arXiv subjects

Jacob B. Simon

Publications and source records attributed to Jacob B. Simon.

At least 19 recordsLinked to original sources

arXiv2026

Azimuthal Dust Polarization from Aerodynamically Aligned Grains as Evidence for the Streaming Instability in Protoplanetary Disks

(Sub)millimeter dust polarization in protoplanetary disks has revealed the presence of large (~ 100 um) dust grains that are aligned along their long axis following the azimuthal direction of the disk. The novel badminton birdie-like aerodynamic alignment predicts large grains to align with their long axes following the direction of gas flow experienced by the dust, denoted as the A-field. With 3D streaming instability (SI) simulations, we find that the A-field is predominantly in the radial direction in regions of low dust-to-gas ratio, but in the azimuthal direction in regions of high dust-to-gas ratio. Through polarized radiation transfer, we find that the resulting polarization angle indeed follows the disk azimuthal direction in the high dust density regions. Therefore, ongoing SI is an attractive candidate for producing the azimuthal dust polarization pattern as observed in an increasing number of disks.

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arXiv2026

A Comparative Study of the Streaming Instability: Unstratified Models with Marginally Coupled Grains

The streaming instability is a leading mechanism for concentrating solids and initiating planetesimal formation in protoplanetary disks. Although numerous studies have explored its linear growth, nonlinear evolution, and implications for planet formation, the diversity of numerical methods and dust treatments used across the literature has made it difficult to assess which features of the instability are physically robust and which arise from code-dependent choices. We present the first systematic comparison of seven hydrodynamic codes--spanning finite-volume and finite-difference schemes and modeling dust either as Lagrangian particles or as a pressureless fluid--applied to the unstratified streaming instability with a dimensionless stopping time of unity. All codes reproduce the characteristic sequence of exponential growth, filament formation, and turbulent saturation, demonstrating broad agreement in the qualitative behavior of the instability. Quantitatively, however, the dust model remains the dominant source of variation at moderate resolution: particle-based simulations reach higher peak densities and exhibit broader high-density tails than fluid-based models at $512^2$ resolution, although increasing the number of particles brings their initial maximum density evolution into close agreement with that of dust-fluid models. At $1024^2$, these differences diminish substantially, indicating better agreement of the saturated-state statistics across dust treatments. In terms of computational performance, most particle implementations suffer from imbalanced parallelized loads, while execution on a GPU is at least two to three times more energy efficient and scales better at higher resolutions than on CPUs. Given the intrinsic stochasticity of this nonlinear system, only statistical diagnostics remain meaningful across codes.

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arXiv2026

Turbulence Can Persist in the Inner Regions of Weakly-Ionized Planet Forming Disks

Identifying the mechanisms responsible for angular momentum transport in protoplanetary disks, and the extent to which those mechanisms produce turbulence, is a crucial problem in understanding planet formation. The bulk of the gas in protoplanetary disks is weakly ionized, which leads to the emergence of three non-ideal effects, Ohmic diffusion, ambipolar diffusion, and the Hall effect. These low-ionization processes can in some cases suppress turbulence driven by the magnetorotational instability (MRI). However, it has recently been shown that these non-ideal terms can also affect the dynamics of the gas in fundamentally different ways than simple diffusion. In order to further study the role of low-ionization on disk gas dynamics, we carry out a 3D local shearing box simulation with both Ohmic diffusion and ambipolar diffusion and an additional simulation with the Hall effect included. The strength of each non-ideal term, when present, is representative of gas at a radius of 5 AU in a realistic protoplanetary disk. We find the Hall effect increases the saturation strength of the magnetic field, but does not necessarily drive turbulence, consistent with previous work. However, interactions between ambipolar diffusion and the Keplerian shear lead to the ambipolar diffusion shear instability (ADSI), which can drive the initial growth, not damping, of magnetic perturbations. To our knowledge, this is the first work that explicitly demonstrates the viability of the ADSI in the non-linear regime within protoplanetary disks. At later times in the disk, the MRI (reduced in strength by ambipolar-diffusion), may also be present in regions of weak magnetic field between strong concentrations of vertical magnetic flux and sustain turbulence locally in protoplanetary disks.

