area, from which we obtain the growth timescales. Integrations are conducted in the potential flow limit (steady, inviscid) and in the Stokes flow regime (steady, viscid). Only particles of stopping time $t_s \ll t_X$ where $t_X\approx10^3$ s experience aerodynamic deflection. Even in that case, the planetesimal's gravity always ensures positive collision factors. The maximum growth timescale occurs typically at around $R\approx100 \ \mathrm{km}$, but is less for colder disks, corresponding to interactions shifting to the Safronov focusing regime. For particles $t_s \gg t_X$ pebble accretion commences only after this phase and is characterized by a steep drop in growth timescales. Consequently, at distances beyond ~10 AU sweepup growth timescales are always longer than $10$ Myr, while in the inner disk (~<3 AU) the viability of the sweepup scenario is determined by the outcome of pebble-planetesimal collisions in the geometric regime. We present analytical fits for the collision efficiency factors and the minimum planetesimal size needed for pebble accretion. (Abridged)

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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

Make Super-Earths, Not Jupiters: Accreting Nebular Gas onto Solid Cores at 0.1 AU and Beyond

Close-in super-Earths having radii 1--4 $R_\oplus$ may possess hydrogen atmospheres comprising a few percent by mass of their rocky cores. We determine the conditions under which such atmospheres can be accreted by cores from their parent circumstellar disks. Accretion from the nebula is problematic because it is too efficient: we find that 10-$M_\oplus$ cores embedded in solar metallicity disks tend to undergo runaway gas accretion and explode into Jupiters, irrespective of orbital location. The threat of runaway is especially dire at $\sim$0.1 AU, where solids may coagulate on timescales orders of magnitude shorter than gas clearing times; thus nascent atmospheres on close-in orbits are unlikely to be supported against collapse by planetesimal accretion. The time to runaway accretion is well approximated by the cooling time of the atmosphere's innermost convective zone, whose extent is controlled by where H$_2$ dissociates. Insofar as the temperatures characterizing H$_2$ dissociation are universal, timescales for core instability tend not to vary with orbital distance --- and to be alarmingly short for 10-$M_\oplus$ cores. Nevertheless, in the thicket of parameter space, we identify two scenarios, not mutually exclusive, that can reproduce the preponderance of percent-by-mass atmospheres for super-Earths at $\sim$0.1 AU, while still ensuring the formation of Jupiters at $\gtrsim 1$ AU. Scenario (a): planets form in disks with dust-to-gas ratios that range from $\sim$20$\times$ solar at 0.1 AU to $\sim$2$\times$ solar at 5 AU. Scenario (b): the final assembly of super-Earth cores from mergers of proto-cores --- a process that completes quickly at $\sim$0.1 AU once begun --- is delayed by gas dynamical friction until just before disk gas dissipates completely.

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