ABSTRACT
Recent ALMA molecular line observations have revealed 3D gas velocity structure in protoplanetary discs, shedding light on mechanisms of disc accretion and structure formation. (1) By ...carrying out viscous simulations, we confirm that the disc’s velocity structure differs dramatically using vertical stress profiles from different accretion mechanisms. Thus, kinematic observations tracing flows at different disc heights can potentially distinguish different accretion mechanisms. On the other hand, the disc surface density evolution is mostly determined by the vertically integrated stress. The sharp disc outer edge constrained by recent kinematic observations can be caused by a radially varying α in the disc. (2) We also study kinematic signatures of a young planet by carrying out 3D planet–disc simulations. The relationship between the planet mass and the ‘kink’ velocity is derived, showing a linear relationship with little dependence on disc viscosity, but some dependence on disc height when the planet is massive (e.g. 10MJ). We predict the ‘kink’ velocities for the potential planets in DSHARP discs. At the gap edge, the azimuthally averaged velocities at different disc heights deviate from the Keplerian velocity at similar amplitudes, and its relationship with the planet mass is consistent with that in 2D simulations. After removing the planet, the azimuthally averaged velocity barely changes within the viscous time-scale, and thus the azimuthally averaged velocity structure at the gap edge is due to the gap itself and not directly caused to the planet. Combining both axisymmetric kinematic observations and the residual ‘kink’ velocity is needed to probe young planets in protoplanetary discs.
Warps and breaks in circumbinary discs Rabago, Ian; Zhu, Zhaohuan; Lubow, Stephen ...
Monthly notices of the Royal Astronomical Society,
07/2024
Journal Article
Recenzirano
Abstract Disc warping, and possibly disc breaking, has been observed in protoplanetary discs around both single and multiple stars. Large warps can break the disc, producing multiple observational ...signatures. In this work, we use comparisons of disc timescales to derive updated formulae for disc breaking, with better predictions as to when and where a disc is expected to break and how many breaks could occur. Disc breaking is more likely for discs with small inner cavities, cooler temperatures, and steeper power-law profiles, such that thin, polar-aligning discs are more likely to break. We test our analytic formulae using 3D grid-based simulations of protoplanetary discs warped by the gravitational torque of an inner binary. We reproduce the expected warp behaviors in different viscosity regimes and observe disc breaking at locations in agreement with our derived equations. As our simulations only show disc breaking when disc viscosity is low, we also consider a viscous criterion for disc breaking, where rapid alignment to the precession vector can prevent a break by reducing the maximum misalignment between neighboring rings. We apply these results to the GW Orionis circumtriple disc, and find that the precession induced from the central stars can break the disc if it is relatively thin. We expect repeated or multiple disc breaking to occur for discs with sufficiently steep power law profiles. We simulate a polar-aligning disc around an eccentric binary with steep power-law profiles, and observe two separate breaking events at locations in rough agreement with our analytical predictions.
ABSTRACT
We describe the first grid-based simulations of the polar alignment of a circumbinary disc. We simulate the evolution of an inclined disc around an eccentric binary using the grid-based code ...athena++
. The use of a grid-based numerical code allows us to explore lower disc viscosities than have been examined in previous studies. We find that the disc aligns to a polar orientation when the α viscosity is high, while discs with lower viscosity nodally precess with little alignment over 1000 binary orbital periods. The time-scales for polar alignment and disc precession are compared as a function of disc viscosity, and are found to be in agreement with previous studies. At very low disc viscosities (e.g. α = 10−5), anticyclonic vortices are observed along the inner edge of the disc. These vortices can persist for thousands of binary orbits, creating azimuthally localized overdensities and multiple pairs of spiral arms. The vortex is formed at ∼3–4 times the binary semimajor axis, close to the inner edge of the disc, and orbits at roughly the local Keplerian speed. The presence of a vortex in the disc may play an important role in the evolution of circumbinary systems, such as driving episodic accretion and accelerating the formation of polar circumbinary planets.