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arXiv2025

The Streaming Instability in 3D: Conditions for Strong Clumping

The streaming instability (SI) is a leading mechanism for planetesimal formation, driving the aerodynamic concentration of solids in protoplanetary disks. The SI triggers strong clumping (i.e., strong enough for clumps to collapse) when the solid-to-gas column density ratio, $Z$, exceeds a threshold, $\Zcrit$. This threshold depends on the dimensionless stopping time, $\tau_s$. Although the strong-clumping threshold has been explored over the last decade, it has been determined largely through 2D axisymmetric simulations. In this work, we perform a suite of 3D, vertically stratified simulations to establish a clumping threshold across $10^{-3} \leq \tau_s \leq 1.0$. Additionally, we study SI-driven concentration that is unique to 3D. We find that $\Zcrit$ is as low as $\approx 0.002$ at $\tau_s=0.1$ and exceeds $\approx 0.03$ at $\tau_s=10^{-3}$. Compared to 2D, our 3D results yield lower $\Zcrit$ for $\tau_s > 0.02$, but higher for $\tau_s \leq 0.02$, with a sharp transition between $\tau_s = 0.02$ and 0.03. This transition correlates with midplane density ratio ($\epsilon$): $\epsilon < 1$ where 3D gives lower thresholds, and $\epsilon > 1$ where 3D gives higher thresholds. We also find a filaments-in-filaments structure when $\epsilon < 1$, which enhances clumping compared to 2D. By contrast, when $\epsilon > 1$ and $\tau_s \leq 0.03$, dust filaments in 3D do not drift inward, suppressing filament mergers and strong clumping. In 2D, filaments drift inward regardless of $\epsilon$, triggering strong clumping easier in this regime. Our results underscore the necessity of 3D simulations for accurately capturing SI-driven concentration and building the strong-clumping threshold.

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arXiv2025

Magnetic Pressure Dominance Stabilizes AGN Disks Against Gravitational Instability

Magnetic effects have long been considered a possible factor in stabilizing the outer regions of active galactic nuclei (AGN) accretion disks against gravitational instability (GI). However, the computational demands of testing this hypothesis have prevented comprehensive study of this problem. Here, we present results from a suite of 6 isothermal magnetohydrodynamics (MHD) shearing box simulations, 3 initialized with strong magnetization ($\beta^{\rm{mid}}_{0} = p_{\rm{gas}} / p_{\rm{mag}} = 10^{2.5}$) and 3 initialized with weak magnetization ($\beta^{\rm mid}_{0} = 10^{4}$). For each magnetization, we performed simulations with both strong ($Q_{0} = 1.0$) and weak ($Q_{0} = 10.0$) self-gravity, where $Q_{0} = \frac{c_{\rm{s}}\Omega}{\pi G \Sigma_{0}}$ is the Toomre stability parameter; we also performed pure MHD simulations for comparison. We find that our strongly magnetized disk stabilized against GI after initialization to critical stability against GI, while our corresponding weakly magnetized disk did not. We show that the strongly magnetized, strongly self-gravitating disk became dominated by magnetic pressure, which led to its stabilization.

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arXiv2025

On the Mass Budget Problem of Protoplanetary Disks: Streaming Instability and Optically Thick Emission

Statistical studies of protoplanetary disks and exoplanet populations often exhibit a "missing mass" problem, where observed dust masses in (sub-)millimeter surveys are significantly lower than expected when compared to the mass of evolved exoplanetary systems. We investigate how the streaming instability and subsequent planetesimal formation in protoplanetary disks might solve this missing mass problem when (sub-)millimeter observations are interpreted under the assumption of optically thin emission. We conduct hydrodynamical simulations of the streaming instability with self-gravity after which radiative transfer calculations with dust scattering are performed to measure the (sub-)millimeter intensity. The measured intensity is then used to estimate the disk mass under the assumption of optically thin emission and compared to the true mass in the simulation to calculate the observational bias via the mass excess. We find that the emission from overdense filaments that emerge due to the streaming instability are optically thick at (sub-)millimeter wavelengths, leading to mass excess factors of $\sim 2-7$, even when the optically thick fraction is low.

