436 research outputs found
Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets
We address disequilibrum abundances of some simple molecules in the
atmospheres of solar composition brown dwarfs and self-luminous extrasolar
giant planets using a kinetics-based 1D atmospheric chemistry model. Our
approach is to use the full kinetics model to survey the parameter space with
effective temperatures between 500 K and 1100 K. In all of these worlds
equilibrium chemistry favors CH4 over CO in the parts of the atmosphere that
can be seen from Earth, but in most disequilibrium favors CO. The small surface
gravity of a planet strongly discriminates against CH4 when compared to an
otherwise comparable brown dwarf. If vertical mixing is like Jupiter's, the
transition from methane to CO occurs at 500 K in a planet. Sluggish vertical
mixing can raise this to 600 K; but clouds or more vigorous vertical mixing
could lower this to 400 K. The comparable thresholds in brown dwarfs are
K. Ammonia is also sensitive to gravity, but unlike CH4/CO, the
NH3/N2 ratio is insensitive to mixing, which makes NH3 a potential proxy for
gravity. HCN may become interesting in high gravity brown dwarfs with very
strong vertical mixing. Detailed analysis of the CO-CH4 reaction network
reveals that the bottleneck to CO hydrogenation goes through methanol, in
partial agreement with previous work. Simple, easy to use quenching relations
are derived by fitting to the complete chemistry of the full ensemble of
models. These relations are valid for determining CO, CH4, NH3, HCN, and CO2
abundances in the range of self-luminous worlds we have studied but may not
apply if atmospheres are strongly heated at high altitudes by processes not
considered here (e.g., wave breaking).Comment: Astrophysical Journal, in press. Clarity improvements throughout and
one new figure. 17 figures, 20 page
Analysis of Spitzer Spectra of Irradiated Planets: Evidence for Water Vapor?
Published mid infrared spectra of transiting planets HD 209458b and HD
189733b, obtained during secondary eclipse by the InfraRed Spectrograph (IRS)
aboard the Spitzer Space Telescope, are predominantly featureless. In
particular these flux ratio spectra do not exhibit an expected feature arising
from water vapor absorption short-ward of 10 um. Here we suggest that, in the
absence of flux variability, the spectral data for HD 189733b are inconsistent
with 8 um-photometry obtained with Spitzer's InfraRed Array Camera (IRAC),
perhaps an indication of problems with the challenging reduction of the IRS
spectra. The IRAC point, along with previously published secondary eclipse
photometry for HD 189733b, are in good agreement with a one-dimensional model
of HD 189733b that clearly shows absorption due to water vapor in the emergent
spectrum. We are not able to draw firm conclusions regarding the IRS data for
HD 209458b, but spectra predicted by 1D and 3D atmosphere models fit the data
adequately, without adjustment of the water abundance or reliance on cloud
opacity. We argue that the generally good agreement between model spectra and
IRS spectra of brown dwarfs with atmospheric temperatures similar to these
highly irradiated planets lends confidence in the modeling procedure.Comment: Revised, Accepted to ApJ Letter
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