arXiv subjects

Rebecca G. Martin

Publications and source records attributed to Rebecca G. Martin.

At least 19 recordsLinked to original sources

arXiv2026

On the formation of retrograde S-type planets in binaries with a polar circumbinary disk

Retrograde S-type planets have been observed in several binary systems, yet their formation pathway remains poorly understood. With high-resolution hydrodynamic simulations, we demonstrate that a polar circumbinary disk around an eccentric, unequal-mass binary can form and sustain a retrograde mini disk around the primary star. This provides a direct in-situ formation channel for retrograde S-type planets. The mini disk forms via a sub-Keplerian accretion stream that is slightly misaligned from the polar disk. The mini disk initially undergoes von Zeipel-Kozai-Lidov (ZKL) oscillations, driving coupled eccentricity and inclination evolution. Rather than oscillating indefinitely, the inner mini disk evolves past the critical ZKL inclination, decouples from the outer disk, and settles into a stable retrograde orbit. This evolution is sensitive to numerical resolution: the retrograde configuration is absent in previous lower-resolution simulations, where the mini disk accretion timescale is too short to sustain ZKL-driven evolution. For a higher disk viscosity, the mini disk remains near-polar due to a shorter accretion timescale. Since protoplanetary disks typically have low viscosity, our results suggest that retrograde S-type planets can form in-situ from retrograde mini disks around polar circumbinary disks, and their occurrence rate may be higher than currently estimated.

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arXiv2026

Stability of circumbinary orbits in misaligned triple star systems

We investigate the stability of circumbinary orbits in hierarchical triple star systems, focusing on the effects of a misaligned outer companion star. Test particles are subject to competing gravitational torques from the inner binary and the outer binary companion. With secular theory we estimate the outer radius of particle stability, where the torques balance. We find good agreement with $n$-body simulations across a wide range of triple star configurations. Stable circumbinary orbits can exist even in strongly misaligned triples. Polar and highly inclined orbits with respect to the inner binary can remain stable over a substantial radial range, which is insensitive to the triple star misalignment. Orbits that are close to coplanar or retrograde coplanar to the inner binary are more susceptible to instability when the mutual inclination between the binaries is large. These findings indicate that misaligned and polar circumbinary disks and planets can survive in triple star systems under a broad set of conditions. The analytic criterion identifies where stable material may exist, with implications for the formation and detection of circumbinary planets in multiple-star systems.

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arXiv2026

2D hydrodynamical simulations of Be star decretion disc formation through boundary layer effects

Be stars are massive main-sequence stars rotating close to their breakup rate. They possess a decretion disc of material built up due to mass loss from the star, however, there is not a consensus to the mechanism responsible for the formation of the disc because of their sub-breakup spin rates. We present the first 2D hydrodynamical simulations of the formation of a Be star decretion disc from a rapidly rotating star due to boundary layer effects that reduce the rotation rate of the disc close to the star. In our simulations with a disc aspect ratio of $h/r=0.1$, a decretion disc forms around a star rotating with $80 \%$ of the breakup rate, but fails when rotating at $70 \%$ of the breakup rate. For a thinner disc, a faster stellar spin may be needed to form a dynamically important decretion disc. We also demonstrate good agreement between 1D and 2D models. Although this work does not consider the presence of magnetic fields and the angular momentum transport is through viscosity, our results robustly show a Be star disc may be built up hydrodynamically through boundary layer effects, and may play an essential role in regulating the stellar spin.

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arXiv2026

Positive superhumps in high mass ratio cataclysmic variables driven by apsidal disk precession

Previously it has been assumed that a cataclysmic variable (CV) disk can only become eccentric and display superhumps if the 3:1 resonance is located within the disk. This requires the binary mass ratio to be $q=M_2/M_1\lesssim 0.33$, where $M_1$ is the mass of the white dwarf and $M_2$ is the mass of the companion star. However, several systems with higher mass ratios have been observed to exhibit positive superhumps, posing a challenge to this picture. We present the first 3D hydrodynamic simulations to show that eccentricity growth can occur in CV disks even when the resonance radius lies outside the disk. The finite width of the 3:1 resonance extends into the outer parts of the disk and drives eccentricity. While high mass ratio CVs more commonly show negative superhumps, our linear analysis reveals that the direction of apsidal precession is highly sensitive to the disk outer radius and surface density distribution. Smoothed particle hydrodynamic simulations tend to suppress eccentricity gradients and favor prograde precession. These results provide a natural explanation for positive superhumps in high mass ratio CVs and show that disk structure, rather than the resonance location alone, controls the emergence of superhumps.

