arXiv subjects

Tilman Birnstiel

Publications and source records attributed to Tilman Birnstiel.

At least 19 recordsLinked to original sources

arXiv2026

Planet formation at the inner edge of the dead zone I. the interplay between accretion outbursts and dust growth

The inner edge of the dead zone in protoplanetary disks has been shown to periodically go unstable, leading to accretion outbursts and annular substructure within the dead zone. While dust opacities play a key role in this process, the thermal and dynamical effects of dust drift and growth have not been fully explored. We investigate the evolution of accretion outbursts in the inner disk and their impact on the formation of dust-rich substructure with a fully dynamic dust model. In doing so, we aim to highlight the importance and limitations of dust growth in forming planets in this region. We carry out radiation hydrodynamics simulations of a protoplanetary disk including prescriptions for the structure of the inner edge of the dead zone, viscous and irradiation heating, radiative cooling, dust-gas dynamics, and dust evolution. We find that accretion outbursts at the inner disk edge can lead to the formation of multiple dust rings that extend deep inside the dead zone (~1 au) and diffuse on viscous timescales (~10 kyr for alpha=1e-4). The rings contain dust masses of up to ~1.6 Earth masses, possibly kickstarting planet formation. Dynamic modeling of dust fragmentation enhances the total opacity during the burst, yielding more intense outbursts that penetrate deeper into the dead zone. Our results highlight the thermal and dynamical importance of treating dust dynamics self-consistently in models of accretion outbursts. Additional modeling is needed to characterize the inevitable nonaxisymmetric structures arising from accretion outbursts and their observational prospects.

astro-ph.EP↗

arXiv2026

Late-infall-induced formation of giant planets, multigenerational planetesimals, and disk substructures

Late infall can replenish the building materials of planets in protoplanetary disks and dramatically alter their structural evolution. The resulting pressure bumps effectively accumulate dust, facilitate grain coagulation, and trigger planetesimal formation via the streaming instability. In this work, we investigated the potential for planetesimal and planet formation, as well as the emergence of observable substructures, in disks undergoing late-stage infall. We utilized a comprehensive modeling framework that couples dust coagulation and dynamics, planetesimal formation, N-body gravity, planetary growth, and planet-disk interactions. Our results show that the abundant dust supply and the migration barrier created by the infalling gas enable the rapid formation of gas giants via pebble and gas accretion within one million years, even at large orbital distances (~70 au). These giants, in turn, exert torques that generate multiple secondary disk substructures, fostering multigenerational planetesimal formation and resulting in diverse planetary system configurations. The planetesimals exhibit distinct dynamical properties that are determined by their formation epoch and environment, which are analogous to the small-body populations in the outer Solar System. Both the infall- and planet-induced substructures are clearly visible in synthetic 1.3-mm continuum observations, closely resembling the multi-ring disks detected in ALMA surveys. Our model provides a new perspective on the origin of distant giant planets, long-lasting planetesimal formation, and the prevalence of disks with multiple substructures.

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arXiv2026

The Influence of Dust Composition on Accretion Outbursts

Context: Episodic accretion outbursts have been shown to occur in protoplanetary discs due to thermal instability at the inner edge of the dead zone. These outbursts periodically heat up the dead zone, significantly altering the composition/chemistry and accretion onto the star. Aims: We investigate how these accretion outbursts affect the inner disc composition and how different dust compositions and properties affect the outbursts' dynamics. Methods: We run vertically integrated axis-symmetric dust and gas evolution models using the TriPoD method, including dust opacity dependent heating and cooling, a dead zone model, and compositional tracking of dust, including the evaporation and condensation of volatiles. Results: When considering dust to be made up of a condensation sequence of silicates and volatiles, the outburst evaporates most of the dust within 0.5 au. The fast re-condensation and subsequent viscous evolution reset the disc between bursts. Additionally, we find that the burst cycle period and maximal accretion rate directly correlate with the dust sublimation temperature.