ABSTRACT
A test particle orbit around an eccentric binary has two stationary states in which there is no nodal precession: coplanar and polar. Nodal precession of a misaligned test particle orbit ...centres on one of these stationary states. A low-mass circumbinary disc undergoes the same precession and moves towards one of these states through dissipation within the disc. For a massive particle orbit, the stationary polar alignment occurs at an inclination less than 90°, which is the prograde-polar stationary inclination. A sufficiently high angular momentum particle has an additional higher inclination stationary state, the retrograde-polar stationary inclination. Misaligned particle orbits close to the retrograde-polar stationary inclination are not nested like the orbits close to the other stationary points. We investigate the evolution of a gas disc that begins close to the retrograde-polar stationary inclination. With hydrodynamical disc simulations, we find that the disc moves through the unnested crescent shape precession orbits and eventually moves towards the prograde-polar stationary inclination, thus increasing the parameter space over which circumbinary discs move towards polar alignment. If protoplanetary discs form with an isotropic orientation relative to the binary orbit, then polar discs may be more common than coplanar discs around eccentric binaries, even for massive discs. This has implications for the alignment of circumbinary planets.
Binary stars are common outcomes of the star formation process, with nearly half of Sun-like stars forming as part of a binary pair. The presence of a second star adds additional complexity and ...dynamical effects to the planet formation process in the surrounding circumbinary disk. In this work, I investigate the behavior of a circumbinary disk using hydrodynamic modeling, specifically in the case where the disk is misaligned to the binary orbital plane. Around eccentric binaries, highly inclined disks can align themselves perpendicular to the binary orbital plane. These "polar disks'' can produce vortices and spiral arms when the disk viscosity is low. These vortices may accelerate the formation of polar-aligned circumbinary planets. Precession induced from the binary can distort the disk and cause it to warp in three-dimensional space. I examine the behavior of disk warping around a central binary using both analytic and numerical methods, deriving a new criterion for disk breaking. These criteria show consistency with both simulations and observed disks.
Recent molecular line observations by ALMA have revealed the 3D velocity structure of protoplanetary disks, providing new insight on the mechanisms of disk accretion and ring/gap formation. Although ...the constant α viscous disk model has been widely used to describe the disk surface density evolution, it may not adequately describe the gas velocity at different disk heights. By carrying out viscous hydrodynamic simulations, we confirm that the vertical velocity structure varies considerably between these models. On the other hand, the surface density evolution of the disk is only dependent on the vertically integrated stress, and does not depend on a particular vertical stress profile. We also include a planet in the 3D simulation to study the kinematic signatures of a young protoplanet. The planet-induced gap causes gas at the gap edge to deviate from Keplerian velocity. This deviation is constant in the vertical direction up to three scale heights in the disk, and the relation between the amplitude of the deviation and the planet mass is consistent with previous relationships derived in 2D simulations. Finally, by removing the planet from the simulation, most of the velocity structure is unchanged and is thus a product of the gap and not the planet. The only notable velocity flows induced by the planet appear in the radial velocity component, away from the planet at the midplane. Overall, axisymmetric observations of the disk can sensitively probe the structure of the rings and gaps, but they cannot exclude other mechanisms besides planets as a process for ring/gap formation
Recent ALMA molecular line observations have revealed 3-D gas velocity structure in protoplanetary disks, shedding light on mechanisms of disk accretion and structure formation. 1) By carrying out ...viscous simulations, we confirm that the disk's velocity structure differs dramatically using vertical stress profiles from different accretion mechanisms. Thus, kinematic observations tracing flows at different disk heights can potentially distinguish different accretion mechanisms. On the other hand, the disk surface density evolution is mostly determined by the vertically integrated stress. The sharp disk outer edge constrained by recent kinematic observations can be caused by a radially varying \(\alpha\) in the disk. 2) We also study kinematic signatures of a young planet by carrying out 3-D planet-disk simulations. The relationship between the planet mass and the "kink" velocity is derived, showing a linear relationship with little dependence on disk viscosity, but some dependence on disk height when the planet is massive, e.g. \(10 M_J\). We predict the "kink" velocities for the potential planets in DSHARP disks. At the gap edge, the azimuthally-averaged velocities at different disk heights deviate from the Keplerian velocity at similar amplitudes, and its relationship with the planet mass is consistent with that in 2-D simulations. After removing the planet, the azimuthally-averaged velocity barely changes within the viscous timescale, and thus the azimuthally-averaged velocity structure at the gap edge is due to the gap itself and not directly caused to the planet. Combining both axisymmetric kinematic observations and the residual "kink" velocity is needed to probe young planets in protoplanetary disks.
