Research Interests
Ian Dobbs-Dixon
iandd at ucolick.org
831.459.2774
Main Page
still working...
I'm working on a number of projects, several of which are described below:
Atmospheric Fluid Dynamics on Giant Planets
Paper: Atmospheric Dynamics of Short-period Extra Solar Gas Giant Planets I: Dependence of Night-Side Temperature on Opacity
More than two dozen short-period Jupiter-mass gas giant planets have
been discovered around nearby solar-type stars in recent years,
several of which undergo transits, making them ideal for the detection
and characterization of their atmospheres. Here we adopt a
three-dimensional radiative hydrodynamical numerical scheme to
simulate atmospheric circulation on close-in gas giant planets. In
contrast to the conventional GCM and shallow water algorithms, this
method does not assume quasi hydrostatic equilibrium and it
approximates radiation transfer from optically thin to thick regions
with flux-limited diffusion. In the first paper of this series, we
consider synchronously-spinning gas giants. We show that a full
three-dimensional treatment, coupled with rotationally modified flows
and an accurate treatment of radiation, yields a clear temperature
transition at the terminator. Based on a series of numerical
simulations with varying opacities, we show that the night-side
temperature is a strong indicator of the opacity of the planetary
atmosphere. Planetary atmospheres that maintain large, interstellar
opacities will exhibit large day-night temperature differences, while
planets with reduced atmospheric opacities due to extensive grain
growth and sedimentation will exhibit much more uniform temperatures
throughout their photosphere's. In addition to numerical results, we
present a four-zone analytic approximation to explain this dependence.
The code was originally written by Willy Kley (Universität Tübingen) and modified Geoff Bryden (JPL). I have parallelized the code using MPI calls and I am now using it in 3D for a number of problems. Below are several plots from our paper exploring the relation between opacity and temperature distribution.
Planetary Migration
To explain the proximity of many extrasolar planets to their host stars it is widely assumed that interactions between a protoplanet and the their surrounding protoplanetary disks cause inward migration of the orbit. The disk interior to the planet exerts a positive torque on the planet, while the outer disk exerts a negative torque; migration occurs because of a slight imbalance in the torques from these two regions. A vast majority of the torque on the planet comes from specific resonant locations within the disk. This realization allowed for the development of an analytic theory that concentrates on the Lindblad and co-rotation regions of the disk. As the planet migrates, it is also accreting material and growing in mass. For this reason, planetary migration has been broken into two sub-categories based on the ability of the planet to open and sustain a gap in the protoplanetary disk: type I without a gap, and type II with a gap. However, there is a problem with migration; it is too effective at moving the planet. When comparing the migration timescale to the formation timescale you find that planets will fall into the star before they have a chance to form. The formation of a gap (and transition to type II) helps things by de-populating those critical Lindblad resonances, but there is still a problem when the planet is too small to open a gap. I have been studying this phase of the migration both analytically and numerically using an inviscid Eulerian code developed by Hui Li and Shengtai Li (Los Alamos). Because of the small mass of the planet, numerical studies of the migration requires very high resolution and have been notoriously difficult. A combination of our improved algorithm and computational resources have allowed us to evolve a highly resolved simulation for hundreds of orbits. Below is an snapshot of the surface density of the disk and the potential vorticity.
Migration Links
TPF/Darwin Talk
Tidal Truncation of Planetary Mass Growth
Paper: Tidal Barrier and the Asymptotic Mass of Proto Gas-Giant Planets
During the later stages of gas-giant formation, runaway gas-accretion increases the total mass of the planet very rapidly. Although the surface density in the vicinity of the protoplanet is reduced as a result of this accretion, there may be sufficient leakage of material into the gap region to allow the planet to grow to masses significantly larger then that of Jupiter. The notable absence of such very massive planets in the both our own solar system and amongst the extra-solar planets, suggest the need for another mechanism to quench accretion. Although global depletion of the disk has been suggested, systems with multiple gas-giant planets argue for another method. We have been studying the later stages of mass growth when the Bondi radius exceeds the Roche radius. During this phase a substantial positive pressure gradient is needed to overcome the planets tidal barrier. This significantly decreases the mass flow onto the planet and may provide a mechanism for limiting planetary masses, particularly for cooler disks. Below is a comparison of the mass vrs time for disks with H/r=0.07 and H/r=0.04.