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arXiv2026

Interpreting the scattering surface in protoplanetary disks

In recent years, extreme adaptive optics have enabled high-resolution, high-contrast scattered-light observations of protoplanetary disks. Interpreting these observations requires an understanding of the scattering surface, which is shaped by the distribution of small dust grains and determines how disks appear in scattered light. We aim to exploit measurements of the scattering surface height to directly constrain the masses of small dust grains in disks. Starting from radiative transfer principles, we developed a semi-analytical model of the stellar radiation path and its interaction with the disk, deriving the height of the scattering surface as a function of disk parameters such as mass, temperature, and opacity. We validated our predictions against the radiative transfer code MCFOST. Using measured scattering heights, we inferred the mass of dust in small grains and the particle size distribution for a sample of ten disks. We confirm previous results indicating that the scattering surface coincides with the region where the integrated optical depth along the stellar path is of order unity. The thermal structure of the disk significantly affects the surface height, while dust settling and anisotropic scattering have comparatively minor effects. Applying our model to observations, we measure global small-dust mass fractions of order (10^{-3}). Using dust-opacity models, we show that these values are consistent with modest grain growth ((a_{\rm max} \gtrsim 0.1,{\rm mm})) and grain-size distribution power-law indices of approximately 3--3.5, as commonly predicted by grain-growth models. Scattering-surface measurements, together with constraints on the disk thermal structure, provide a powerful method for determining the small-dust content of protoplanetary disks.

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arXiv2026

Substructures in Planet-Forming Disks with the SKAO

Disks of gas and dust orbiting young stars are the arenas and material reservoirs for planet formation. Over the past decade, multiwavelength observations, from infrared to radio, have resolved the spatial distribution of hundreds of protoplanetary disks in nearby star-forming regions, revealing a diverse zoo of substructures. These substructures are morphological features such as rings, gaps, spirals, vortices, asymmetries, warps, or clumps that trace variations in density, temperature, or composition relative to an otherwise smooth distribution of gas and dust. Many unknowns persist as to the origin of these substructures, their role in planet assembly, and their true properties. SKA-Mid Band 5b continuum observations, offering angular resolutions of $\sim 0.05''$ ($\sim 0.15''$) with AA4 (AA*) at $12.5$ GHz / $2.4$ cm, will enable new progress at this frontier. In this chapter, we outline the open questions in the field of disk substructure that SKA-Mid is uniquely poised to address, with a lens on dust thermal emission.

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arXiv2026

Planet formation at the inner edge of the dead zone II. Outbursts, rings, vortices, and suppression of planetesimal formation

Accretion outbursts have been observed in a variety of young stellar objects, but models of their dynamical evolution have been largely limited to axisymmetric models due to their computational cost. We investigate the azimuthal stability of accretion outbursts and the formation of planetesimals during these events. We performed high-resolution 2D, vertically integrated multifluid radiation-hydrodynamical simulations of the inner 10 au of protoplanetary disks with a dynamically growing dust population, including radiation transport and a realistic dust opacity model. Accretion outbursts are highly unstable to the Rossby-wave instability, with the burst front quickly diffusing into a large number of small-scale vortices that coalesce over time into a single, compact vortex and inducing azimuthal asymmetries. Vortices act as a source of vigorous turbulent diffusion, strongly suppressing planetesimal formation. Our results suggest that azimuthal asymmetries associated with accretion outbursts should be both common and detrimental to planet formation. Nevertheless, planetesimal formation will resume post-burst, as the burst-induced vortices eventually decay and the disk returns to a quiescent state featuring a pressure bump at ~1 au.

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arXiv2026

A tension between dust and gas radii: the role of substructures and external photoevaporation in protoplanetary disks

Protoplanetary disk substructures are thought to play a crucial role in disk evolution and planet formation. Population studies of disks large-sample size surveys show that substructures, and their rapid formation, are needed to reproduce the observed spectral indices. Moreover, they enable the simultaneous reproduction of the observed spectral index and size-luminosity distributions. This study aims to investigate the necessity of substructures and predict their characteristics to reproduce gas-to-dust size ratios observed in the Lupus star-forming region. We performed a population synthesis study of gas and dust evolution in disks using a two-population model (two-pop-py) and the DustPy code. We considered the effects of viscous evolution, dust growth, fragmentation, transport, and external photoevaporation. The simulated population distributions were obtained by post-processing the resulting disk profiles of surface density, maximum grain size, and disk temperature. Although substructures help reduce the discrepancy between simulated and observed disk gas-to-dust size ratios; even when accounting for external photoevaporation, they do not fully resolve it. Only specific initial conditions in disks undergoing viscous evolution with external photoevaporation can reproduce the observations, highlighting a fine-tuning problem. While substructured disks reproduce dust size and spectral index, they tend to overestimate gas radii. The results ultimately highlight the main challenge of simultaneously reproducing gas and dust sizes. One possible explanation is that the outermost substructure is linked to the disk truncation radius, which determines the gas radius, or that substructures are frequent enough to always be near the gas outer radius.

