arXiv subjects

David G. Rea

Publications and source records attributed to David G. Rea.

5 recordsLinked to original sources

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.

astro-ph.EP↗

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.

astro-ph.EP↗

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, $τ_s$. 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 $10^{-3} \leq τ_s \leq 1.0$. Additionally, we study SI-driven concentration that is unique to 3D. We find that $\Zcrit$ is as low as $\approx 0.002$ at $τ_s=0.1$ and exceeds $\approx 0.03$ at $τ_s=10^{-3}$. Compared to 2D, our 3D results yield lower $\Zcrit$ for $τ_s > 0.02$, but higher for $τ_s \leq 0.02$, with a sharp transition between $τ_s = 0.02$ and 0.03. This transition correlates with midplane density ratio ($ε$): $ε< 1$ where 3D gives lower thresholds, and $ε> 1$ where 3D gives higher thresholds. We also find a filaments-in-filaments structure when $ε< 1$, which enhances clumping compared to 2D. By contrast, when $ε> 1$ and $τ_s \leq 0.03$, dust filaments in 3D do not drift inward, suppressing filament mergers and strong clumping. In 2D, filaments drift inward regardless of $ε$, 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.

astro-ph.EP↗

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.

astro-ph.EP↗

arXiv2023

Streaming Instability and Turbulence: Conditions for Planetesimal Formation

The streaming instability (SI) is a leading candidate for planetesimal formation, which can concentrate solids through two-way aerodynamic interactions with the gas. The resulting concentrations can become sufficiently dense to collapse under particle self-gravity, forming planetesimals. Previous studies have carried out large parameter surveys to establish the critical particle to gas surface density ratio ($Z$), above which SI-induced concentration triggers planetesimal formation. The threshold $Z$ depends on the dimensionless stopping time ($τ_s$, a proxy for dust size). However, these studies neglected both particle self-gravity and external turbulence. Here, we perform 3D stratified shearing box simulations with both particle self-gravity and turbulent forcing, which we characterize via $α_D$ that measures turbulent diffusion. We find that forced turbulence, at amplitudes plausibly present in some protoplanetary disks, can increase the threshold $Z$ by up to an order of magnitude. For example, for $τ_s = 0.01$, planetesimal formation occurs when $Z \gtrsim 0.06$, $\gtrsim 0.1$, and $\gtrsim 0.2$ at $α_D = 10^{-4}$, $10^{-3.5}$, and $10^{-3}$, respectively. We provide a single fit to the critical $Z$ as a function of $α_D$ and $τ_s$ required for the SI to work (though limited to the range $τ_s = 0.01$--0.1). Our simulations also show that planetesimal formation requires a mid-plane particle-to-gas density ratio that exceeds unity, with the critical value being independent of $α_D$. Finally, we provide the estimation of particle scale height that accounts for both particle feedback and external turbulence.

astro-ph.EP↗