The Theory of Brown Dwarfs and Extrasolar Giant Planets

Straddling the traditional realms of the planets and the stars, objects below the edge of the main sequence have such unique properties, and are being discovered in such quantities, that one can rightly claim that a new field at the interface of planetary science and and astronomy is being born. In this review, we explore the essential elements of the theory of brown dwarfs and giant planets, as well as of the new spectroscopic classes L and T. To this end, we describe their evolution, spectra, atmospheric compositions, chemistry, physics, and nuclear phases and explain the basic systematics of substellar-mass objects across three orders of magnitude in both mass and age and a factor of 30 in effective temperature. Moreover, we discuss the distinctive features of those extrasolar giant planets that are irradiated by a central primary, in particular their reflection spectra, albedos, and transits. Aspects of the latest theory of Jupiter and Saturn are also presented. Throughout, we highlight the effects of condensates, clouds, molecular abundances, and molecular/atomic opacities in brown dwarf and giant planet atmospheres and summarize the resulting spectral diagnostics. Where possible, the theory is put in its current observational context.Comment: 67 pages (including 36 figures), RMP RevTeX LaTeX, accepted for publication in the Reviews of Modern Physics. 30 figures are color. Most of the figures are in GIF format to reduce the overall size. The full version with figures can also be found at: http://jupiter.as.arizona.edu/~burrows/papers/rm

Carbothermal reduction of titania in different gas atmospheres

The synthesis of titanium oxycarbide by carbothermal reduction of titania was studied in hydrogen, argon, and helium in isothermal and temperature programmed reduction experiments in a tube reactor with continuously flowing gas. In the temperature range of 1000 C to 1500 C, the reduction rate increased with increasing temperature. Formation of titaniumoxycarbide started at 1200 C in all three gases. The reduction was the fastest in hydrogen. Formation of titanium oxycarbide in hydrogen was close to completion in 120 minutes at 1300 C, 60 minutes at 1400 C, and less than 30 minutes at 1500 C. The reduction in argon and helium had similar rates and reached 90 to 95 pct after a 300-minute reduction at 1400 C to 1500 C. Faster carbothermal reduction of titania in hydrogen than in argon and helium was attributed to involvement of hydrogen in the reaction. Hydrogen reduced titania to titanium suboxides and reacted with carbon, forming methane, which reduced titaniumsuboxides to titaniumoxycarbide. Titanium oxycarbide synthesized in hydrogen for 180 minutes at 1300 C contained 13 mol pct TiO. At 1500 C, oxygen concentration decreased to a degree corresponding to 1.4 mol pct TiO. In the titanium oxycarbide produced by a 300-minute reduction at 1600 C, the TiO content was 0.6 mol pct