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arXiv2025

On Linking Planet Formation Models, Protoplanetary Disk Properties, and Mature Gas Giant Exoplanet Atmospheres

Measuring a single elemental ratio (e.g., carbon-to-oxygen) provides insufficient information for understanding the formation mechanisms and evolution that affect our observations of gas giant planet atmospheres. Although the fields of planet formation, protoplanetary disks, and exoplanets are well established and interconnected, our understanding of how to self-consistently and accurately link the theoretical and observational aspects of these fields together is lacking. To foster interdisciplinary conversations, the Max-Planck Institut f\"ur Astronomie (MPIA) hosted a week-long workshop called, "Challenge Accepted: Linking Planet Formation with Present-Day Atmospheres." Here, we summarize the latest theories and results in planet formation modeling, protoplanetary disk observations, and atmospheric observations of gas giant atmospheres to address one of the challenges of hosting interdisciplinary conferences: ensuring everyone is aware of the state-of-the-art results and technical language from each discipline represented. Additionally, we highlight key discussions held at the workshop. Our main conclusion is that it is unclear what the ideal observable is to make this link between formation scenarios and exoplanet atmospheres, whether it be multiple elemental abundance ratios, measuring refractory budgets, or something else. Based on discussions held throughout the workshop, we provide several key takeaways of what the workshop attendees feel need the most improvement and exploration within each discipline.

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arXiv2025

Bridging Unstratified and Stratified Simulations of the Streaming Instability for $\tau_s=0.1$ Grains

The streaming instability (SI), driven by aerodynamic coupling between solids and the gas under a global radial pressure gradient, concentrates solids and facilitates planetesimal formation. Unstratified simulations are commonly used to study the SI, based on the assumption that they approximate conditions near the disk midplane. However, it remains unclear how accurately these unstratified simulations capture the midplane dust-gas dynamics in stratified disks. To address this, we examine the saturated state of the SI in stratified simulations and compare dust-gas dynamics to those in unstratified simulations across various radial pressure gradients. To this end, we consider a dimensionless dust stopping time ($\tau_s$) of 0.1 and perform 2D axisymmetric, stratified simulations. We find that the formation of dust filaments during dust settling exhibits morphological similarities to those in unstratified simulations. Vertical gravity acts to redistribute momentum vertically in response to momentum flux, resulting in midplane velocities in the center-of-mass frame that are consistent with those from unstratified models at any given pressure gradient. Furthermore, the velocity dispersions and density distributions of the gas and dust near the midplane of our stratified simulations closely match those in unstratified simulations. While further exploration across the parameter space is needed, our results suggest that, for $\tau_s=0.1$, unstratified simulations represents well the midplane dust--gas dynamics in stratified disks before any strong clumping occurs. Consequently, our results confirm that in the saturated state, the streaming turbulence in stratified simulations behaves similarly to that in unstratified simulations for the parameter values explored here.

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arXiv2025

Turbulence Inhibits Planetesimal Formation in Class 0/I Disks Subject to Infall

There is growing evidence that planet formation begins early, within the $\lesssim 1$Myr Class 0/I phase, when infall dominates disk dynamics. Our goal is to determine if Class 0/I disks reach the conditions needed to form planetesimals ($\sim 100$km planet building blocks) by the streaming instability (SI). We focus on a recent suggestion that early infall causes an ``inflationary'' phase in which dust grains are advected outward. We modified the \texttt{DustPy} code to build a 1D disk that includes dust evolution, infall, and heating and cooling sources. We ran six models and examined the implications for the SI, taking into account recent works on how the SI responds to external turbulence. In line with other works, we find that grains are advected outward, which leads to ``advection-condensation-drift'' loop that greatly enhances the dust density at the water snowline. However, we do not see this process at the silicate line. Instead, we find a new pile up at the edge of the expanding disk. However, despite these localized enhancements, even a modest amount of turbulence ($\alpha = 10^{-3}$) leaves planetesimal formation far out of reach. The midplane dust-to-gas ratio is at least an order of magnitude below the SI threshold, even taking into account recent results on how dust coagulation boosts the SI. For planetesimals to form in the Class 0/I phase may require a way to transport angular momentum without turbulence (e.g., disk winds) or a non-SI mechanism to form planetesimals.

