arXiv subjects

Jeremy L. Smallwood

Publications and source records attributed to Jeremy L. Smallwood.

At least 19 recordsLinked to original sources

arXiv2026

Dust Growth in Binary Systems: Inhibition of dust settling and growth in circumbinary discs

Stellar multiplicity alters the density structure of protoplanetary discs and thereby the initial conditions for planet formation. Yet, the interplay between companion-disc interactions and dust growth remains poorly understood. The goal of this work is to investigate to what extent the density structure of a disc undergoing tidal interactions with a companion star promotes or inhibits the growth of dust grains. We perform a set of hydrodynamical simulations of protoplanetary discs orbiting one or both stars of a binary, including dust growth and fragmentation. We explore a range of companion orbits and compare the results with a single-star reference case. We find that dust growth is mainly driven by local accumulations of dust. In circumbinary discs, the maximum grain size is up to five times smaller than in isolated discs. This result likely originates from the perturbations caused by the inner binary, which prevent dust grains from properly settling and drifting. As a consequence, the conditions required to trigger strong clumping driven by the streaming instability are difficult to achieve. In contrast, circumstellar discs in binary systems exhibit grain sizes similar to those in isolated discs, leading to comparable conditions for strong clumping by the streaming instability. Planet formation through core accretion seems challenging in circumbinary discs harbouring binaries larger than a few au, suggesting that circumbinary planets observed near the dynamical stability limit did not form in situ. Conversely, perturbations from external companions only marginally affect density-driven dust growth compared to isolated systems.

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arXiv2026

Misaligned circumbinary discs around unequal-mass eccentric binaries: alignment, morphology, and binary accretion variability

Binary systems are ubiquitous in the Universe and often host circumbinary discs that are misaligned with the binary orbital plane. Such misalignments can affect disc evolution and binary accretion variability. We here present 3D hydrodynamical simulations of circumbinary discs with initial tilts $i_0$ from $0^\circ$ to $180^\circ$, around eccentric binaries with secondary-to-primary mass ratios of $0.11-0.67$. We find that both the initial tilt and mass ratio can affect the long-term accretion variability in our simulations. Discs evolving towards polar and coplanar retrograde generally favour accretion onto the primary star, while discs evolving towards coplanar prograde generally favour accretion onto the secondary. We find preferential accretion ratio $η=\langle\dot{M_2}\rangle/\langle\dot{M_\mathrm{b}}\rangle$ to be a non-monotonic function of the mass ratio. For discs close to coplanar prograde alignment, $η$ increases with decreasing mass ratio, whereas for discs with $30^\circ \le i_0 \le 135^\circ$, $η$ decreases for smaller mass ratios. Polar discs show the lowest mass loss rates, slightly lower than those of coplanar prograde discs, while retrograde discs lose mass faster than their prograde counterparts. Discs that undergo strong warping or breaking experience rapid mass loss. Our findings provide insights into observed circumbinary discs and have implications for circumbinary planet formation.

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arXiv2026

Evolution of dust in a protoplanetary disc driven by stellar flybys: implications for the streaming instability

Stellar flybys are a common dynamical process in young stellar clusters and can significantly reshape protoplanetary discs. However, their impact on dust dynamics remains poorly understood, particularly in the weakly coupled regime (St$\gg$1). We present three-dimensional hydrodynamical simulations of parabolic stellar flybys-both coplanar and inclined-interacting with a gaseous and dusty protoplanetary disc. Dust species with Stokes numbers ranging from 15 to 100, corresponding to four grain sizes under a uniform initial gas surface density, are included. Perturber masses of 0.1 and 1$\mathrm{M}_{\odot}$ are considered. The induced spiral structures exhibit distinct dynamical behaviours in gas and dust: dust spirals retain a nearly constant pattern speed, while gas spirals gradually decelerate. The pitch angles of both components decrease over time, with dust evolving more rapidly. In the weakly coupled regime, gas and dust spirals are spatially offset, facilitating dust accumulation around both structures. Equal-mass flybys truncate the disc at approximately $\sim$0.55$r_{\mathrm{Hill}}$, producing tightly wound, ring-like spirals that promote dust concentration. By mapping the streaming instability growth rates in the solid abundance-Stokes number space across three evolutionary phases, we find that a low-mass flyby suppresses dust concentration below the critical clumping threshold after periastron and maintains this suppression over time, indicating long-lasting inhibition of dust clumping. An equal-mass flyby raises local solid abundance well above the threshold, suggesting that such encounters may foster conditions favourable for dust clumping. Flyby-induced spirals play a central role in shaping dust evolution, leading to distinct spatial and temporal behaviours in weakly coupled discs.

