Research

Observations of protoplanetary disks using ALMA have revealed that they often exhibit multiple concentric gap and ring substructures. An exciting possible explanation for these substructures is that they are carved by embedded planets. Recent numerical studies have shown that a single planet can produce multiple rings and gaps in a disk. However, a simplifying assumption often used in these studies, that of a fixed temperature profile, may lead to anomalous results for the structure of the rings and gaps.

In Miranda & Rafikov (2019b), we demonstrated that the effect of a planet on disk substructure formation is overestimated when a fixed temperature profile is assumed, due to a modification of the dynamics of the density waves excited by the planet (see the AAS research highlight on this work).

Next, in Miranda & Rafikov (2020a) , we developed the full linear theory of planet-disk interaction with more realistic thermodynamics, in which the temperature of the gas in the disk relaxes towards a prescribed profile with a finite cooling timescale. By modifying the conservation of the angular momentum flux for the waves excited by the planet, cooling has a substantial influence on the ring/gap structures that are produced. In particular, the formation of multiple gaps is suppressed in favor of a single gap around the orbit of the planet when the cooling timescale falls within a critical range. The code used in this work to produce numerical solutions of the linear perturbation equations for planet-disk interaction with cooling can be found here.

Finally, in Miranda & Rafikov (2020b), we presented a detailed estimate for the cooling timescale in protoplanetary disks, taking into account both cooling from the disk surface as well as along the plane of the disk. By including this realistic cooling prescription in numerical simulations, we explored the variety of planet-induced substructures that arise under different physical conditions for the disk. This work highlights the fact that poorly constrained parameters in disk models, such as temperature, mass and opacity, play important roles in substructure formation by planets, making modeling of this process more uncertain than previously realized.

The interactions of a planets with a disk produces spiral arm patterns. Evidence for these patterns has recently been observed in several protoplanetary disks. Numerical simulations have shown that a single planet can produce multiple spiral arms in the disk, in contrast to the the conventional expectation of a single spiral. These multiple spiral arms are linked to the formation of multiple gaps and rings. In Miranda & Rafikov (2019a), we showed unambiguously that multiple spirals are the result of the interference of wave modes with different azimuthal numbers, as predicted by linear perturbation theory, ruling out more complicated nonlinear mechanisms that have been proposed. Multiple spirals are a generic outcome of wave propagation in disks.

Several gaseous debris disks around white dwarfs, produced by disrupted planets, exhibit variability on timescales of years to tens of years. It has been suggested that the variability is the result of the precession of an eccentric pattern in the disk. In Miranda & Rafikov (2018), we showed that the range of possible precession timescales, resulting from the combined effects of pressure and general relativistic effects, is consistent with the timescales of the observed variabilities. In particular, the rapid 1.4 year variability of HE 1349–2305 can be explained by precession. This strengthens the eccentirc disk hypothesis for the variability of these systems.

Circumbinary disks are found in a variety of astrophysical settings, including around young stellar binaries and supermassive black hole binaries. Binaries surrounded by disks have been thought to shrink as a result of gravitational torques between the binary and disk. In Miranda, Muñoz, and Lai (2017) (see also Muñoz, Miranda, and Lai 2019), we showed that gravitational torques are subdominant to the angular momentum gained by accretion of the disk materal. As a result, accreting binaries tend to experience orbital expansion, rather than orbital shrinkage. This may cast doubt on the role of gas accretion in facilitating the merger of supermassive black hole binaries, i.e., as a solution of the "final parsec problem".

The interaction of low-mass planets with disks leads to rapid inward "Type I" migration, potentially leading to a pileup of planets at the inner disk edge. Migration traps, locations in the disk where Type I migration can be halted, can modify this picture. In Miranda & Lai (2018), following a theoretical idea put forth by Tsang (2011), I tested the ability of a reflective inner disk edge, possibly resulting from magnetospheric truncation of the disk, to halt Type I migration. In 2D simulations, this mechanism can trap sufficiently low-mass planets at several times the inner disk radius, even if the disk viscosity parameter, which tends to weaken the effect, is large.