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arXiv2025

Positive Feedback II: How Dust Coagulation inside Vortices Can Form Planetesimals at Low Metallicity

The origin of planetesimals ($\sim$100 km planet building blocks) has confounded astronomers for decades, as numerous growth barriers appear to impede their formation. In a recent paper we proposed a novel interaction where the streaming instability (SI) and dust coagulation work in tandem, with each one changing the environment in a way that benefits the other. This mechanism proved effective at forming planetesimals in the fragmentation-limited inner disk, but much less effective in the drift-limited outer disk, concluding that dust traps may be key to forming planets at wide orbital separations. Here we explore a different hypothesis: That vortices host a feedback loop in which a vortex traps dust, boosting dust coagulation, which in turn boosts vortex trapping. We combine an analytic model of vortex trapping with an analytic model of fragmentation limited grain growth that accounts for how dust concentration dampens gas turbulence. We find a powerful synergy between vortex trapping and dust growth. For $\alpha \le 10^{-3}$ and solar-like metallicity this feedback loop consistently takes the grain size and dust density into the planetesimal formation region of the streaming instability (SI). Only in the regime of strong turbulence ($\alpha \ge 3\times 10^{-3}$) does the system often converge to a steady state below the SI criterion. The combination of vortex trapping with dust coagulation is an even more powerful mechanism than the one involving the SI. It is effective at lower metallicity and across the whole disk -- anywhere that vortices form.

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arXiv2025

Positive Feedback: How a Synergy Between the Streaming Instability and Dust Coagulation Forms Planetesimals

One of the most important open questions in planet formation is how dust grains in a protoplanetary disk manage to overcome growth barriers and form the $\sim$100km planet building blocks that we call planetesimals. There appears to be a gap between the largest grains that can be produce by coagulation, and the smallest grains that are needed for the streaming instability (SI) to form planetesimals. Here we explore a novel hypothesis: That dust coagulation and the SI work in tandem. That they form a feedback loop where each one boosts the action of the other to bridge the gap between dust grains and planetesimals. We develop a semi-analytical model of dust concentration due to the SI, and an analytic model of how the SI affects the fragmentation and radial drift barriers. We then combine those to model our proposed feedback loop. In the fragmentation-limited regime, we find a powerful synergy between the SI and dust growth that drastically increases both grain sizes and densities. We find that a midplane dust-to-gas ratio of $\epsilon \ge 0.3$ is a sufficient condition for the feedback loop to reach the planetesimal-forming region for turbulence values $10^{-4} \le \alpha \le 10^{-3}$ and grain sizes $0.01 \le {\rm St} \le 0.1$. In contrast, the drift-limited regime only shows grain growth, without significant dust accumulation. Planet formation in the drift-limited portion of the disk may require other processes (particle traps) to halt radial drift.

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arXiv2024

Probing Conditions for Strong Clumping by the Streaming Instability: Small Dust Grains and Low Dust-to-gas Density Ratio

The streaming instability (SI) is a leading mechanism for concentrating solid particles into regions dense enough to form planetesimals. Its efficiency in clumping particles depends primarily on the dimensionless stopping time ($\tau_s$, a proxy for particle size) and dust-to-gas surface density ratio ($Z$). Previous simulations identified a critical $Z$ ($Z_{\rm{crit}}$) above which strong clumping occurs, where particle densities exceed the Hill density (thus satisfying a condition for gravitational collapse), over a wide range of $\tau_s$. These works found that for $\tau_s \leq 0.01$, $Z_{\rm{crit}}$ was above the ISM value $(\sim 0.01)$. In this work, we reexamine the clumping threshold using 2D axisymmetric, stratified simulations at high resolution and with relatively large (compared to many previous simulations) domain sizes. Our main results are as follows: First, when $\tau_s = 0.01$, strong clumping occurs even at $Z \lesssim 0.01$, lower than $Z_{\rm{crit}}$ found in all previous studies. Consequently, we revise a previously published fit to the $Z_{\rm{crit}}$ curve to account for this updated $Z_{\rm{crit}}$. Second, higher resolution results in a thicker dust layer, which may result from other instabilities manifesting, such as the vertical shearing streaming instability. Third, despite this thicker layer, higher resolution can lead to strong clumping even with lower midplane dust-to-gas density ratios (which results from the thicker particle layer) so long as $Z \gtrsim Z_{\rm{crit}}$. Our results demonstrate the efficiency of the SI in clumping small particles at $Z \sim 0.01$, which is a significant refinement of the conditions for planetesimal formation by the SI.