Semiconvection in Massive Stars
The presence of a molecular weight gradient in a fluid will inhibit the ability of that fluid to mix via convection. The region will break up into a series of layers; fully convective regions separated by sharp boundaries across which material can only pass via diffusion. Subsequent evolution requires destruction of the boundaries and merging of the layers. This process is though to be important near the contracting cores of massive stars, giant planets, and even the earths oceans. I am utilizing a One-Dimensional Turbulence (ODT) model developed by Alan Kerstein
to study the mixing time-scales in this region and nature of the fluxes across boundaries. The time-scale of semiconvective mixing will have drastic effects on the further evolution of massive stars as they progress toward core-collapse. ODT is a mixing length theory developed by Alan Kerstein with a variable mixing length calculated from the energetics of the flow. Because it is one-dimensional it should be possible to resolve scales ranging from the diffusive scale up to the entire convective region of the star. I have modified the theory to explain compressible stellar material. The goal is to find the material diffusion coefficient is semiconvective regions.
There have been many laboratory experiments illustrating the double diffusive instability. Both Kato's unstable oscillations and, the formation of a cellular structure are nicely illustrated here .
Here is more detail into the theory if you want it...
Spin-Orbit Coupling in Short Period Planets
Paper: Spin-Orbit Evolution of Short-Period Planets
The negligible eccentricity of all extra solar planets with periods less than six days can be accounted for by dissipation of tidal
disturbances within their envelopes which are induced by their host stars. In the period range of 7-21 days, planets with circular orbits coexist with planets with eccentric orbits. These will be referred to as the 'borderline planets'. We propose that this discrepancy can be attributed to the variation in spin-down rates of young stars. In particular, prior to spin-down, dissipation of a planet's tidal disturbance within the envelope of a sufficiently rapidly spinning star can excite eccentricity growth, and for a more slowly spinning star, at least reduce the eccentricity damping rate. In contrast, tidal dissipation within the envelope of a slowly spinning low-mass mature star can enhance the eccentricity damping process. Based on these results, we suggest that short-period planets around relatively young stars may have a much larger dispersion in eccentricity than those around mature stars. We also suggest that because the rate of angular momentum loss from G and K dwarfs via stellar winds is much faster than the tidal transfer of angular momentum between themselves and their very-short (3-4 days) period planets, they cannot establish a dynamical configuration in which the stellar and planetary spins are approximately parallel and synchronous with the orbital frequency. In principle, however, such configurations may be established for planets (around G and K dwarfs) with orbital periods of up to several weeks. In contrast to G and K dwarfs, the angular momentum loss due to stellar winds is much weaker in F dwarfs. It is therefore possible for synchronized short-period planets to exist around such stars. The planet around Tau Boo is one such example.
Mass Transfer and Eccentricity Evolution in SAX J1808.4-3658
Observations of SAX J1808.4-3658 have revealed a 0.05M_sun brown dwarf with a period of 2.01 hours orbiting the central pulsar. Timing measurements set an upper limit to the eccentricity of e<5x10^-4, while the rate of reoccurring bursts in the system indicates an accretion rate of approximately 5x10^-12 M_sun/yr. We explore the possibility that the tidal damping of the eccentricity within the secondary provides the energy for inflation and subsequent mass transfer. To maintain this heating source the eccentricity must be excited through the process of mass transfer and interaction with the disk. We develop such a model and utilize it, in conjunction with an equilibrium tidal model with a dissipation of parameter (Q-parameter) of 10^5, to predict an equilibrium eccentricity for SAX J1808.4-3658 of approximately 2x10^-4. Conversely, an accurate measurement of the eccentricity for this system will yield a measurement of the Q-parameter.
Core-Mantle Mixing in Giant Planets
I'm just starting this, but here's a great picture comparing the interior of planets
Atmospheric Fluid Dynamics
Migration