Disc warping, and possibly disc breaking, has been observed in protoplanetary discs around both single and multiple stars. Large warps can break the disc, producing multiple observational signatures. ...In this work, we use comparisons of disc timescales to derive updated formulae for disc breaking, with better predictions as to when and where a disc is expected to break and how many breaks could occur. Disc breaking is more likely for discs with small inner cavities, cooler temperatures, and steeper power-law profiles, such that thin, polar-aligning discs are more likely to break. We test our analytic formulae using 3D grid-based simulations of protoplanetary discs warped by the gravitational torque of an inner binary. We reproduce the expected warp behaviors in different viscosity regimes and observe disc breaking at locations in agreement with our derived equations. As our simulations only show disc breaking when disc viscosity is low, we also consider a viscous criterion for disc breaking, where rapid alignment to the precession vector can prevent a break by reducing the maximum misalignment between neighboring rings. We apply these results to the GW Orionis circumtriple disc, and find that the precession induced from the central stars can break the disc if it is relatively thin. We expect repeated or multiple disc breaking to occur for discs with sufficiently steep power law profiles. We simulate a polar-aligning disc around an eccentric binary with steep power-law profiles, and observe two separate breaking events at locations in rough agreement with our analytical predictions.
We describe the first grid-based simulations of the polar alignment of a circumbinary disk. We simulate the evolution of an inclined disk around an eccentric binary using the grid-based code ...ATHENA++. The use of a grid-based numerical code allows us to explore lower disk viscosities than have been examined in previous studies. We find that the disk aligns to a polar orientation when the \(\alpha\) viscosity is high, while disks with lower viscosity nodally precess with little alignment over 1000 binary orbital periods. The timescales for polar alignment and disk precession are compared as a function of disk viscosity, and are found to be in agreement with previous studies. At very low disk viscosities (e.g. \(\alpha = 10^{-5}\)), anticyclonic vortices are observed along the inner edge of the disk. These vortices can persist for thousands of binary orbits, creating azimuthally localized overdensities as well as multiple pairs of spiral arms. The vortex is formed at \(\sim 3-4\) times the binary semi-major axis, close to the inner edge of the disk, and orbits at roughly the local Keplerian speed. The presence of a vortex in the disk may play an important role in the evolution of circumbinary systems, such as driving episodic accretion and accelerating the formation of polar circumbinary planets.
A test particle orbit around an eccentric binary has two stationary states in which there is no nodal precession: coplanar and polar. Nodal precession of a misaligned test particle orbit centres on ...one of these stationary states. A low mass circumbinary disc undergoes the same precession and moves towards one of these states through dissipation within the disc. For a massive particle orbit, the stationary polar alignment occurs at an inclination less than \(90^{\circ}\), this is the prograde-polar stationary inclination. A sufficiently high angular momentum particle has an additional higher inclination stationary state, the retrograde-polar stationary inclination. Misaligned particle orbits close to the retrograde-polar stationary inclination are not nested like the orbits close to the other stationary points. We investigate the evolution of a gas disc that begins close to the retrograde-polar stationary inclination. With hydrodynamical disc simulations, we find that the disc moves through the unnested crescent shape precession orbits and eventually moves towards the prograde-polar stationary inclination thus increasing the parameter space over which circumbinary discs move towards polar alignment. If protoplanetary discs form with an isotropic orientation relative to the binary orbit, then polar discs may be more common than coplanar discs around eccentric binaries, even for massive discs. This has implications for the alignment of circumbinary planets.
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