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arXiv2026

Starlight-driven flared-staircase geometry in radiation hydrodynamic models of protoplanetary disks

Protoplanetary disks observed in millimeter continuum and scattered light show a variety of substructures. Various physical processes in the disk could trigger such features -- one of which that has been previously theorized for passive disks is the thermal wave instability -- the flared disk may become unstable as directly illuminated regions puff up and cast shadows behind them. This would manifest as bright and dark rings, and a staircase-like structure in the disk optical surface. We provide a realistic radiation hydrodynamic model to test the limits of the thermal wave instability in irradiated disks. We carry out global axisymmetric 2D hydrostatic and dynamic simulations including radiation transport with frequency-dependent ray-traced irradiation and flux-limited diffusion (FLD). We found that starlight-driven shadows are most prominent in optically thick, slow cooling disks, shown by our models with high surface densities and dust-to-gas ratios of sub-micron grains of 0.01. We recover that thermal waves form and propagate inwards in the hydrostatic limit. In contrast, our hydrodynamic models show bumps and shadows within 30 au that converge to a quasi-steady state on several radiative diffusion timescales -- indicating a long-lived staircase structure. We find that existing thermal pressure bumps could produce and enhance this effect, forming secondary shadowing downstream. Hydrostatic models with self-consistent dust settling instead show a superheated dust irradiation absorption surface with a radially smooth temperature profile without staircases. We conclude that one can recover thermally induced flared-staircase structures in radiation hydrodynamic simulations of irradiated protoplanetary disks using flux-limited diffusion. We highlight the importance of modeling dust dynamics consistently to explain starlight-driven shadows.

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arXiv2025

Grain growth in protoplanetary disks in the Upper Scorpius revealed by millimeter-wave spectral indices

The measurement of dust size from millimeter-wavelength spectra provides direct constraints on grain growth in protoplanetary disks. The spectral indices between 0.88 mm and 2.9 mm have been measured in multiple young star-forming regions, such as Taurus, Ophiuchus, and Lupus, which have ages of 1-3 Myr. These spectral indices are as low as 2-3, suggesting that grains in disks are much larger than those in the interstellar medium. In this study, we analyze the ALMA archival data of 23 disks in the Upper Scorpius region. The observed wavelength is 2.9 mm in Band 3, the angular resolution is 3.3 arcsec x 2.1 arcsec, which is not high enough to resolve the targets, and the rms noise is below 0.075 mJy beam$^{-1}$ for almost all sources. Together with the literature values of the Band 7 fluxes of the same targets, we find that the average spectral index of the disks in the Upper Scorpius region is $α_\mathrm{mm}=2.09 \pm 0.10$, which is equal to or slightly smaller than those at the other younger regions. To explain the relationship between the fluxes and spectral indices of the disks in the Taurus, Ophiuchus, Lupus, and Upper Scorpius regions, we construct simple disk evolution models. The observations are best reproduced by models in which the inner radius of the disk increases. This suggests that a substantial amount of dust mass must persist in the outer disk regions where the dust temperature is lower than 20 K even at late evolutionary stages. These findings offer key insights into the grain growth and the temporal evolution of protoplanetary disks.