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arXiv2026

Circumbinary disk formation through AGB star winds

Circumbinary disks are commonly observed around post-asymptotic giant branch (post-AGB) star binaries, yet their formation and especially their long-term evolution remain unclear. We investigate this process using smoothed-particle hydrodynamics simulations of AGB star wind-binary interactions across different wind velocities, binary eccentricities, and mass ratios. When the wind is fast, or the companion is relatively low-mass, corresponding to the Bondi-Hoyle regime, the outflow remains largely unbound and forms a spiral density pattern. In contrast, slower winds and more massive companions lead to wind Roche-lobe overflow (WRLOF), where a circumsingle disk forms around the companion and efficiently transfers angular momentum to the outflow, producing a circumbinary disk. We perform simulations for over 600 binary orbital periods and find that the resulting disk properties depend sensitively on binary parameters, with higher eccentricities producing more extended and eccentric circumbinary disks, while lower mass companions reduce the disk density and growth rate. We further find that angular momentum transport within the circumbinary disk is dominated by spiral structures and shocks generated by the binary-wind interaction, corresponding to effective stresses comparable to or larger than the imposed viscosity. These results show that wind-binary interactions can naturally generate diverse circumbinary disk morphologies observed in AGB and post-AGB star systems.

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arXiv2026

Resolving Oblique Star-Disk Collisions in Quasi-Periodic Eruptions: Numerical Requirements and the Importance of Geometry

Star-disk collisions have been proposed as a promising mechanism for producing quasi-periodic eruptions (QPEs) in galactic nuclei. Because the stellar atmospheric scale height is orders of magnitude smaller than the stellar radius, studying the shock launching by stars poses a significant numerical challenge. We implement an immersed solid-boundary method in Athena++ to study bow-shock formation and ejecta launching when a solid sphere crosses an accretion disk at supersonic speed. After validating the method against experimental results for solid bodies in uniform flows, we perform two- and three-dimensional adiabatic simulations of star-disk collisions. We find that resolving the bow-shock stand-off distance during the compression phase is essential: under-resolved simulations severely underestimate the ejecta mass and energy. When adequately resolved, the ejecta properties agree well with analytical estimates. We further show that collision geometry plays a critical role. Oblique encounters, which arise naturally due to disk rotation, allow easier shock breakout from the disk's backside and substantially reduce the luminosity contrast between forward and backward ejecta compared to perpendicular collisions. These results emphasize the importance of both numerical resolution and three-dimensional geometry in modeling star-disk collisions and interpreting QPEs.

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arXiv2026

Relativistic Effects on Circumbinary Orbit Stability

With n-body simulations and analytic approximations we study the dynamics and stability of low eccentricity misaligned test particles around binary systems with varying mass fraction and eccentricity. General relativity (GR) plays a primary role in determining the motion of an outer particle since it drives apsidal precession of the binary orbit. The effects of GR can drive particle instability close to the binary orbit, depending upon the binary parameters and the initial inclination of the particle. For the binary parameters we consider, we find instability up to a semimajor axis of about 8 ab, where ab is the binary semimajor axis. In particular, we identify and analyse three different regions of instability that are driven by GR in the phase plane of the initial semimajor axis and the initial inclination of the particle. The results have implications for circumbinary orbits and circumbinary disks on all scales, but are particularly important around supermassive black hole binaries where the effects of GR can be strong.

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arXiv2026

Negative superhumps in cataclysmic variables driven by retrograde apsidal disk precession

Negative superhumps are photometric modulations in cataclysmic variables with periods slightly shorter than the orbital period. They are usually attributed to retrograde nodal precession of a tilted accretion disk, although the origin and persistence of the tilt remains unexplained. We propose instead that negative superhumps arise from retrograde apsidal precession of an eccentric disk. Using linear eccentric disk theory, we show that the direction of apsidal precession is highly sensitive to disk size and temperature, and that pressure effects can drive retrograde precession even in cool disks. In low mass ratio systems where the 3:1 resonance is within the disk, disk expansion during outbursts may produce opposite precession directions in the inner and outer disk, allowing the temporary coexistence of positive and negative superhumps, and driving dissipation in an extended superoutburst. In higher mass ratio systems where the resonance location is outside of the disk, the resonance width can still extend into the outer parts of the disk, excite eccentricity, and drive apsidal precession. This mechanism explains the prevalence of negative superhumps across a wide range of mass ratios and accretion states, without requiring a long-lived disk tilt. It may also explain how positive superhumps can occur in high mass ratio systems if the disk density builds up in the outer parts of the disk.

