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Research

At Stampede's dedication, Austin TX, April 2013. Each of these towers has 3,000 processors.

My current research interests revolve around the processes taking place in circumstellar disks, with emphasis on their importance for planet formation. As these disks are gaseous, hydrodynamics is a component of paramount importance. Since the central equation of hydrodynamics (the Navier-Stokes equation) is non-linear, we have to resort to numerical simulations in order to progress in our understanding. You can find here some movies of my computer models, along with brief explanations of the research highlights, and links to the respective publications.

Most simulations were done with the Pencil Code, an open source, collaborative, a multipurpose high-order finite-difference magnetohydrodynamics code. I'm a co-owner and co-developer of Pencil since 2005.

The videos are also available in my YouTube channel.

 

 

Planet formation

The current paradigm in planet formation theory is developed around a hierarchical growth of solid bodies, from interstellar dust grains to rocky planetary cores. A particularly difficult phase in the process is the growth from cm-size pebbles to planetary embryos of the size of our Moon or Mars. Subject to angular momentum loss through nebular drag, peebles are expected to drift extremely rapid in a protoplanetary disk, so that they would generally fall into the central star well before larger bodies can form.

Ways around this problem have focused on the development of pressure traps in the disk. The way these pressure traps work are shown in the figures below. While a negative pressure gradient slows down the gas, a positive one speeds it up. Particles thus feel a headwind in a negative pressure gradient, and a tailwind in a positive one. The result is that particles drift toward pressure maxima.  

 
 

 
Turbulence is a way to generate several of such pressure maxima. The simulation below shows the saturated state of turbulence brought about by magnetorotational instability (MRI) in a 3D stratified wedge of a protoplanetary disk. The quantity shown in the ratio of thermal to magnetic pressure (known as "plasma beta").  

 

 

 
The movie below shows the behaviour of pebbles in MRI turbulence. Structure forms as several transient high pressure regions form as a result of the turbulence, trapping particles in them. The color code refers to the bulk density of solids. The bright (yellow-white) is saturated, meaning that these areas are much denser than they appear.  

 

Publication: Lyra et al. 2008a, A&A, 479, 883. [arXiv] [ADS]
 

 
But turbulence is not present throughout the whole extent of the disk. Because sufficient ionization is a main requirement, a significant portion of the cold, dense disk, will be neutral and therefore unmagnetized. In this region, the disk will be "dead" to the MRI and we can expect laminar motion. At the boundary between active and dead zone, however, the transition in turbulent viscosity will trigger Rossy wave instability (RWI), the form that the Kelvin-Helmholtz instability takes in differentially rotating disks. The saturated state of the RWI are vortices, that are very effective in concentrating particles. The figure below shows the particle concentration that results from vortex trapping in this scenario. It leads, in this model, to the formation of over three hundred bound clumps of particles -- 20 of them more massive than Mars.  

 

Publications: Lyra et al. 2008b, A&A, 491, L41.[A&A highlight] [arXiv] [ADS]
Publications: Lyra et al. 2009a, A&A, 497, 869. [arXiv] [ADS]  

 
In the video below, a Jupiter-mass planet is perturbing a planetesimal disk of meter-sized bodies. Particles are trapped in the Lagrangian points of the planet and at the border of the gas gap, where large vortices form. In 50 orbits the particles achieve critical density and collapse. The mass attained by the most massive planet is around 5 Earth masses.  

 


Publication: Lyra et al 2009b, A&A, 493, 1125.
[A&A cover] [arXiv] [ADS]

 

 

Disk vortices

Vortices are hydrodynamical features similar to the Great Red Spot. Long hypothesized to occur in protoplanetary disks, vortices have two properties that make them attractive locations for planet formation. The first is that they are equilibrium solutions to the equations of hydrodynamics, which makes them persistent structures. The second is that under the influence of the drag force, solid bodies inside a vortex experience a radial net force, making them sink to its center. This has the convenient side effect of dramatically enhancing the solids-to-gas ratio locally.