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arXiv2025

Detailed Microwave Continuum Spectra from Bright Protoplanetary Disks in Taurus

We present new observations that densely sample the microwave (4-360 GHz) continuum spectra from eight young systems in the Taurus region. Multi-component, empirical model prescriptions were used to disentangle the contributions from their dust disks and other emission mechanisms. We found partially optically thick, free-free emission in all these systems, with positive spectral indices (median $α_{\rm c} \approx 1$ at 10 GHz) and contributing 5-50% of the 43 GHz fluxes. There is no evidence for synchrotron or spinning dust grain emission contributions for these targets. The inferred dust disk spectra all show substantial curvature: their spectral indices decrease with frequency, from $α_{\rm d} \approx 2.8$-4.0 around 43 GHz to 1.7-2.1 around 340 GHz. This curvature suggests that a substantial fraction of the (sub)millimeter ($\gtrsim$ 200 GHz) dust emission may be optically thick, and therefore the traditional metrics for estimating dust masses are flawed. Assuming the emission at lower frequencies (43 GHz) is optically thin, the local spectral indices and fluxes were used to constrain the disk-averaged dust properties and estimate corresponding dust masses. These masses are roughly an order of magnitude higher ($\approx 1000 \, M_\oplus$) than those found from the traditional approach based on (sub)millimeter fluxes. These findings emphasize the value of broad spectral coverage - particularly extending to lower frequencies ($\sim$cm-band) - for accurately interpreting dust disk emission; such observations may help reshape our perspective on the available mass budgets for planet formation.

astro-ph.SR↗

arXiv2025

Spirals, rings, and vortices shaped by shadows in protoplanetary disks: from radiative hydrodynamical simulations to observable signatures

Numerous protoplanetary disks exhibit shadows in scattered light observations. These shadows are typically cast by misaligned inner disks and are associated with observable structures in the outer disk such as bright arcs and spirals. Investigating the dynamics of the shadowed outer disk is therefore essential in understanding the formation and evolution of these structures. We carry out twodimensional radiation hydrodynamics simulations that include radiative diffusion and dust-gas dynamics to study the formation of substructure in shadowed disks. We find that spiral arms are launched at the edge of each shadow, permeating the entire disk. The local dissipation of these spirals results in an angular momentum flux, opening multiple gaps and leading to a series of concentric, regularly-spaced rings We find that ring formation is favored in weakly turbulent disks where dust growth is taking place. These conditions are met for typical class-II disks, in which bright rings should form well within a fraction of their lifetime (0.1-0.2 Myr). For hotter disks gap opening is more efficient, such that the gap edges quickly collapse into vortices that can appear as bright arcs in continuum emission before decaying into rings or merging into massive, long-lived structures. Synthetic observations show that these structures should be observable in scattered light and millimeter continuum emission, providing a new way to probe the presence of substructure in protoplanetary disks. Our results suggest that the formation of rings and gaps is a common process in shadowed disks, and can explain the rich radial substructure observed in several protoplanetary disks.

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arXiv2025

Planetesimal formation in a pressure bump induced by infall

Infall of interstellar material is a potential non-planetary origin of pressure bumps in protoplanetary disks. While pressure bumps arising from other mechanisms have been numerically demonstrated to promote planet formation, the impact of infall-induced pressure bumps remains unexplored. We aim to investigate the potential for planetesimal formation in an infall-induced pressure bump, starting with sub-micrometer-sized dust grains, and to identify the conditions most conducive to triggering this process. We developed a numerical model that integrates axisymmetric infall, dust drift, and dust coagulation, along with planetesimal formation via streaming instability. Our parameter space includes gas viscosity, dust fragmentation velocity, initial disk mass, characteristic disk radius, infall rate and duration, as well as the location and width of the infall region. An infall-induced pressure bump can trap dust from both the infalling material and the outer disk, promoting dust growth. The locally enhanced dust-to-gas ratio triggers streaming instability, forming a planetesimal belt inside the central infall location until the pressure bump is smoothed out by viscous gas diffusion. Planetesimal formation is favored by a massive, narrow streamer infalling onto a low-viscosity, low-mass, and spatially extended disk containing dust with a high fragmentation velocity. This configuration enhances the outward drift speed of dust on the inner side of the pressure bump, while also ensuring the prolonged persistence of the pressure bump. Planetesimal formation can occur even if the infalling material consists solely of gas. A pressure bump induced by infall is a favorable site for dust growth and planetesimal formation, and this mechanism does not require a preexisting massive planet to create the bump.

