arXiv subjects

Eric B. Ford

Publications and source records attributed to Eric B. Ford.

222 records · Page 13Linked to original sources

arXiv2002

Dynamical Instabilities in Extrasolar Planetary Systems

Instabilities and strong dynamical interactions between multiple giant planets have been proposed as a possible explanation for the surprising orbital properties of extrasolar planetary systems. In particular, dynamical instabilities seem to provide a natural mechanism for producing the highly eccentric orbits seen in many systems. Previously, we performed numerical integrations for the dynamical evolution of planetary systems containing two giant planets of equal masses initially in nearly circular orbits very close to the dynamical stability limit. We found the ratio of collisions to ejections in these simulations was greater than the ratio of circular orbits to eccentric orbits among the known extrasolar planets. Further, the mean eccentricity of the planets remaining after an ejection was larger than the mean eccentricity of the known extrasolar planets. Recently, we have performed additional integrations, generalizing to consider two planets of unequal masses. Our new simulations reveal that the two-planet scattering model can produce a distribution of eccentricities consistent with the observed eccentricity distribution for plausible mass distributions. Additionally, this model predicts a maximum eccentricity of about 0.8, in agreement with observations. Early results from simulations of three equal-mass planets also reveal a reduced frequency of collisions and a broad range of final eccentricities for the retained inner planet.

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arXiv2002

A Model of the Temporal Variability of Optical Light from Extrasolar Terrestrial Planets

The light scattered by an extrasolar Earth-like planet's surface and atmosphere will vary in intensity and color as the planet rotates; the resulting light curve will contain information about the planet's properties. Since most of the light comes from a small fraction of the planet's surface, the temporal flux variability can be quite significant, $\sim$ 10-100%. In addition, for cloudless Earth-like extrasolar planet models, qualitative changes to the surface (such as ocean fraction, ice cover) significantly affect the light curve. Clouds dominate the temporal variability of the Earth but can be coherent over several days. In contrast to Earth's temporal variability, a uniformly, heavily clouded planet (e.g. Venus), would show almost no flux variability. We present light curves for an unresolved Earth and for Earth-like model planets calculated by changing the surface features. This work suggests that meteorological variability and the rotation period of an Earth-like planet could be derived from photometric observations. The inverse problem of deriving surface properties from a given light curve is complex and will require much more investigation.

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arXiv2001

Evolution of the Cluster Mass Function: Gpc^3 Dark Matter Simulations

High-resolution N-body simulations of four popular Cold Dark Matter cosmologies (LCDM, OCDM, QCDM, and tilted SCDM), each containing 10^5 clusters of galaxies in a cubic gigaparsec volume, are used to determine the evolution of the cluster mass function from z=3 to z=0. The large volume and high resolution of these simulations allow an accurate measure of the evolution of cosmologically important (but rare) massive clusters at high redshift. The simulated mass function is presented for cluster masses within several radii typically used observationally (R=0.5, 1.0, and 1.5 Mpc/h, both comoving and physical) in order to enable direct comparison with current and future observations. The simulated evolution is compared with current observations of massive clusters at redshifts 0.3 0.3 and no massive clusters at z>0.5, in stark contradiction with observations. The Omega_m=0.3 models- LCDM, OCDM, and QCDM- all exhibit considerably weaker evolution and are consistent with current data. Among these low density models, OCDM evolves the least. These trends are enhanced at high redshift and can be used to discriminate between flat and open low density models. The simulated mass functions are compared with the Press-Schechter approximation. Standard Press-Schechter predicts too many low mass clusters at z=0, and too few clusters at higher redshift. We modify the approximation by a simple parameterization of the density contrast threshold for collapse, which has a redshift dependence. This modified Press-Schechter approximation provides a good fit to the simulated mass functions.

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arXiv2000

Dynamical Instabilities in Extrasolar Planetary Systems Containing Two Giant Planets

Instabilities and strong dynamical interactions between several giant planets have been proposed as a possible explanation for the surprising orbital properties of extrasolar planetary systems. In particular, dynamical instabilities would seem to provide a natural mechanism for producing the highly eccentric orbits seen in many systems. Here we present results from numerical integrations for the dynamical evolution of planetary systems containing two identical giant planets in nearly circular orbits very close to the dynamical stability limit. We determine the statistical properties of the three main types of systems resulting from the development of an instability: systems containing one planet, following either a collision between the two initial planets, or the ejection of one of them to infinity, and systems containing two planets in a new, quasi-stable configuration. We discuss the implications of our results for the formation and evolution of observed extrasolar planetary systems. We conclude that the distributions of eccentricities and semimajor axes for observed systems cannot be explained easily by invoking dynamical interactions between two planets initially on circular orbits. While highly eccentric orbits can be produced naturally by these interactions, collisions between the two planets, which occur frequently in the range of observed semimajor axes, would result in many more nearly circular orbits than in the observed sample.

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arXiv2000

Secular Evolution of Hierarchical Triple Star Systems

We derive octupole-level secular perturbation equations for hierarchical triple systems, using classical Hamiltonian perturbation techniques. By extending previous work done to leading (quadrupole) order to octupole level (i.e., including terms of order alpha^3, where alpha = a_1/a_2 < 1 is the ratio of semimajor axes) we obtain expressions that are applicable to a much wider range of parameters. For triple systems containing a close inner binary, we also discuss the possible interaction between the classical Newtonian perturbations and the general relativistic precession of the inner orbit. In some cases we show that this interaction can lead to resonances and a significant increase in the maximum amplitude of eccentricity perturbations. We establish the validity of our analytic expressions by providing detailed comparisons with the results of direct numerical integrations of the three-body problem obtained for a large number of representative cases. We also discuss applications of the theory in the context of several observed triple systems of current interest, including the millisecond pulsar PSR B1620-26 in M4, the giant planet in 16 Cygni, and the protostellar binary TMR-1. [short version]

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arXiv1998

Structure and Evolution of Nearby Stars with Planets. I. Short-Period Systems

Using the Yale stellar evolution code, we have calculated theoretical models for nearby stars with planetary-mass companions in short-period nearly circular orbits: 51 Pegasi, Tau Bootis, Upsilon Andromedae, Rho Cancri, and Rho Coronae Borealis. We present tables listing key stellar parameters such as mass, radius, age, and size of the convective envelope as a function of the observable parameters (luminosity, effective temperature, and metallicity), as well as the unknown helium fraction. For each star we construct best models based on recently published spectroscopic data and the present understanding of galactic chemical evolution. We discuss our results in the context of planet formation theory, and, in particular, tidal dissipation effects and stellar metallicity enhancements.

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