The image below is a high resolution model of 2D vortices in disks, with particles. The upper left panel shows the vorticity in the gas phase. The lower left panels the particles in a model without drag force backreaction; the right panel includes backreaction (which becomes important as St approaches 1). Reproduced from the thesis work of Natalie Raettig, with my co-supervision (Raettig, 2013; also Raettig et al. 2013, 2015).


 

However, although an inverse cascade makes it simple to form and sustain vortices in two dimensions from small scale noise, in three dimensions the direct cascade dissipates their energy fairly rapidly. Therefore the vorticity lost must be replenished by some other mechanism. Two such mechanisms are known, and described below.


 

 

Rossby wave instability

As accretion in protoplanetary disks is enabled by turbulent viscosity, the border between active and inactive (dead) zones constitutes a location where there is an abrupt change in the accretion flow. The gas accumulation that ensues constitutes a localized pressure bump, that behaves as a resonant cavity. Waves are trapped in this cavity if the density transition is sufficiently sharp. For a smooth jump the waves will tunnel away, but for sharp, high amplitude jumps, bound states exist. In resonance, the wave amplitudes will grow exponentially. The saturated state are anticyclonic vortices.

The movie below show the transition between MRI active (outer disk) and MRI dead (inner disk) zones. The transition is as smooth as 15 scale heights in resistivity (from 10-30 AU), following the smooth decrease in ionization due to attenuation of stellar X-rays. At some point the Lundquist number drops rises above 1 and the MRI is abruptly triggered (irrespectively of how smooth the resistivity jump is). Notice a Rossby vortex forming in the transition (the yellowish non-axisymmetric structure at the transition zone by the end of the simulation).

This simulation is the first to show that a smooth jump in resistivity can still trigger the RWI. Previous alpha disks models (Lyra et al. 2009a) suggested that only jumps sharper than 2H would do the trick.  

 


Publication: Lyra et al. (2014), A&A, accepted. [arXiv]
 

 

The model below was the first model of the RWI with a properly modeled active zone. By that we mean with MRI in a MHD disk instead of viscosity jumps in an alpha disk. It represents the transition from the inner active zone to the dead zone, at 0.1 AU, when temperature drops below 900K and collisions are not energetic enough to ionize the alkali metals.  

 


Publication: Lyra & Mac Low (2012), ApJ, 756, 62.[arXiv] [ADS]

 

 

Convective overstability

A hydrodynamical linear overstability exists in protoplanetary disks, powered by buoyancy in the presence of thermal relaxation. We model the system numerically, reproducing the linear growth rate for all cases studied. The figure shows the linear and nonlinear evolution of the overstability. Upper panels: Convergence study with resolution (left) and initial amplitude of perturbation (right). Resolution of 64 points per scale height is enough to resolve the overstability, and initial amplitudes as small as sound speed (c0) are enough to lead to growth, demonstrating the linear nature of the process. The linear growth rate (black dashed curve) is well reproduced in all cases. Lower panels: With the linear overstability raising the amplitude of the initial fluctuations to nonlinear levels, a large-scale vortex is generated in the course of the next 100 orbits. This was the first simulation that generated a self-sustained 3D vortex from linear amplitude perturbation of a quiescent base state.
 

Publication: Lyra (2014), ApJ, 789, 77. [arXiv] [ADS]  

 
Before the linearity of the instability had been recognized (Klahr & Hubbard 2014), however, much about the saturated state had been deduced, yet under the assumption that the instability was nonlinear in nature. We used to call it "baroclinic instability", for reasons that have nothing to do with atmospheric physics. It is a misnomer that was around for long enough to make it to a number of publications. To discriminate between that and the geophysical one, we call the former "subcritical baroclinic instability" (SBI) nowadays, following suggestion by Geoffroy Lesur. So, the saturated state of the COS is the SBI. The movie shows the development of the SBI (BI) in a 2D local shearing box model. The box used was small, and only one vortex remained. In general, vortices do not grow past the sonic scale, as they shock, radiating excess vorticity and thus shrinking. The SBI is shown to provide a hydrodynamical source of turbulence, allowing for some degree of accretion (alpha of the order of 5e-3) in the absense of magnetization.

