8914218
Rossby Wave Instability of Keplerian Accretion Disks
We find a linear instability of nonaxisymmetric Rossby waves in a thin nonmagnetized Keplerian disk when there is a local maximum in the radial profile of a key function (r)≡(r)S2/Γ(r), where −1=(∇×v)/Σ is the potential vorticity, S=P/ΣΓ is the entropy, Σ is the surface mass density, P is the vertically integrated pressure, and Γ is the adiabatic index. We consider in detail the special case where there is a local maximum in the disk entropy profile S(r). This maximum acts to trap the waves in its vicinity if its height-to-width ratio max(S)/Δr is larger than a threshold value. The pressure gradient derived from this entropy variation provides the restoring force for the wave growth. We show that the trapped waves act to transport angular momentum outward. A plausible way to produce an entropy variation is when an accretion disk is starting from negligible mass and temperature, therefore, negligible entropy. As mass accumulates by either tidal torquing, magnetic torquing, or Roche-lobe overflow, confinement of heat will lead to an entropy maximum at the outer boundary of the disk. Possible nonlinear developments from this instability include the formation of Rossby vortices and the formation of spiral shocks. What remains to be determined from hydrodynamic simulations is whether or not Rossby wave packets (or vortices) "hold together" as they propagate radially inward.
2655191
A proposed origin for chondrule-forming shocks in the solar nebula
We propose that the nebular shocks currently favored as a model to form chondrules and other annealed silicates in the solar nebula originate in the dynamical activity present in the envelope of forming Jovian planets. In contrast to the classic `core accretion model', our 3D hydrodynamic simulations show that this envelope is not a 1D hydrostatic structure but is instead vigorously active and contains densities and temperatures that appear similar in magnitude and spatial extent to those thought to be responsible for the production of chondrules.
117045776
Dynamics and observational appearance of circumstellar disks
In my thesis I present a study of the dynamics and observational characteristics of massive circumstellar disks in two dimensions ($r$, $\phi$) using two complimentary hydrodynamic codes: a `Smoothed Particle Hydrodynamic' (SPH) code and a `Piecewise Parabolic Method' (PPM) code. I also study the detection limits available to radial velocity searches for low mass companions to main sequence stars. The thesis is organized as a series of published or submitted papers, connected by introductory and concluding material and is archived `for posterity'. I recommend that readers of this abstract obtain the published versions of each of these papers. I first outline the progress which has been made in the modeling of the structure and origins of the solar system, then in chapter 2 (ApJ 502,p342) I proceed with numerical simulations of circumstellar disks using both hydrodynamic codes assuming a `locally isothermal' equation of state. In chapter 4 (ApJ submitted) I relax the locally isothermal evolution assumption and instead include simple heating and cooling prescriptions for the system. In chapter 6 (ApJ 500,p940) I examine the limits which may be placed upon the detection of planets, brown dwarfs and low mass stellar companions using radial velocity measurements. In chapter 7, I outline several problems which may be profitably addressed by building on this work.
15725335
A Resolution Requirement for Disk Simulations Modeling Collapse
The accurate simulation of collapsed objects requires that a huge range of spatial scales be well resolved if the result is not to be contaminated by numerically induced fragmentation. In this context, `insufficient resolution' means comparable to the local instability wavelength in the system. I define a minimum resolution criterion for simulations of disk systems, using the critical wavelength appropriate for disks.