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arXiv2025

Formation of super-Earths around low-mass stars: evolution of an icy dead zone

Exoplanet observations show that close-in super-Earths are more common around M-dwarfs than around solar mass stars. Since the snow line in a protoplanetary disc plays a crucial role in determining the amount of solid material available for planet formation, we explore the icy regions of protoplanetary discs around stars with masses 0.1, 0.5 and 1 $\rm M_\odot$. In a protoplanetary disc, a dead zone, where the magneto-rotational instability (MRI) is suppressed, provides a quiescent region for solids to settle to the mid-plane and planets to form. Viscosity may be driven in the dead zone by gravitational instability if enough material builds up. Heating from the gravitational instability can trigger the MRI and an accretion outburst onto the star. There may be two icy regions in a disc: (1) far from the star and (2) in the dead zone close to the star. We solve the 1D disc equations to find steady state solutions and time-dependent evolution with different values for the critical surface density in the MRI-active surface layers. Larger surface density in the MRI-active surface layers reduces the extent and lifetime of the inner icy region. The inner icy region in the dead zone around a solar mass star is small and short-lived. Around M-dwarfs, the size of the inner icy region is more persistent and oscillates between the accretion outbursts in the region 0.1-1 au. An extended icy region within the dead zone of a disc around M-dwarfs may promote the formation of more numerous and massive close-in super-Earths.

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arXiv2025

Primordial planet spin driven by boundary layer effects in a decretion disc

Accretion of material from a protoplanetary disc on to a forming giant planet can spin the planet up to close to its breakup rate, $Ω_{\rm b}=(G M_{\rm p}/R_{\rm p}^3)$, where $M_{\rm p}$ is the mass and $R_{\rm p}$ is the radius of the planet. After the protoplanetary disc dissipates, the rapidly rotating planet may eject a decretion (outflowing) disc in a similar way to a Be star. Boundary layer effects in a hydrodynamic disc allow for decretion disc formation at spin rates below the breakup spin rate of the planet. The decretion disc exerts a torque on the planet that slows its spin to an equilibrium value that is sensitive to the planet temperature. By considering steady state circumplanetary decretion disc solutions, we show that the equilibrium spin rate for planets is around $0.4\,Ω_{\rm b}$ for $H/R=0.2$ and around $0.2\,Ω_{\rm b}$ for $H/R=0.3$, where $H$ is the disc scale height at radius $R$. These values are in line with the spins of the giant planets in the solar system and observed exoplanet spins.

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arXiv2025

Planet-disc interactions around eccentric binaries and misaligned ring formation

We explore the evolution of a giant planet that interacts with a circumbinary disc that orbits a misaligned binary by means of analytic models and hydrodynamical simulations. Planet-disc interactions lead to mutual tilt oscillations between the planet and the disc. Even if circumbinary gas discs form with an isotropic mutual misalignment to the binary, planet-disc interactions can cause giant planets to evolve towards coplanar or polar alignment. For a low-mass disc, the binary dominates the dynamical evolution of the planet leading to a wide range of circumbinary planet inclinations. For a high-mass disc, the disc dominates the dynamical evolution of the planet and planet inclinations move towards coplanar or polar alignment to the binary orbit, depending upon the initial disc inclination and the binary eccentricity. In addition, for a high-mass disc ($\sim 50\, M_{\rm p}$) and a high initial disc inclination, the planet can undergo Kozai-Lidov oscillations that can result in the planet being ejected from the system. For initially highly misaligned systems, the non-coplanarity of the planet and the disc can lead to long-lived inner misaligned disc rings that can become highly eccentric.

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arXiv2025

The fraction of polar aligned circumbinary disks

Circumbinary gas disks that are misaligned to the binary orbital plane evolve toward either a coplanar or a polar-aligned configuration with respect to the binary host. The preferred alignment depends on the dynamics of the disk: whether it undergoes librating or circulating nodal precession, with librating disks evolving to polar inclinations and circulating disks evolving to coplanar. We quantify the fraction of binary star systems whose disks are expected to have polar orbits $f_\text{polar}$, extending previous work to include disks with non-zero mass. Our results suggest that, for low mass disks, the polar fraction is highly sensitive to the distribution of binary eccentricity with a higher fraction expected for higher binary eccentricities, $f_{\rm polar}\sim e_{\rm b}$. However, for massive discs, the fraction is independent of the binary eccentricity and $f_{\rm polar}\approx 0.37$. The value of $f_\text{polar}$ is always reduced in a population with a greater preference for low initial mutual inclination. We also explore the consequences of the finite lifetime and non-zero radial extent of a real disk, both of which affect a disk's ability to complete its evolution to a stationary configuration. Our findings can be used to make predictions given populations with well-understood distributions of binary eccentricity, initial mutual inclination, and disk angular momentum.

