We calculate self-consistent extrasolar giant planet (EGP) phase functions and light curves for orbital distances ranging from 0.2 to 15 AU. We explore the dependence on wavelength, cloud ...condensation, and Keplerian orbital elements. We find that the light curves of EGPs depend strongly on wavelength, the presence of clouds, and cloud particle sizes. Furthermore, the optical and infrared colors of most EGPs are phase-dependent, tending to be reddest at crescent phases in V - R and R - I. Assuming circular orbits, we find that at optical wavelengths most EGPs are 3-4 times brighter near full phase than near greatest elongation for highly inclined (i.e., close to edge-on) orbits. Furthermore, we show that the planet/star flux ratios depend strongly on the Keplerian elements of the orbit, particularly inclination and eccentricity. Given a sufficiently eccentric orbit, an EGP's atmosphere may make periodic transitions from cloudy to cloud-free, an effect that may be reflected in the shape and magnitude of the planet's light curve. Such elliptical orbits also introduce an offset between the time of the planet's light-curve maximum and the time of full planetary phase, and for some sets of orbital parameters, this light-curve maximum can be a steeply increasing function of eccentricity. We investigate the detectability of EGPs by proposed space-based direct-imaging instruments.
Using a model for refractory clouds, a novel algorithm for handling them, and the latest gas-phase molecular opacities, we have produced a new series of L and T dwarf spectral and atmosphere models ...as a function of gravity and metallicity, spanning the T sub(eff) range from 2200 to 700 K. The correspondence with observed spectra and infrared colors for early and mid-L dwarfs and for mid- to late T dwarfs is good. We find that the width in infrared color-magnitude diagrams of both the T and L dwarf branches is naturally explained by reasonable variations in gravity and therefore that gravity is the "second parameter" of the L-T dwarf sequence. We investigate the dependence of theoretical dwarf spectra and color-magnitude diagrams on various cloud properties, such as particle size and cloud spatial distribution. In the region of the L 1 T transition, we find that no single combination of cloud particle size and gravity can be made to fit all the observed data. Our results suggest that current ignorance of detailed cloud meteorology renders ambiguous the extraction of various physical quantities such as T sub(eff) and gravity for mid-L to early T dwarfs. Nevertheless, for decreasing T sub(eff), we capture with some accuracy the major spectral features and signatures observed. We speculate that the subdwarf branch of the L dwarfs would be narrower in effective temperature and that for low enough metallicity the L dwarfs would disappear altogether as a spectroscopic class. Furthermore, we note that the new, lower solar oxygen abundances of Allende-Prieto and coworkers produce better fits to brown dwarf data than do the older values. Finally, we discuss various issues in cloud physics and modeling and speculate on how a better correspondence between theory and observation in the problematic L 1 T transition region could be achieved.
This work is a detailed study of extrasolar giant planet (EGP) atmospheres and spectra. Models representative of the full range of systems known today are included, from the extreme close-in EGPs to ...Jovian-like planets at large orbital radii. Using a self-consistent planar atmosphere code along with the latest atomic and molecular cross sections, cloud models, Mie theory treatment of grain scattering and absorption, and incident stellar fluxes, I produce an extensive set of theoretical EGP atmosphere models and emergent spectra. The emergent spectra of EGPs strongly depend upon their outer atmospheric chemical compositions, which in turn depend upon the run of temperature and pressure with atmospheric depth. Because of qualitative similarities in the compositions and spectra of objects within several broad temperature ranges, EGPs fall naturally into five groups, or composition classes. Such a classification scheme, however preliminary, brings a degree of order to the rich variety of EGP systems known to exist today. Generic models that represent the EGP classes, as well as a set of specific models for a number of important systems that have been detected, are provided. Furthermore, the effects on emergent EGP spectra of varying key parameters such as surface gravity, cloud particle sizes, orbital distance, etc. are modeled. A discussion of current and future ground-based and space-based missions to detect and characterize EGPs in light of theoretical spectral models is included to facilitate an understanding of which systems are most likely to be studied successfully.
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