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Owen J.E.,Canadian Institute for Theoretical Astrophysics
Astrophysical Journal | Year: 2014

We investigate under what circumstances an embedded planet in a protoplanetary disk may sculpt the dust distribution such that it observationally presents as a "transition" disk. We concern ourselves with "transition" disks that have large holes (≳ 10 AU) and high accretion rates (10-9-10-8 M ⊙ yr -1), particularly, those disks which photoevaporative models struggle to explain. Adopting the observed accretion rates in "transition" disks, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small (s ≲ 1 μm) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disk with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a "transition" disk (NIR dip in the spectral energy distribution (SED), millimeter cavity, and high accretion rate). At suitably high planet masses (≳ 3-4 MJ ), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disk that is optically thin to infrared radiation. The planet-disk system will present as a "transition" disk with a dip in the SED only when the planet mass and planetary accretion rate are high enough. At other times, it will present as a disk with a primordial SED, but with a cavity in the millimeter, as observed in a handful of protoplanetary disks. © 2014. The American Astronomical Society. All rights reserved.

Hansen B.M.S.,University of California at Los Angeles | Murray N.,Canadian Institute for Theoretical Astrophysics
Astrophysical Journal | Year: 2012

We demonstrate that the observed distribution of "hot Neptune"/"super-Earth" systems is well reproduced by a model in which planet assembly occurs in situ, with no significant migration post-assembly. This is achieved only if the amount of mass in rocky material is 50-100 M ⊕ interior to 1AU. Such a reservoir of material implies that significant radial migration of solid material takes place, and that it occurs before the stage of final planet assembly. The model not only reproduces the general distribution of mass versus period but also the detailed statistics of multiple planet systems in the sample. We furthermore demonstrate that cores of this size are also likely to meet the criterion to gravitationally capture gas from the nebula, although accretion is rapidly limited by the opening of gaps in the gas disk. If the mass growth is limited by this tidal truncation, then the scenario sketched here naturally produces Neptune-mass objects with substantial components of both rock and gas, as is observed. The quantitative expectations of this scenario are that most planets in the "hot Neptune/super-Earth" class inhabit multiple-planet systems, with characteristic orbital spacings. The model also provides a natural division into gas-rich (hot Neptune) and gas-poor (super-Earth) classes at fixed period. The dividing mass ranges from 3 M ⊕ at 10day orbital periods to 10 M ⊕ at 100day orbital periods. For orbital periods <10days, the division is less clear because a gas atmosphere may be significantly eroded by stellar radiation. © 2012. The American Astronomical Society. All rights reserved..

Lin M.-K.,Canadian Institute for Theoretical Astrophysics
Astrophysical Journal | Year: 2012

Numerical calculations of the linear Rossby wave instability (RWI) in global three-dimensional (3D) disks are presented. The linearized fluid equations are solved for vertically stratified, radially structured disks with either a locally isothermal or polytropic equation of state, by decomposing the vertical dependence of the perturbed hydrodynamic quantities into Hermite and Gegenbauer polynomials, respectively. It is confirmed that the RWI operates in 3D. For perturbations with vertical dependence assumed above, there is little difference in growth rates between 3D and two-dimensional (2D) calculations. Comparison between 2D and 3D solutions of this type suggests the RWI is predominantly a 2D instability and that 3D effects, such as vertical motion, can be interpreted as a perturbative consequence of the dominant 2D flow. The vertical flow around corotation, where vortex formation is expected, is examined. In locally isothermal disks, the expected vortex center remains in approximate vertical hydrostatic equilibrium. For polytropic disks, the vortex center has positive vertical velocity, whose magnitude increases with decreasing polytropic index n. © 2012. The American Astronomical Society. All rights reserved.

Palenzuela C.,Canadian Institute for Theoretical Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

This work presents an implementation of the resistive magnetohydrodynamic equations for a generic algebraic Ohm's law which includes the effects of finite resistivity within full General Relativity. The implementation naturally accounts for magnetic field induced anisotropies and, by adopting a phenomenological current, is able to accurately describe electromagnetic fields in the star and in its magnetosphere. We illustrate the application of this approach in interesting systems with astrophysical implications: the aligned rotator solution and the collapse of a magnetized rotating neutron star to a black hole. © 2013 The Author. Published by Oxford University Press on behalf of the Royal Astronomical Society.

Lin M.K.,Canadian Institute for Theoretical Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

The linear Rossby wave instability (RWI) in global, 3D polytropic discs is revisited with a much simpler numerical method than that previously employed by the author. The governing partial differential equation is solved with finite differences in the radial direction and spectral collocation in the vertical direction. RWI modes are calculated subject to different upper disc boundary conditions. These include free surface, solid boundaries and variable vertical domain size. Boundary conditions that oppose vertical motion increase the instability growth rate by a few per cent. The magnitude of vertical flow throughout the fluid column can be affected but the overall flow pattern is qualitatively unchanged. Numerical results support the notion that the RWI is intrinsically two dimensional. This implies that inconsistent upper disc boundary conditions, such as vanishing enthalpy perturbation, may inhibit the RWI in 3D.© 2012 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.

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