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arXiv2025

The polar debris disc around 99 Herculis: A potential signpost for polar circumbinary planets

The nearby binary star system 99 Herculis (99 Her) is host to the only known polar-aligned circumbinary debris disc. We investigate the hypothesis that the narrow structure of this circumbinary disc is sculpted by the gravitational influence of one or more unseen polar circumbinary planets. We first establish the theoretically viable parameter space for a sculpting planet by considering dynamical stability and clearing mechanisms, including the chaotic zone, Hill radius, diffusion, and polar alignment timescales. We then use $N$-body simulations to test three specific architectures: a single planet interior to the disc, a single planet exterior, and a two-planet system bracketing the disc. Our simulations demonstrate that single-planet models are insufficient to reproduce the observed morphology, as they can only truncate one edge of the disc while leaving the other dynamically extended. In contrast, the two-planet shepherding model successfully carves both the inner and outer edges, confining the debris into a narrow, stable polar ring consistent with observations. We conclude that the structure of the 99 Her debris disc is most plausibly explained by the presence of two shepherding, polar circumbinary planets. We present a specific, testable model for this unique system, which elucidates the pivotal role of planetary bodies in sculpting the architecture of debris discs.

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arXiv2025

Polar alignment of a circumbinary disc around a brown dwarf binary

Inspired by recent observations suggesting that the retrograde precession of the brown dwarf binary 2M1510 AB is consistent with induction by a polar circumbinary planet, we investigate the formation of such planets by studying the evolution of a primordial misaligned circumbinary disc around a brown dwarf binary. Analytical calculations show that a critical tilt angle of $i_{\rm crit} \gtrsim 50^\circ$ for moderately eccentric binaries is needed for polar alignment of circumbinary discs in systems with low disc-to-binary angular momentum ratios. For higher ratios, this angle converges to the Kozai-Lidov instability threshold of $\sim 39^\circ$. We identify disc parameters, such as viscosity ($α= 10^{-4}$) and aspect ratio ($H/r = 0.05$), that enable polar alignment within typical disc lifetimes. Notably, a circumbinary disc around a low-mass binary, such as a brown dwarf binary, will require more time to achieve polar alignment compared to higher-mass systems. A hydrodynamical simulation confirms that an initially inclined disc around a brown dwarf evolves towards a polar state, creating favorable conditions for polar planet formation. Using these results, we finish by placing 2M1510 AB into a wider context and speculate why such a polar circumbinary configuration has not been identified before.

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arXiv2025

Disc-planet misalignment from an unstable triple system: IRAS04125

The IRAS01425+2902 wide binary system was recently reported to have both a young planet and a puzzling geometric arrangement, where the planet and binary both orbit edge-on, but misaligned by 60 deg to the circumprimary disc. This is the youngest transiting planet yet to be detected but its misalignment to the disc is difficult to explain. In this paper we explore the dissolution of an unstable triple system as a potential mechanism to produce this system. We simulate the effects of an ejection interaction in models using a highly inclined, retrograde flyby centred on the primary star of IRAS01425. The escaping star of ~0.35 solar masses inclines both the disc and binary orbits such that they have a relative misalignment of greater than 60 deg, as inferred from observations. The planet orbit also becomes inclined relative to the disc, and our interpretation predicts that the binary should have a highly eccentric orbit (e > 0.5 from our simulations). We additionally demonstrate that despite the high relative misalignment of the disc it is unlikely to be vulnerable to von Zeipel-Kozai-Lidov oscillations.

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arXiv2024

Shedding light on the origin of the broken misaligned circumtriple disk around GW Ori

We revisit the origin of the observed misaligned rings in the circumtriple disk around GW Ori. Previous studies appeared to disagree on whether disk breaking is caused by the differential precession driven in the disk by the triple star system. In this letter, we show that the previous studies are in agreement with each other when using the same set of parameters. But for observationally motivated parameters of a typical protoplanetary disk, the disk is unlikely to break due to interactions with the triple star system. We run 3-dimensional hydrodynamical simulations of a circumtriple disk around GW Ori with different disk aspect ratios. For a disk aspect ratio typical of protoplanetary disks, $H/r \gtrsim 0.05$, the disk does not break. An alternative scenario for the gap's origin consistent with the expected disk aspect ratio involves the presence of giant circumtriple planets orbiting GW Ori.

