Rebecca G. Martin

The Kozai-Lidov Mechanism in Hydrodynamical Disks

We use three-dimensional hydrodynamical simulations to show that a highly misaligned accretion disk around one component of a binary system can exhibit global Kozai-Lidov cycles, where the inclination and eccentricity of the disk are interchanged periodically. This has important implications for accreting systems on all scales, for example, the formation of planets and satellites in circumstellar and circumplanetary disks, outbursts in X-ray binary systems, and accretion onto supermassive black holes.

Planet–disc evolution and the formation of Kozai–Lidov planets

SHL acknowledges support from NASA grant NNX11AK61G. CN was supported by the Science and Technology Facilities Council (grant number ST/M005917/1). Research in theoretical astrophysics at Leicester is supported by an STFC Consolidated Grant. PJA acknowledges support from NASA through grant NNX13AI58G and from the NSF through grant AST 1313021.

Giant Outbursts in Be/X-RAY Binaries

Be/X-ray binary systems exhibit both periodic (Type I) X-ray outbursts and giant (Type II) outbursts, whose origins have remained elusive. We suggest that Type II X-ray outbursts occur when a highly misaligned decretion disk around the Be star becomes eccentric, allowing the compact object companion to capture a large amount of material at periastron. Using three-dimensional smoothed particle hydrodynamics simulations, we model the long-term evolution of a representative Be/X-ray binary system. We find that periodic (Type I) X-ray outbursts occur when the neutron star is close to periastron for all disk inclinations. Type II outbursts occur for large misalignment angles and are associated with eccentricity growth which occurs on a timescale of about 10 orbital periods. Mass capture from the eccentric decretion disk results in an accretion disk around the neutron star whose estimated viscous time is long enough to explain the extended duration of Type II outbursts. Previous studies suggested that the outbursts are caused by a warped disk but our results suggest that this is not sufficient; the disk must be both highly misaligned and eccentric to initiate a Type II accretion event.

Tidal Warping of Be Star Decretion Discs

Rapidly rotating Be stars are observed as shell stars when the decretion disc is viewed edge on. Transitions between the two implies that the discs may be warped and precessing. Type II X-ray outbursts are thought to occur when the warped disc interacts with the fast stellar wind. We suggest that tides from a misaligned companion neutron star can cause the observed effects. We make numerical models of a Be star decretion disc in which the spin of the Be star is misaligned with the orbital axis of a neutron star companion. Tidal torques from the neutron star truncate the disc at a radius small enough that the neutron star orbit does not intersect the disc unless the eccentricity or misalignment is very large. A magnetic torque from the Be star that is largest at the equator, where the rotation is fastest, is approximated by an inner boundary condition. There are large oscillations in the mass and inclination of the disc as it moves towards a steady state. These large variations may explain the observed changes from Be star to Be shell star and vice versa and also the Type II X-ray outbursts. We find the tidal time-scale on which the disc warps, precesses and reaches a steady state to be around a year up to a few hundred years. If present, the oscillations in mass and disc inclination occur on a fraction of this time-scale depending on the orbital parameters of the binary. The time-scales associated with the tidal torque for observed Be star binaries suggest that these effects are important in all but the longest period binaries.

On the evolution of the snow line in protoplanetary discs – II. Analytic approximations

We examine the evolution of the snow line in a protoplanetary disc that contains a dead zone (a region of zero or low turbulence). The snow line is within a self-gravitating part of the dead zone, and we obtain a fully analytic solution for its radius. Our formula could prove useful for future observational attempts to characterise the demographics of planets outside the snow line. External sources such as comic rays or X-rays from the central star can ionise the disc surface layers and allow the magneto-rotational instability to drive turbulence there. We show that provided that the surface density in this layer is less than about 50 g/cm^2, the dead zone solution exists, after an initial outbursting phase, until the disc is dispersed by photoevaporation. We demonstrate that the snow line radius is significantly larger than that predicted by a fully turbulent disc model, and that in our own solar system it remains outside of the orbital radius of the Earth. Thus, the inclusion of a dead zone into a protoplanetary disc model explains how our Earth formed with very little water.

Tidal Torques on Misaligned Disks in Binary Systems

We extend previous studies of the tidal truncation of coplanar disks in binary systems to the more general case of noncoplanar disks. As in the prograde coplanar case, Lindblad resonances play a key role in tidal truncation. We analyze the tidal torque acting on a misaligned nearly circular disk in a circular orbit binary system. We concentrate on the 2:1 inner Lindblad resonance associated with the m=2 tidal forcing (for azimuthal wavenumber m) that plays a major role in the usual coplanar case. We determine the inclination dependence of this torque, which is approximately cos^8(i/2) for misalignment angle i. Compared to the prograde coplanar case (i=0), this torque decreases by a factor of about 2 for i = pi/6 and by a factor of about 20 for i=pi/2. The Lindblad torque decreases to zero for a tilt angle of pi (counter-rotation), consistent with previous investigations. The effects of higher order resonances associated with m>2 tidal forcing may contribute somewhat, but are much more limited than in the i=0 case. These results suggest that misaligned disks in binary systems can be significantly extended compared to their coplanar counterparts. In cases where a disk is sufficiently inclined and viscous, it can overrun all Lindblad resonances and overflow the Roche lobe of the disk central object.

On the formation and evolution of asteroid belts and their potential significance for life

Suggestions have been made that asteroid belts may be important both for the existence of life and perhaps even for the evolution of complex life on a planet. Using numerical models for protoplanetary discs, we calculate the location of the snow line, and we propose that asteroid belts are most likely to form in its vicinity. We then show that observations of warm dust in exosolar systems, thought to be produced by collisions between asteroids in a belt, indicate that asteroid belts (when they exist) indeed coincide with the radial location and the temperature of the snow line. Giant planets form outside the snow line and prevent planet formation just inside of their orbit, creating an asteroid belt there. However, the migration of giant planets through the asteroid belt likely disperses the compact formation. We examine existing observations of giant exoplanets and find that less than 4 per cent are at radial locations outside of the snow line. This definitely may be the consequence of observational selection effects. However, with this caveat in mind, we point out that the dearth of giant planets outside the snow line may also suggest that compact asteroid belts are not common, and more speculatively that complex life may not be expected in most of the currently observed systems.

The gravo-magneto disc instability with a viscous dead zone

We consider the evolution of accretion discs that contain some turbulence within a disc dead zone, a region about the disc midplane of a disc that is not sufficiently ionised for the magneto-rotational instability (MRI) to drive turbulence. In particular, we determine whether additional sources of turbulence within a dead zone are capable of suppressing gravo-magneto (GM) disc outbursts that arise from a rapid transition from gravitationally produced to MRI produced turbulence. With viscous

FORMATION OF CIRCUMBINARY PLANETS IN A DEAD ZONE

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Propagation of the gravo-magneto disc instability

disc models we consider two mechanisms that may drive turbulence within the dead zone. First, we examine a constant