An official website of the United States government
Abstract 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 thresholdZdepends on the dimensionless stopping time (τs, 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 a turbulent diffusion parameter,αD. We find that forced turbulence, at amplitudes plausibly present in some protoplanetary disks, can increase the thresholdZby up to an order of magnitude. For example, forτs= 0.01, planetesimal formation occurs whenZ≳ 0.06, ≳0.1, and ≳0.2 atαD= 10−4, 10−3.5, and 10−3, respectively. We provide a single fit to the criticalZrequired for the SI to work as a function ofαDandτs(although limited to the rangeτs= 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 largely insensitive toαD. Finally, we provide an estimation of particle scale height that accounts for both particle feedback and external turbulence.
more »
« less
This content will become publicly available on March 6, 2026
Probing Conditions for Strong Clumping by the Streaming Instability: Small Dust Grains and Low Dust-to-gas Density Ratio
Abstract 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 (τs, a proxy for particle size) and dust-to-gas surface density ratio (Z). Previous simulations identified a criticalZ(Zcrit) above which strong clumping occurs, where particle densities exceed the Hill density (thus satisfying a condition for gravitational collapse), over a wide range ofτs. These works found that, forτs≤ 0.01,Zcritwas above the interstellar medium value (∼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, whenτs = 0.01, strong clumping occurs even atZ ≲ 0.01, lower thanZcritfound in all previous studies. Consequently, we revise a previously published fit to theZcritcurve to account for this updatedZcrit. Second, higher resolution results in a thicker dust layer, which may result from other instabilities manifesting, such as the vertically shearing SI. Third, despite this thicker layer, higher resolution can lead to strong clumping even with a lower midplane dust-to-gas density ratios (which results from the thicker particle layer) so long asZ ≳ Zcrit. Our results demonstrate the efficiency of the SI in clumping small particles atZ ∼ 0.01, which is a significant refinement of the conditions for planetesimal formation by the SI.
more »
« less
- Award ID(s):
- 2007422
- PAR ID:
- 10621838
- Publisher / Repository:
- American Astronomical Society
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 981
- Issue:
- 2
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 160
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACT Observations indicate dust populations vary between galaxies and within them, suggesting a complex life cycle and evolutionary history. Here we investigate the evolution of galactic dust populations across cosmic time using a suite of cosmological zoom-in simulations from the Feedback in Realistic Environments project, spanning $$M_{\rm vir}=10^{9-12}{M}_{\odot };\, M_{*}=10^{6-11}\, {M}_{\odot }$$. Our simulations incorporate a dust evolution model that accounts for the dominant sources of dust production, growth, and destruction and follows the evolution of specific dust species. All galactic dust populations in our suite exhibit similar evolutionary histories, with gas–dust accretion being the dominant producer of dust mass for all but the most metal-poor galaxies. Similar to previous works, we find the onset of efficient gas–dust accretion occurs above a ‘critical’ metallicity threshold (Zcrit). Due to this threshold, our simulations reproduce observed trends between galactic D/Z and metallicity and element depletion trends in the interstellar medium. However, we find Zcrit varies between dust species due to differences in key element abundances, dust physical properties, and life cycle processes resulting in $$Z_{\rm crit}\sim 0.05{\rm Z}_{\odot },\, 0.2{\rm Z}_{\odot },\, 0.5{\rm Z}_{\odot }$$ for metallic iron, silicates, and carbonaceous dust, respectively. These variations could explain the lack of small carbonaceous grains observed in the Magellanic Clouds. We also find a delay between the onset of gas–dust accretion and when a dust population reaches equilibrium, which we call the equilibrium time-scale (τequil). The relation between τequil and the metal enrichment time-scale of a galaxy, determined by its recent evolutionary history, can contribute to the scatter in the observed relation between galactic D/Z and metallicity.more » « less
-
ABSTRACT The streaming instability, a promising mechanism to drive planetesimal formation in dusty protoplanetary discs, relies on aerodynamic drag naturally induced by the background radial pressure gradient. This gradient should vary in discs, but its effect on the streaming instability has not been sufficiently explored. For this purpose, we use numerical simulations of an unstratified disc to study the non-linear saturation of the streaming instability with mono-disperse dust particles and survey a wide range of gradients for two distinct combinations of the particle stopping time and the dust-to-gas mass ratio. As the gradient increases, we find most kinematic and morphological properties increase but not always in linear proportion. The density distributions of tightly coupled particles are insensitive to the gradient whereas marginally coupled particles tend to concentrate by more than an order of magnitude as the gradient decreases. Moreover, dust–gas vortices for tightly coupled particles shrink as the gradient decreases, and we note higher resolutions are required to trigger the instability in this case. In addition, we find various properties at saturation that depend on the gradient may be observable and may help reconstruct models of observed discs dominated by streaming turbulence. In general, increased dust diffusion from stronger gradients can lower the concentration of dust filaments and can explain the higher solid abundances needed to trigger strong particle clumping and the reduced planetesimal formation efficiency previously found in vertically stratified simulations.more » « less
-
Abstract Radio images of protoplanetary disks demonstrate that dust grains tend to organize themselves into rings. These rings may be a consequence of dust trapping within gas pressure maxima, wherein the local high dust-to-gas ratio is expected to trigger the formation of planetesimals and eventually planets. We revisit the behavior of dust near gas pressure perturbations enforced by a planet in two-dimensional, shearing-box simulations. While dust grains collect into generally long-lived rings, particles with a small Stokes parameter τ s < 0.1 tend to advect out of the ring within a few drift timescales. Scaled to the properties of ALMA disks, we find that rings composed of larger particles ( τ s ≥ 0.1) can nucleate a dust clump massive enough to trigger pebble accretion, which proceeds to ingest the entire dust ring well within ∼1 Myr. To ensure the survival of the dust rings, we favor a nonplanetary origin and typical grain size τ s ≲ 0.05–0.1. Planet-driven rings may still be possible but if so we would expect the orbital distance of the dust rings to be larger for older systems.more » « less
-
Abstract Observations and measurements show that crystals remain relatively compact at low ice supersaturations, but become increasingly hollowed and complex as the ice supersaturation rises. Prior measurements at temperatures >−25°C indicate that the transition from compact, solid ice to morphologically complex crystals occurs when the excess vapor density exceeds a threshold value of about 0.05 g m−3. A comparable threshold is not available at low temperatures. A temperature-dependent criterion for the excess vapor density threshold (Δρthr) that defines morphological transformations to complex ice is derived from laboratory measurements of vapor grown ice at temperatures below −40°C. This criterion depends on the difference between the equilibrium vapor density of liquid () and ice (ρei) multiplied by a measurement-determined constant,. The new criterion is consistent with prior laboratory measurements, theoretical estimates, and it reproduces the classical result of about 0.05 g m−3above −25°C. Since Δρthrdefines the excess vapor density above which crystals transition to a morphologically complex (lower density) growth mode, we can estimate the critical supersaturation (scrit) for step nucleation during vapor growth. The derived values ofscritare consistent with previous measurements at temperatures above −20°C. No direct measurements ofscritare available for temperatures below −40°C; however, our derived values suggest some measurement-based estimates may be too high while estimates from molecular dynamics simulations may be too low.more » « less