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arXiv2024

Circumbinary accretion as a diagnostic for binary--disc misalignment

Binary star systems can accrete material originating from a circumbinary disc. Since it is common for the circumbinary disc to be tilted with respect to the binary orbital plane, we test whether the accretion dynamics can be a diagnostic for binary-disc misalignment. We present hydrodynamical simulations to model the accretion flow from a circumbinary disc around an eccentric binary with initial tilts ranging from $0^\circ$ to $180^\circ$ in increments of $15^\circ$. Based on the initial tilt, the circumbinary disc will align towards three different configurations: prograde coplanar, polar, or retrograde coplanar. For discs with initial tilts evolving towards prograde coplanar alignment, the accretion rates onto the primary and secondary stars exhibit alternating preferential accretion. Circumbinary discs evolving towards polar alignment exhibit no alternating preferential accretion onto the binary unless the initial tilt is close to the critical tilt that sets the boundary between coplanar or polar alignment. Such cases cause strong disc warping, leading to disc breaking. The inner disc becomes eccentric, leading to alternating preferential accretion onto the binary. As the break propagates outward, the disc tilt damps towards a polar state and the disc eccentricity decreases. As the disc re-circularizes, the accretion rate transitions back from alternating preferential accretion to non-alternating accretion. Lastly, no alternating preferential accretion exists for discs undergoing retrograde coplanar alignment. From the summary of the accretion rates from our suite of SPH simulations, it is evident that the accretion rate evolution can be affected by the initial tilt and subsequent evolution of the circumbinary disc.

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arXiv2024

Observational Signatures of Dust Traffic Jams in Polar-Aligning Circumbinary Disks

Misaligned circumbinary disks will produce dust traffic jams during alignment or anti-alignment to the binary orbital plane. We conduct a hydrodynamical simulation of an initially misaligned circumbinary disk undergoing polar alignment with multiple dust species. Due to differential precession between the gas and dust components, multiple dust traffic jams are produced within the disk during polar alignment. The radial locations of the dust traffic jams depend on the Stokes number of the grains, which depends on grain size. We compute the dust temperature structure using post-processing radiative transfer to produce continuum images at cm-wavelengths. Multiple distinct rings emerge in the continuum images, corresponding to the dust traffic jams. The angular resolution of upcoming observations from SKA and ngVLA will be sufficient to detect centimeter-sized grains in protoplanetary disks and resolve the widths of dust traffic jams. Therefore, dust traffic jams resulting from the differential precession of gas and dust in misaligned circumbinary disks will be a prime target for more extended wavelength observations.

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arXiv2024

Polar alignment of a dusty circumbinary disc -- II. Application to 99 Herculis

We investigate the formation of dust traffic jams in polar-aligning circumbinary discs. In our first paper, we found as the circumbinary disc evolves towards a polar configuration perpendicular to the binary orbital plane, the differential precession between the gas and dust components leads to multiple dust traffic jams. These dust traffic jams evolve to form a coherent dust ring. In part two, we use 3D smoothed particle hydrodynamical simulations of gas and dust to model an initially highly misaligned circumbinary disc around the 99 Herculis (99 Her) binary system. Our results reveal that the formation of these dust rings is observed across various disc parameters, including the disc aspect ratio, viscosity, surface density power law index, and temperature power law index. The dust traffic jams are long-lived and persist even when the disc is fully aligned polar. The midplane dust-to-gas ratio within the rings can surpass unity, which may be a favourable environment for planetesimal formation. Using 2D inviscid shearing box calculations with parameters from our 3D simulations, we find streaming instability modes with significant growth rates. The streaming instability growth timescale is less than the tilt oscillation timescale during the alignment process. Therefore, the dust ring will survive once the gas disc aligns polar, suggesting that the streaming instability may aid in forming polar planets around 99 Her.