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arXiv2024

Exploring the Complex Ionization Environment of the Turbulent DM Tau Disk

Ionization drives important chemical and dynamical processes within protoplanetary disks, including the formation of organics and water in the cold midplane and the transportation of material via accretion and magneto-hydrodynamic (MHD) flows. Understanding these ionization-driven processes is crucial for understanding disk evolution and planet formation. We use new and archival ALMA observations of HCO+, H13CO+, and N2H+ to produce the first forward-modeled 2D ionization constraints for the DM Tau protoplanetary disk. We include ionization from multiple sources and explore the disk chemistry under a range of ionizing conditions. Abundances from our 2D chemical models are post-processed using non-LTE radiative transfer, visibility sampling, and imaging, and are compared directly to the observed radial emission profiles. The observations are best fit by a modestly reduced CR ionization rate ($\zeta_{CR}$ ~ 10$^{-18}$ s$^{-1}$) and a hard X-ray spectrum (hardness ratio [HR] = 0.3), which we associate with stellar flaring conditions. Our best-fit model under-produces emission in the inner disk, suggesting that there may be an additional mechanism enhancing ionization in DM Tau's inner disk. Overall, our findings highlight the complexity of ionization in protoplanetary disks and the need for high resolution multi-line studies.

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arXiv2024

Rapid protoplanet formation in vortices: three-dimensional local simulations with selfgravity

Disk vortices, seen in numerical simulations of protoplanetary disks and found observationally in ALMA and VLA images of these objects, are promising sites for planet formation given their pebble trapping abilities. Previous works have shown strong concentration of pebbles in vortices, but gravitational collapse has only been shown in low-resolution, two-dimensional, global models. In this letter, we aim to study the pebble concentration and gravitational collapse of pebble clouds in vortices via high-resolution, three-dimensional, local models. We performed simulations of the dynamics of gas and solids in a local shearing box where the gas is subject to convective overstability, generating a persistent giant vortex. We find that the vortex produces objects of Moon and Mars mass, with mass function of power law $d\ln N/d\ln M=-1.6\pm 0.3$. The protoplanets grow rapidly, doubling in mass in about 5 orbits, following pebble accretion rates. The mass range and mass doubling rate are in broad agreement with previous low resolution global models. We conclude that Mars-mass planetary embryos are the natural outcome of planet formation inside the disk vortices seen in millimeter and radio images of protoplanetary disks.

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arXiv2024

Evidence for Non-zero Turbulence in the Protoplanetary disc around IM Lup

The amount of turbulence in protoplanetary discs around young stars is critical for determining the efficiency, timeline, and outcomes of planet formation. It is also difficult to measure. Observations are still limited, but direct measurements of the non-thermal, turbulent gas motion are possible with the Atacama Large Millimeter/submillimeter Array (ALMA). Using CO(2-1)/$^{13}$CO(2-1)/C$^{18}$O(2-1) ALMA observations of the disc around IM Lup at ~0.4" (~60 au) resolution we find evidence of significant turbulence, at the level of $\delta v_{\rm turb}=(0.18-0.30)$c$_s$. This result is robust against systematic uncertainties (e.g., amplitude flux calibration, midplane gas temperature, disc self-gravity). We find that gravito-turbulence as the source of the gas motion is unlikely based on the lack of an imprint on the rotation curve from a massive disc, while magneto-rotational instabilities and hydrodynamic instabilities are still possible, depending on the unknown magnetic field strength and the cooling timescale in the outer disc.

