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.
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
282379
Planet Formation is Unlikely in Equal-Mass Binary Systems with a ~ 50 AU
We show that planet formation via both gravitational collapse and core accretion is unlikely to occur in equal-mass binary systems with moderate (~50 AU) semimajor axes. Internal thermal energy generation in the disks is sufficient to heat the gas everywhere so that spiral structures quickly decay rather than grow or fragment. This same heating will inhibit dust coagulation because the temperatures rise above the vaporization temperatures of many volatile materials. We consider other processes not included in the model and conclude that our temperatures are conservatively estimated (low), i.e., planet formation is less likely in real systems than in the model.
17175349
Numerical requirements for simulations of self-gravitating and non-self-gravitating discs
We define three requirements for accurate simulations that attempt to model circumstellar discs and the formation of collapsed objects (e.g. planets) within them. First, we define a resolution requirement based on the wavelength for neutral stability of self-gravitating waves in the disc, where a Jeans analysis does not apply. For particle-based or grid-based simulations, this criterion takes the form, respectively, of a minimum number of particles per critical (‘Toomre’) mass or maximum value of a ‘Toomre number’, T=δx/λT, where the wavelength, λT, is the wavelength for neutral stability for waves in discs. The requirements are analogues of the conditions for cloud collapse simulations as discussed in Bate & Burkert and Truelove et al., where the required minimum resolution was shown to be twice the number of neighbours per Jeans mass or four–five times the local Jeans wavelength, λJ, for particle or grid simulations, respectively. 
 
 
 
We apply our criterion to particle simulations of disc evolution and find that in order to prevent numerically induced fragmentation of the disc, the Toomre mass must be resolved by a minimum of six times the average number of neighbour particles used. We investigate the origin of the apparent discrepancy between the number of particles required by the cloud and disc fragmentation criteria and find that it is due largely to ambiguities in the definition of the Jeans mass, as used by different authors. We reconcile the various definitions, and when an identical definition of the Jeans mass is used, the condition that J≲ 1/4 in the Truelove condition is equivalent to requiring about 10–12 times the average number of neighbour particles per Jeans mass in a smoothed particle hydrodynamics (SPH) simulation, reducing the difference between simulations of discs and clouds to about two. While the numbers of particles per critical mass are similar for both the Jeans and Toomre formalisms, the Toomre requirement is more restrictive than the Jeans requirement when the local value of the Toomre stability parameter Q falls below about one half. 
 
 
 
Second, we require that particle-based simulations with self-gravity use a variable gravitational softening, in order to avoid inducing fragmentation by an inappropriate choice of softening length. We show that using a fixed gravitational softening length for all particles can lead either to artificial suppression or enhancement of structure (including fragmentation) in a given disc, or both in different locations of the same disc, depending on the value chosen for the softening length. Unphysical behaviour can occur whether or not the system is properly resolved by the new Toomre criterion. 
 
 
 
Third, we require that three-dimensional SPH simulations resolve the vertical structure with at least ∼4 particle smoothing lengths per scaleheight at the disc mid-plane, a value which implies a substantially larger number per vertical column because the disc itself extends over many scaleheights. We suggest that a similar criterion applies to grid-based simulations. We demonstrate that failure to meet this criterion leads to underestimates in the mid-plane density of up to 30–50 per cent at resolutions common in the literature. As a direct consequence, gas pressures will be dramatically underestimated and simulations of self-gravitating systems may artificially and erroneously inflate the likelihood of fragmentation. We outline an additional condition on the vertical resolution in simulations that include radiative transfer in order to ensure a correct description of the cooling, specifically that the temperature structure near the disc photosphere must be well resolved. As an example, we demonstrate that for an isentropic vertical structure, the criterion translates into resolution comparable to H/20 near the disc photosphere, to avoid serious errors in transfer rates of thermal energy in and out of the disc. 
 