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arXiv2024

Polar alignment of a dusty circumbinary disc -- I. Dust ring formation

We investigate the formation of dust traffic jams in polar-aligning circumbinary discs. We use 3D smoothed particle hydrodynamical simulations of both gas and dust to model an initially highly misaligned circumbinary disc around an eccentric binary. As the circumbinary disc evolves to a polar configuration (perpendicular to the binary orbital plane), the difference in the precession between the gas and dust produces dust traffic jams, which become dense dust rings. We find the formation of dust rings exists for different Stokes number, binary eccentricity, and initial disc tilt. Dust rings are only produced while the circumbinary disc is misaligned to the binary orbital plane. When the disc becomes polar aligned, the dust rings are still present and long-lived. Once these dust rings are formed, they drift inward. The drift timescale depends on the Stokes number. The lower the Stokes number, the faster the dust ring drifts near the inner edge of the disc. The dust rings will have an increased midplane dust-to-go ratio, which may be a favourable environment for the steaming instability to operate.

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arXiv2023

Outcomes of Planetary Collisions: Importance of Gravity and Material Properties

The final sizes, composition, and angular momenta of solid planetary bodies depend on the outcomes of collisions between planetary embryos. The most common numerical method for simulating embryo collisions is to combine a gravity solver with a hydrodynamic solver, allowing pressure gradients, shock waves, and gravitational torques to loft material into orbit. Here, we perform the first direct comparison between hydrodynamic methods and a simplified method employing only gravity and a quadratic repulsive force. The formation of Earth's Moon, perhaps the most heavily simulated planetary collision, is used as a test case. Many of the main features of a collision between two planetary embryos, including collisions in which an orbiting disc of material and/or intact moons are formed, are controlled solely by gravitational forces. Comparison of the methods shows that the mass and orbit of the satellite, as well as the extent of physical mixing between the protoearth and impactor, are similar regardless of the inclusion of the inclusion of hydrodynamic effects or the equation of state employed. The study of thermal and chemical effects of the impact, and determining the time scale for lunar accretion, still require a full hydrodynamic calculation. The simplified gravity plus quadratic repulsive force approach allows rapid testing of various initial conditions to identify cases for further detailed study.

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arXiv2023

Formation of misaligned second-generation discs through flyby encounters

Observations reveal protoplanetary discs being perturbed by flyby candidates. We simulate a scenario where an unbound perturber, i.e., a flyby, undergoes an inclined grazing encounter, capturing material and forming a second-generation protoplanetary disc. We run $N$--body and three-dimensional hydrodynamical simulations of a parabolic flyby grazing a particle disc and a gas-rich protoplanetary disc, respectively. In both our $N$--body and hydrodynamic simulations, we find that the captured, second-generation disc forms at a tilt twice the initial flyby tilt. This relationship is robust to variations in the flyby's tilt, position angle, periastron, and mass. We extend this concept by also simulating the case where the flyby has a disc of material prior to the encounter but we do not find the same trend. An inclined disc with respect to the primary disc around a misaligned flyby is tilted by a few degrees, remaining close to its initial disc tilt. Therefore, if a disc is present around the flyby before the encounter, the disc may not tilt up to twice the perturber tilt depending on the balance between the angular momentum of the circumsecondary disc and captured particles. In the case where the perturber has no initial disc, analyzing the orientation of these second-generation discs can give information about the orbital properties of the flyby encounter.

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arXiv2023

Formation of the warped debris disc around $β$ Pictoris

In light of the recent confirmation of an eccentric orbit giant planet, $β$ Pic c, I revisit the formation and evolution of the warped debris disc in the system. $β$ Pic c is interior to $β$ Pic b, and the debris disc is exterior to both planets. Previous $N$-body simulations have shown that $β$ Pic b is responsible for exciting the inclination of the debris disc. With hydrodynamical simulations, I model a protoplanetary gas disc misaligned with the planets. I find that the gas disc does not exhibit significant long lasting inclination excitation from the planets even for the observed disc size. The warp that is excited by the planets propagates through the entire disc with a timescale much less than the gas disc lifetime. Therefore, the observed warp in the debris disc must be produced after the gas disc has dispersed. With analytical secular theory calculations, I show that two secular resonances are exterior to $β$ Pic b, located at $\sim 20\, \rm au$ and $\sim 25\, \rm au$. This agrees with my $N$-body simulations that show that these secular resonances shape the inner edge of the $β$ Pic debris disc at a radius that agrees with observations.