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arXiv2024

The Limited Role of the Streaming Instability During Moon and Exomoon Formation

It is generally accepted that the Moon accreted from the disk formed by an impact between the proto-Earth and impactor, but its details are highly debated. Some models suggest that a Mars-sized impactor formed a silicate melt-rich (vapor-poor) disk around Earth, whereas other models suggest that a highly energetic impact produced a silicate vapor-rich disk. Such a vapor-rich disk, however, may not be suitable for the Moon formation, because moonlets, building blocks of the Moon, of 100 m-100 km may experience strong gas drag and fall onto Earth on a short timescale, failing to grow further. This problem may be avoided if large moonlets ($\gg 100$ km) form very quickly by streaming instability, which is a process to concentrate particles enough to cause gravitational collapse and rapid formation of planetesimals or moonlets. Here, we investigate the effect of the streaming instability in the Moon-forming disk for the first time and find that this instability can quickly form $\sim 100$ km-sized moonlets. However, these moonlets are not large enough to avoid strong drag and they still fall onto Earth quickly. This suggests that the vapor-rich disks may not form the large Moon, and therefore the models that produce vapor-poor disks are supported. This result is applicable to general impact-induced moon-forming disks, supporting the previous suggestion that small planets ($<1.6 R_\oplus$) are good candidates to host large moons because their impact-induced disks would be likely vapor-poor. We find a limited role of streaming instability in a satellite formation in an impact-induced disk, whereas it plays a key role during planet formation.

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arXiv2024

Magnetically Driven Turbulence in the Inner Regions of Protoplanetary Disks

Given the important role turbulence plays in the settling and growth of dust grains in protoplanetary disks, it is crucial that we determine whether these disks are turbulent and to what extent. Protoplanetary disks are weakly ionized near the mid-plane, which has led to a paradigm in which largely laminar magnetic field structures prevail deeper in the disk, with angular momentum being transported via magnetically launched winds. Yet, there has been little exploration on the precise behavior of the gas within the bulk of the disk. We carry out 3D, local shearing box simulations that include all three low-ionization effects (Ohmic diffusion, ambipolar diffusion, and the Hall effect) to probe the nature of magnetically driven gas dynamics 1-30 AU from the central star. We find that gas turbulence can persist with a generous yet physically motivated ionization prescription (order unity Elsasser numbers). The gas velocity fluctuations range from 0.03-0.09 of the sound speed $c_s$ at the disk mid-plane to $\sim c_s$ near the disk surface, and are dependent on the initial magnetic field strength. However, the turbulent velocities do not appear to be strongly dependent on the field polarity, and thus appear to be insensitive to the Hall effect. The mid-plane turbulence has the potential to drive dust grains to collision velocities exceeding their fragmentation limit, and likely reduces the efficacy of particle clumping in the mid-plane, though it remains to be seen if this level of turbulence persists in disks with lower ionization levels.

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arXiv2024

A solution for the density dichotomy problem of Kuiper Belt objects with multi-species streaming instability and pebble accretion

Kuiper belt objects show an unexpected trend, whereby large bodies have increasingly higher densities, up to five times greater than their smaller counterparts. Current explanations for this trend assume formation at constant composition, with the increasing density resulting from gravitational compaction. However, this scenario poses a timing problem to avoid early melting by decay of $^{26}$Al. We aim to explain the density trend in the context of streaming instability and pebble accretion. Small pebbles experience lofting into the atmosphere of the disk, being exposed to UV and partially losing their ice via desorption. Conversely, larger pebbles are shielded and remain more icy. We use a shearing box model including gas and solids, the latter split into ices and silicate pebbles. Self-gravity is included, allowing dense clumps to collapse into planetesimals. We find that the streaming instability leads to the formation of mostly icy planetesimals, albeit with an unexpected trend that the lighter ones are more silicate-rich than the heavier ones. We feed the resulting planetesimals into a pebble accretion integrator with a continuous size distribution, finding that they undergo drastic changes in composition as they preferentially accrete silicate pebbles. The density and masses of large KBOs are best reproduced if they form between 15 and 22\,AU. Our solution avoids the timing problem because the first planetesimals are primarily icy, and $^{26}$Al is mostly incorporated in the slow phase of silicate pebble accretion. Our results lend further credibility to the streaming instability and pebble accretion as formation and growth mechanisms.

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