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arXiv2024

High Resolution ALMA Observations of Richly Structured Protoplanetary Disks in $σ$ Orionis

ALMA has detected substructures in numerous protoplanetary disks at radii from a few to over a hundred au. These substructures are commonly thought to be associated with planet formation, either by serving as sites fostering planetesimal formation or arising as a consequence of planet-disk interactions. Our current understanding of substructures, though, is primarily based on observations of nearby star-forming regions with mild UV environments, whereas stars are typically born in much harsher UV environments, which may inhibit planet formation in the outer disk through external photoevaporation. We present high resolution ($\sim8$ au) ALMA 1.3 mm continuum images of eight disks in $σ$ Orionis, a cluster irradiated by an O9.5 star. Gaps and rings are resolved in the images of five disks. The most striking of these is SO 1274, which features five gaps that appear to be arranged nearly in a resonant chain. In addition, we infer the presence of gap or shoulder-like structures in the other three disks through visibility modeling. These observations indicate that substructures robustly form and survive at semi-major axes of several tens of au or less in disks exposed to intermediate levels of external UV radiation as well as in compact disks. However, our observations also suggest that disks in $σ$ Orionis are mostly small and thus millimeter continuum gaps beyond a disk radius of 50 au are rare in this region, possibly due to either external photoevaporation or age effects.

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arXiv2024

A Dust-Trapping Ring in the Planet-Hosting Disk of Elias 2-24

Rings and gaps are among the most widely observed forms of substructure in protoplanetary disks. A gap-ring pair may be formed when a planet carves a gap in the disk, which produces a local pressure maximum following the gap that traps inwardly drifting dust grains and appears as a bright ring due to the enhanced dust density. A dust-trapping ring would provide a promising environment for solid growth and possibly planetesimal production via the streaming instability. We present evidence of dust trapping in the bright ring of the planet-hosting disk Elias 2-24, from the analysis of 1.3 mm and 3 mm ALMA observations at high spatial resolution (0.029 arcsec, 4.0 au). We leverage the high spatial resolution to demonstrate that larger grains are more efficiently trapped and place constraints on the local turbulence ($8 \times 10^{-4} < α_\mathrm{turb} < 0.03$) and the gas-to-dust ratio ($Σ_g / Σ_d < 30$) in the ring. Using a scattering-included marginal probability analysis we measure a total dust disk mass of $M_\mathrm{dust} = 13.8^{+0.7}_{-0.5} \times 10^{-4} \ M_\odot$. We also show that at the orbital radius of the proposed perturber, the gap is cleared of material down to a flux contrast of 10$^{-3}$ of the peak flux in the disk.

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arXiv2024

Population Synthesis Models Indicate a Need for Early and Ubiquitous Disk Substructures

Large mm surveys of star forming regions enable the study of entire populations of planet-forming disks and reveal correlations between their observable properties. Population studies of disks have shown that the correlation between disk size and millimeter flux could be explained either through disks with strong substructure, or alternatively by the effects of radial inward drift of growing dust particles. This study aims to constrain the parameters and initial conditions of planet-forming disks and address the question of the need for the presence of substructures in disks and, if needed, their predicted characteristics, based on the large samples of disk sizes, millimeter fluxes, and spectral indices available. We performed a population synthesis of the continuum emission of disks, exploiting a two-population model (two-pop-py), considering the influence of viscous evolution, dust growth, fragmentation, and transport varying the initial conditions of the disk and substructure to find the best match to the observed distributions. We show that the observed distributions of spectral indices, sizes, and luminosities together can be best reproduced by disks with significant substructure, namely a perturbation strong enough to be able to trap particles, and that is formed early in the evolution of the disk, that is within 0.4Myr. Agreement is reached by relatively high initial disk masses ($10^{-2.3}M_{\star}\leqslant M_{disk}\leqslant10^{-0.5}M_{\star}$) and moderate levels of turbulence ($10^{-3.5}\leqslantα\leqslant 10^{-2.5}$). Other disk parameters play a weaker role. Only opacities with high absorption efficiency can reproduce the observed spectral indices. Our results extend to the whole population that substructure is likely ubiquitous, so far assessed only in individual disks and implies that most "smooth" disks hide unresolved substructure.