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arXiv2023

Exciting spiral arms in protoplanetary discs from flybys

Spiral arms are observed in numerous protoplanetary discs. These spiral arms can be excited by companions, either on bound or unbound orbits. We simulate a scenario where an unbound perturber, i.e. a flyby, excites spiral arms during a periastron passage. We run three-dimensional hydrodynamical simulations of a parabolic flyby encountering a gaseous protoplanetary disc. The perturber mass ranges from $10\, \rm M_J$ to $1\, \rm M_{\odot}$. The perturber excites a two-armed spiral structure, with a more prominent spiral feature for higher mass perturbers. The two arms evolve over time, eventually winding up, consistent with previous works. We focus on analysing the pattern speed and pitch angle of these spirals during the whole process. The initial pattern speed of the two arms are close to the angular velocity of the perturber at periastron, and then it decreases over time. The pitch angle also decreases over time as the spiral winds up. The spirals disappear after several local orbital times. An inclined prograde orbit flyby induces similar disc substructures as a coplanar flyby. A solar-mass flyby event causes increased eccentricity growth in the protoplanetary disc, leading to an eccentric disc structure which dampens over time. The spirals' morphology and the disc eccentricity can be used to search for potential unbound stars or planets around discs where a flyby is suspected. Future disc observations at high resolution and dedicated surveys will help to constrain the frequency of such stellar encounters in nearby star-forming regions.

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arXiv2023

Formation of polar circumstellar discs in binary star systems

We investigate the flow of material from highly misaligned and polar circumbinary discs that feed the formation of circumstellar discs around each binary component. With three-dimensional hydrodynamic simulations we consider equal mass binaries with low eccentricity. We also simulate inclined test particles and highly-misaligned circumstellar discs around one binary component for comparison. During Kozai-Lidov (KL) cycles, the circumstellar disc structure is altered through exchanges of disc eccentricity with disc tilt. Highly inclined circumstellar discs and test particles around individual binary components can experience very strong KL oscillations. The continuous accretion of highly misaligned material from the circumbinary disc allows the KL oscillations of circumstellar discs to be long-lived. In this process, the circumbinary material is continuously delivered with a high inclination to the lower inclination circumstellar discs. We find that the simulation resolution is important for modeling the longevity of the KL oscillations. An initially polar circumbinary disc forms nearly polar, circumstellar discs that undergo KL cycles. The gas steams accreting onto the polar circumstellar discs vary in tilt during each binary orbital period, which determines how much material is accreted onto the discs. The long-lived KL cycles in polar circumstellar discs may lead to the formation of polar S-type planets in binary star systems.

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arXiv2022

Accretion onto a binary from a polar circumbinary disc

We present hydrodynamical simulations to model the accretion flow from a polar circumbinary disc onto a high eccentricity ($e=0.78$) binary star system with near unity mass ratio ($q=0.83$), as a model for binary HD 98800 BaBb. We compare the polar circumbinary disc accretion flow with the previously studied coplanar case. In the coplanar case, the circumbinary disc becomes eccentric and the accretion alternates from being dominant onto one binary member to the other. For the polar disc case involving a highly eccentric binary, we find that the circumbinary disc retains its initially low eccentricity and that the primary star accretion rate is always about the same as the secondary star accretion rate. Recent observations of the binary HD 98800 BaBb, which has a polar circumbinary disc, have been used to determine the value of the $\rm Hα$ flux from the brighter component. From this value, we infer that the accretion rate is much lower than for typical T Tauri stars. The eccentric orbit of the outer companion HD 98800 A increases the accretion rate onto HD 98800 B by $\sim 20$ per cent after each periastron passage. Our hydrodynamical simulations are unable to explain such a low accretion rate unless the disc viscosity parameter is very small, $α< 10^{-5}$. Additional observations of this system would be useful to check on this low accretion rate.

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arXiv2021

GW Ori: circumtriple rings and planets

GW Ori is a hierarchical triple star system with a misaligned circumtriple protoplanetary disc. Recent ALMA observations have identified three dust rings with a prominent gap at $100\, \rm au$ and misalignments between each of the rings. A break in the gas disc may be driven either by the torque from the triple star system or a planet that is massive enough to carve a gap in the disc. Once the disc is broken, the rings nodally precess on different timescales and become misaligned. We investigate the origins of the dust rings by means of $N$-body integrations and 3-dimensional hydrodynamic simulations. We find that for observationally-motivated parameters of protoplanetary discs, the disc does not break due to the torque from the star system. We suggest that the presence of a massive planet (or planets) in the disc separates the inner and outer disc. We conclude that the disc breaking in GW Ori is likely caused by undetected planets -- the first planet(s) in a circumtriple orbit.

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