Publications: Lyra & Klahr (2011), A&A, 527, 138. [arXiv] [ADS]
Publications: Raettig et al. (2013), ApJ, 765, 115. [arXiv] [ADS]

 

 

Magneto-elliptic instability

Phenomenologically, turbulence can be described as a series of bifurcations, starting with a primary instability that converts shear into vorticity, creating vortices. This is followed by another bifurcation, a secondary instability, to break these vortices into lesser vortical structures. These in turn shall experience a sequence of inertial instabilities, leading to a cascade. Though the Kelvin-Helmholtz instability and the Rayleigh-Taylor instability are well established as examples of primary instabilities, the highly successful theory of the turbulent cascade put forth by Kolmogorov rested on a heuristic picture of secondary instability, established by early experiments. It was not until the 80s that the elliptic instability was introduced as a mechanism for secondary instability. A fluid in rigid rotation supports a spectrum of stable inertial waves, the simplest case being circularly polarized transverse plane waves oscillating at twice the frequency of the base flow. Strain is introduced when the streamlines pass from circular to elliptical, and some modes find resonance with the strain field, leading to de-stabilization. This is called "elliptic instability".

In this simulation we examined the magnetic properties of vortices produced by the SBI. When magnetic fields are introduced in the problem, the addition of Alfven waves enriches the families of unstable modes. When background rotation is introduced, the same as had occurred to the elliptic instability en- sues. When this background rotation runs opposite to the rotation of the vortex (anti-cyclonic motion), a fluid parcel becomes subject to an intense effective shear. Since the magnetic tension resists shear, leading to instability a powerful unstable in-plane mode appears. This instability is of course the magneto-rotational instability, in generalized form. This provides an interesting unification, explaining the magneto-elliptic and magneto-rotational instabilities as different manifestations of the same magneto-elliptic-rotational instability

We find that vortices survive the hydrodynamical elliptic instability, but they fall prey to the more violent magneto-elliptic instability. The conclusion is that vortices are restricted to the dead zones of accretion disks.
 

Publications: Lyra & Klahr (2011), A&A, 527, 138. [arXiv] [ADS]
Publications: Mizerski & Lyra (2012), JFM, 698, 358. [arXiv] [ADS]

 

 

Transitional disks

The Atacama Large Millimeter Array (ALMA) has been returning images of transitional disks in which large asymmetries are seen in the distribution of mm-sized dust in the outer disk. The explanation in vogue borrows from the vortex literature by suggesting that these asymmetries are the result of dust trapping in giant vortices, excited via Rossby wave instability (RWI) at planetary gap edges. Due to the drag force, dust trapped in vortices will accumulate in the center, and diffusion is needed to maintain a steady state over the lifetime of the disk. While previous work derived semi-analytical models of the process, in this paper we provide analytical steady-state solutions. Exact solutions exist for certain vortex models. The solution is determined by the vortex rotation profile, the gas scale height, the vortex aspect ratio, and the ratio of dust diffusion to gas-dust friction. In principle, all these quantities can be derived from observations, which would give validation of the model, also giving constrains on the strength of the turbulence inside the vortex core. Based on our solution, we derive quantities such as the gas-dust contrast, the trapped dust mass, and the dust contrast at the same orbital location. We apply our model to the recently imaged Oph IRS 48 system, finding values within the range of the observational uncertainties.

Publication: Lyra & Lin 2013.


 

 

Photoelectric instability

Structure seen in debris disks - sharp rings, eccentricities, asymmetric ansae - are usually taken as evidence for the gravitational influence of unseen planets, as these disks are mostly gas-free systems of dust and leftover planetesimals. In this simulation we show that if the system has gas, hydrodynamical forces at work suffice to explain the structure seen in debris disks. This gas comes from outgassing of planetesimals and dust grains via sublimation, photodesorption, or collisions, generating a system of dust-to-gas ratio close to unity, where hydrodynamics cannot be ignored. The backreaction of the drag force from the gas onto the dust shepherds rings. The eccentricity can be simply explained by a standing wave propagating along the ring. The planet possibility, though thrilling, is not necessarily required to explain these systems.


 


 


 

Publication: Lyra & Kuchner 2013.