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arXiv2025

Excitation of post-AGB Star Binary Eccentricity by Massive Polar-Aligned Circumbinary Disks

Many post-AGB star binaries are observed to have relatively high orbital eccentricities (up to 0.6). Recently, AC Her was observed to have a polar-aligned circumbinary disk. We perform hydrodynamic simulations to explore the impact of a polar-aligned disk on the eccentricity of a binary. For a binary system with central masses of 0.73 M_sun and 1.4 M_sun, we find that a disk with a total mass of 0.1 M_sun can enhance the binary eccentricity from 0.2 to 0.7 within 5000 years, or from 0.01 to 0.65 within 15000 years. Even if the disk mass is as low as 0.01 M_sun, the binary eccentricity grows within our simulation time while the system remains stable. These eccentricity variations are associated with the variations of the inclination between the disk and the binary orbit due to von Zeipel-Kozai-Lidov oscillations. The oscillations eventually damp and leave the binary eccentricity at a high value. The numerical results are in good agreement with analytical estimates. In addition, we examine the AC-Her system and find that the disk mass should be on the order of 10^(-3)M_sun for the disk to remain polar.

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arXiv2025

Disc breaking through forced eccentricity growth

Motivated by misaligned discs observed in eccentric orbit Be/X-ray binaries, we examine the evolution of a retrograde disc around one component of an eccentric binary with hydrodynamic simulations, $n$-body simulations and linear theory. Forced eccentricity growth from the eccentric orbit binary causes the initially circular disk to undergo eccentricity oscillations. A retrograde disc becomes more radially extended, more highly eccentric and undergoes more rapid apsidal precession compared to a prograde disc. We find that a retrograde disc can be subject to disc breaking where the disc forms two rings with different eccentricities and longitude of periastrons while remaining coplanar. This could have implications for the lightcurves and the X-ray outbursts observed in Be/X-ray binaries.

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arXiv2025

Polar circumtriple planets and disks around misaligned hierarchical triple stars

Observations of hierarchical triple star systems show that misalignments are common both between the angular momentum vector of the inner binary and the outer companion orbit, and between the outer binary orbit and a circumtriple gas disk. With analytic methods and n-body simulations we explore the dynamics of circumtriple orbits around a misaligned hierarchical triple star. Circumtriple test particle orbits nodally precess either about the outer binary angular momentum vector (circulating orbits) or about a stationary inclination that depends upon the binary properties (librating orbits). For a coplanar (or retrograde coplanar) triple star, the apsidal precession rate is maximal and the critical orbital radius outside of which all orbits are circulating is minimal. Polar alignment of a circumtriple gas disk requires nodal libration and therefore it can be more likely if there is a large misalignment between the inner and outer binary orbits. There are two values of the mutual misalignment, i_c and 180-i_c, for which the apsidal precession rate of the triple star is zero and polar alignment is possible at all orbital radii. For a circular inner binary orbit i_c=55, and it changes with eccentricity of the inner binary while being insensitive to other triple star parameters.

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arXiv2025

Formation of Be star decretion discs through boundary layer effects

Be stars are rapidly rotating, with angular frequency around $0.7-0.8$ of their Keplerian break up frequency, as a result of significant accretion during the earlier stellar evolution of a companion star. Material from the equator of the Be star is ejected and forms a decretion disc, although the mechanism for the disc formation has remained elusive. We find one-dimensional steady state decretion disc solutions that smoothly transition from a rapidly rotating star that is in hydrostatic balance. Boundary layer effects in a geometrically thick disc which connects to a rotationally flattened star enable the formation of a decretion disc at stellar spin rates below the break up rate. For a disc with an aspect ratio $H/R\approx 0.1$ at the inner edge, the torque from the disc on the star slows the stellar spin to the observed range and mass ejection continues at a rate consistent with observed decretion rates. The critical rotation rate, to which the star slows down to, decreases as the disc aspect ratio increases. More generally, steady state accretion and decretion disc solutions can be found for all stellar spin rates. The outcome for a particular system depends upon the balance between the decretion rate and any external infall accretion rate.

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arXiv2025

Observational Signatures of a Previous Dynamical Instability in Multi-planet M-Dwarf Systems

We identify observational signatures suggesting a history of dynamical instability in 26 out of 34 M-dwarf multi-planet systems containing no large planets. These systems may have primarily formed in a gas-rich environment, potentially hosted more planets and were more compact. We extend previous simulations of the formation of the TRAPPIST-1 system to 100 Myr to test the stability of these systems without gas. We find the absence of a strong mean motion resonance in the innermost planet pair and the absence of three body resonances throughout the system are likely to result in the merging and ejection of planets after the gas disk disperses. The runs that experience such an instability tend to produce final systems with lower multiplicities, period ratios larger than two, increased orbital spacings, higher planetary angular momentum deficits, and slightly smaller mass ratios between adjacent planets. Remarkably, we find these same trends in the observations of M-dwarf multi-planet systems containing no large planets. Our work allows us to identify specific systems that may have experienced an instability and suggests that only ~25% of these systems formed in their current observed state while most systems were likely more compact and multiplicitous earlier in time. Previous research indicates that systems that have experienced a late stage giant impact may host planets potentially more habitable than the systems that did not. With this in mind, we suggest systems around M-dwarfs that contain period ratios larger than two be given priority in the search for habitable worlds.

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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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