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arXiv2023

Dust growth and evolution in protoplanetary disks

Over the past decade, advancement of observational capabilities, specifically the Atacama Large Millimeter/submillimeter Array (ALMA) and SPHERE instrument, alongside theoretical innovations like pebble accretion, have reshaped our understanding of planet formation and the physics of protoplanetary disks. Despite this progress, mysteries persist along the winded path of micrometer-sized dust, from the interstellar medium, through transport and growth in the protoplanetary disk, to becoming gravitationally bound bodies. This review outlines our current knowledge of dust evolution in circumstellar disks, yielding the following insights: $\bullet$ Theoretical and laboratory studies have accurately predicted the growth of dust particles to sizes that are susceptible to accumulation through transport processes like radial drift and settling. $\bullet$ Critical uncertainties in that process remain the level of turbulence, the threshold collision velocities at which dust growth stalls, and the evolution of dust porosity. $\bullet$ Symmetric and asymmetric substructure are widespread. Dust traps appear to be solving several long-standing issues in planet formation models, and they are observationally consistent with being sites of active planetesimal formation. $\bullet$ In some instances, planets have been identified as the causes behind substructures. This underlines the need to study earlier stages of disks to understand how planets can form so rapidly. In the future, better probes of the physical conditions in optically thick regions, including densities, turbulence strength, kinematics, and particle properties will be essential for unraveling the physical processes at play.

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arXiv2023

The impact of dust evolution on the dead zone outer edge in magnetized protoplanetary disks

[Abridged] Aims. We provide an important step toward a better understanding of the magnetorotational instability (MRI)-dust coevolution in protoplanetary disks by presenting a proof of concept that dust evolution ultimately plays a crucial role in the MRI activity. Methods. First, we study how a fixed power-law dust size distribution with varying parameters impacts the MRI activity, especially the steady-state MRI-driven accretion, by employing and improving our previous 1+1D MRI-driven turbulence model. Second, we relax the steady-state accretion assumption in this disk accretion model, and partially couple it to a dust evolution model in order to investigate how the evolution of dust (dynamics and grain growth processes combined) and MRI-driven accretion are intertwined on million-year timescales. Results. Dust coagulation and settling lead to a higher gas ionization degree in the protoplanetary disk, resulting in stronger MRI-driven turbulence as well as a more compact dead zone. On the other hand, fragmentation has an opposite effect because it replenishes the disk in small dust particles. Since the dust content of the disk decreases over million years of evolution due to radial drift, the MRI-driven turbulence overall becomes stronger and the dead zone more compact until the disk dust-gas mixture eventually behaves as a grain-free plasma. Furthermore, our results show that dust evolution alone does not lead to a complete reactivation of the dead zone. Conclusions. The MRI activity evolution (hence the temporal evolution of the MRI-induced $α$-parameter) is controlled by dust evolution and occurs on a timescale of local dust growth, as long as there is enough dust particles in the disk to dominate the recombination process for the ionization chemistry. Once it is no longer the case, it is expected to be controlled by gas evolution and occurs on a viscous evolution timescale.

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arXiv2023

Leaky dust traps: How fragmentation impacts dust filtering by planets

The nucleosynthetic isotope dichotomy between carbonaceous (CC) and non-carbonaceous (NC) meteorites has been interpreted as evidence for spatial separation and the coexistence of two distinct planet-forming reservoirs for several million years in the solar protoplanetary disk. The rapid formation of Jupiter's core within one million years after the formation of calcium-aluminium-rich inclusions (CAIs) has been suggested as a potential mechanism for spatial and temporal separation. In this scenario, Jupiter's core would open a gap in the disk and trap inward-drifting dust grains in the pressure bump at the outer edge of the gap, separating the inner and outer disk materials from each other. We performed simulations of dust particles in a protoplanetary disk with a gap opened by an early-formed Jupiter core, including dust growth and fragmentation as well as dust transport, using the dust evolution software DustPy. Our numerical experiments indicate that particles trapped in the outer edge of the gap rapidly fragment and are transported through the gap, contaminating the inner disk with outer disk material on a timescale that is inconsistent with the meteoritic record. This suggests that other processes must have initiated or at least contributed to the isotopic separation between the inner and outer Solar System.

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