 
 
Finally, we discuss results in the literature that purport to form collapsed objects and conclude that many are likely to have violated one or more of our criteria, and have therefore made incorrect conclusions regarding the likelihood for fragmentation and planet formation.
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.
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.
122261326
Constraints on a new vector boson coupled to baryons
Abstract We consider several processes which are sensitive to the existence of a light gauge boson coupled to baryon number. In the mass region between 10 Mev and M π ≈140 MeV the best limits on the coupling come from rare π 0 decays, while a boson with mass between M π and 2 M π may be observed in K + decays, or η decays. Heavier gauge bosons will decay hadronically and are harder to find, also limits on their coupling are subject to strong interaction uncertainties.
14555932
Fermion masses and gauge mediated supersymmetry breaking from a single U(1)
We present a supersymmetric model of flavor. A single U(1) gauge group is responsible for both generating the flavor spectrum and communicating supersymmetry breaking to the visible sector. The problem of flavor changing neutral currents is overcome, in part using an {open_quotes}effective supersymmetry{close_quotes} spectrum among the squarks, with the first two generations very heavy. All masses are generated dynamically and the theory is completely renormalizable. The model contains a simple Froggatt-Nielsen sector and communicates supersymmetry breaking via gauge mediation without requiring a separate messenger sector. By forcing the theory to be consistent with SU(5) grand unification, the model predicts a large tan{beta} and a massless up quark. While respecting the experimental bounds on {ital CP} violation in the {ital K} system, the model leads to a large enhancement of {ital CP} violation in B-{bar B} mixing as well as in {ital B} decay amplitudes. {copyright} {ital 1999} {ital The American Physical Society}
119222841
Apparent CPT violation in neutrino oscillation experiments
We consider searching for light sterile fermions and new forces by using long baseline oscillations of neutrinos and antineutrinos. A new light sterile state and/or a new force can lead to apparent CPT violation in muon neutrino and antineutrino oscillations. As an example, we present an economical model of neutrino masses containing a sterile neutrino. The potential from the standard model weak neutral current gives rise to a difference between the disappearance probabilities of neutrinos and antineutrinos, when mixing with a light sterile neutrino is considered. The addition of a B — L interaction adds coherently to the neutrino current potential and increases the difference between neutrino and antineutrino disappearance. We find that this model can improve the fit to the results of MINOS for both neutrinos and antineutrinos, without any CPT violation, and that the regions of parameter space which improve the fit are within experimental constraints.
119104232
Effects of mass varying neutrinos on cosmological parameters as determined from the cosmic microwave background
In models with a light scalar field (the `acceleron') coupled to neutrinos, neutrino masses depend on neutrino density. The resulting coupled system of mass varying neutrinos (MaVaNs) and the acceleron can act as a negative pressure fluid and is a candidate for dark energy \cite{Fardon:2003eh} . MaVaNs also allow for higher $\Sigma$m$_\nu$ than terrestrial bounds, giving late forming warm dark matter. In this paper we study the effect of increasing neutrino mass on the CMB spectrum, implementing MaVaNs cosmology using CMBEASY. We find that the CMB spectrum is only affected at very low multipoles. Cosmic variance allows for significant warm dark matter at late times. This implies that in MaVaNs cosmology $\sigma_8$ as determined by the CMB may not be a good determinant of structure evolution at late times, potentially reducing the tension between $\sigma$$_{8}$ as reported by Planck Collaboration \cite{PlanckSZ:2013} without increasing the tension in the Planck determined value of Hubble's constant. In addition, in MaVaNs theories, CMB data do not necessarily constrain possible neutrino mass results in terrestrial experiments.
19576396
Short baseline neutrino oscillations and a new light gauge boson
12 pages, 4 figures.-- PACS number: 14.60.Pq.-- Submitted on 8 Nov 2007 (v1), last revised 21 Dec 2007 (this version, v3): References added, minor change to coupling constraints, expanded sections on astrophysical and cosmological constraints.-- Published in: Phys. Rev. D 77, 033001 (2008) (12 pages).-- Final full-text version of the paper available at: http://dx.doi.org/10.1103/PhysRevD.77.033001.