16491613
VINE—A NUMERICAL CODE FOR SIMULATING ASTROPHYSICAL SYSTEMS USING PARTICLES. II. IMPLEMENTATION AND PERFORMANCE CHARACTERISTICS
We continue our presentation of VINE. In this paper, we begin with a description of relevant architectural properties of the serial and shared memory parallel computers on which VINE is intended to run, and describe their influences on the design of the code itself. We continue with a detailed description of a number of optimizations made to the layout of the particle data in memory and to our implementation of a binary tree used to access that data for use in gravitational force calculations and searches for SPH neighbor particles. We describe the modifications to the code necessary to obtain forces efficiently from special purpose ‘GRAPE’ hardware, the interfaces required to allow transparent substitution of those forces in the code instead of those obtained from the tree, and the modifications necessary to use both tree and GRAPE together as a fused GRAPE/tree combination. We conclude with an extensive series of performance tests, which demonstrate that the code can be run efficiently and without modification in serial on small workstations or in parallel using the OpenMP compiler directives on large scale, shared memory parallel machines. We analyze the effects of the code optimizations and estimate that they improve its overall performance by more than an order of magnitude over that obtained by many other tree codes. Scaled parallel performance of the gravity and SPH calculations, together the most costly components of most simulations, is nearly linear up to at least 120 processors on moderate sized test problems using the Origin 3000 architecture, and to the maximum machine sizes available to us on several other architectures. At similar accuracy, performance of VINE, used in GRAPE-tree mode, is approximately a factor two slower than that of VINE, used in host-only mode. Further optimizations of the GRAPE/host communications could improve the speed by as much as a factor of three, but have not yet been implemented in VINE. Finally, we find that although parallel performance on small problems may reach a plateau beyond which more processors bring no additional speedup, performance never decreases, a factor important for running large simulations on many processors with individual time steps, where only a small fraction of the total particles require updates at any given moment. Subject headings: methods: numerical — methods: N-body simulations
64863479
Dynamics of Core Accretion
We perform 3-dimensional hydrodynamic simulations of gas flowing around a plan- etary core of mass Mpl=10Membedded in a near Keplerian background flow, using a modified shearing box approximation. We assume an ideal gas behavior following an equation of state with a fixed ratio of the specific heats, γ = 1.42, consistent with the conditions of a moderate temperature background disk with solar composition. No radiative heating or cooling is included in the models. We employ a nested grid hydrodynamic code implementing the 'Piecewise Parabolic Method' with as many as six fixed nested grids, providing spatial resolution on the finest grid comparable to the present day diameters of Neptune and Uranus. We find that a strongly dynamically active flow develops such that no static en- velope can form. The activity is not sensitive to plausible variations in the rotation curve of the underlying disk. It is sensitive to the thermodynamic treatment of the gas, as modeled by prescribed equations of state (either 'locally isothermal' or 'locally isentropic') and the temperature of the background disk material. The activity is also sensitive to the shape and depth of the core's gravitational potential, through its mass and gravitational softening coefficient. Each of these factors influence the magnitude and character of hydrodynamic feedback of the small scale flow on the background, and we conclude that accurate modeling of such feedback is critical to a complete understanding of the core accretion process. The varying flow pattern gives rise to large, irregular eruptions of matter from the region around the core which return matter to the background flow: mass in the envelope at one time may not be found in the envelope at any later time. No net mass accretion into the envelope is observed over the course of the simulation and none is expected, due to our neglect of cooling. Except in cases of very rapid cooling however, as defined by locally isothermal or isentropic treatments, any cooling that does affect the envelope material will have limited consequences for the dynamics, since the flow quickly carries cooled material out of the core's environment entirely. The angular momentum of material in the envelope, relative to the core, varies both in magnitude and in sign on time scales of days to months near the core and on time scales a few years at distances comparable to the Hill radius. The dynamical activity contrasts with the largely static behavior typically assumed within the framework of the core accretion model for Jovian planet formation. We show that material entering the dynamically active environment may suffer intense heating and cooling events the durations of which are as short as a few hours to a few days. Shorter durations are not observable in our work due to the limits of our resolution. Peak temperatures in these events range from T � 1000 K to as high as T � 3 4000 K, with densities ρ � 10 9 10 8 g/cm 3 . These time scales, densities and temperatures span a range consistent with those required for chondrule formation in the nebular shock model. We therefore propose that dynamical activity in the Jovian planet formation environment could be responsible for the production of chondrules and other annealed silicates in the solar nebula.
16070485
Non-Abelian Monopole and Dyon Solutions in a Modified Einstein-Yang-Mills-Higgs System
We have studied a modied Yang-Mills-Higgs system coupled to Einstein gravity. The modication of the Einstein-Hilbert action involves a direct coupling of the Higgs eld to the scalar curvature. In this modied system we are able to write a Bogomol’nyi type condition in curved space and demonstrate that the positive static energy functional is bounded from below. We then investigate non-Abelian spherically symmetric static solutions in a similar fashion to the ’t Hooft-Polyakov monopole. After reviewing previously studied monopole solutions of this type, we extend the formalism to included electric charge and we present dyon solutions.