866 research outputs found
Bulk Composition of GJ 1214b and other sub-Neptune exoplanets
GJ1214b stands out among the detected low-mass exoplanets, because it is, so
far, the only one amenable to transmission spectroscopy. Up to date there is no
consensus about the composition of its envelope although most studies suggest a
high molecular weight atmosphere. In particular, it is unclear if hydrogen and
helium are present or if the atmosphere is water dominated. Here, we present
results on the composition of the envelope obtained by using an internal
structure and evolutionary model to fit the mass and radius data. By examining
all possible mixtures of water and H/He, with the corresponding opacities, we
find that the bulk amount of H/He of GJ1214b is at most 7% by mass. In general,
we find the radius of warm sub-Neptunes to be most sensitive to the amount of
H/He. We note that all (Kepler-11b,c,d,f, Kepler-18b, Kepler-20b, 55Cnc-e,
Kepler-36c and Kepler-68b) but two (Kepler-11e and Kepler-30b) of the
discovered low-mass planets so far have less than 10% H/He. In fact, Kepler-11e
and Kepler-30b have 10-18% and 5-15% bulk H/He. Conversely, little can be
determined about the H2O or rocky content of sub-Neptune planets. We find that
although a 100% water composition fits the data for GJ1214b, based on formation
constraints the presence of heavier refractory material on this planet is
expected, and hence, so is a component lighter than water required. A robust
determination by transmission spectroscopy of the composition of the upper
atmosphere of GJ1214b will help determine the extent of compositional
segregation between the atmosphere and envelope.Comment: Updated the masses and radii of the Kepler-11 system, added
Kepler-30b as well in the analysis. Accepted in ApJ, 39 pages, 9 figure
The atmospheric circulation of the super Earth GJ 1214b: Dependence on composition and metallicity
We present three-dimensional atmospheric circulation models of GJ 1214b, a
2.7 Earth-radius, 6.5 Earth-mass super Earth detected by the MEarth survey.
Here we explore the planet's circulation as a function of atmospheric
metallicity and atmospheric composition, modeling atmospheres with a low
mean-molecular weight (i.e., H2-dominated) and a high mean-molecular weight
(i.e. water- and CO2-dominated). We find that atmospheres with a low
mean-molecular weight have strong day-night temperature variations at pressures
above the infrared photosphere that lead to equatorial superrotation. For these
atmospheres, the enhancement of atmospheric opacities with increasing
metallicity lead to shallower atmospheric heating, larger day-night temperature
variations and hence stronger superrotation. In comparison, atmospheres with a
high mean-molecular weight have larger day-night and equator-to-pole
temperature variations than low mean-molecular weight atmospheres, but
differences in opacity structure and energy budget lead to differences in jet
structure. The circulation of a water-dominated atmosphere is dominated by
equatorial superrotation, while the circulation of a CO2-dominated atmosphere
is instead dominated by high-latitude jets. By comparing emergent flux spectra
and lightcurves for 50x solar and water-dominated compositions, we show that
observations in emission can break the degeneracy in determining the
atmospheric composition of GJ 1214b. The variation in opacity with wavelength
for the water-dominated atmosphere leads to large phase variations within water
bands and small phase variations outside of water bands. The 50x solar
atmosphere, however, yields small variations within water bands and large phase
variations at other characteristic wavelengths. These observations would be
much less sensitive to clouds, condensates, and hazes than transit
observations.Comment: 12 pages, 11 figures, 2 tables, accepted to Ap
Retrieval of atmospheric properties of cloudy L dwarfs
© 2017 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.We present the first results from applying the spectral inversion technique in the cloudy L dwarf regime. Our new framework provides a flexible approach to modelling cloud opacity which can be built incrementally as the data requires, and improves upon previous retrieval experiments in the brown dwarf regime by allowing for scattering in two stream radiative transfer. Our first application of the tool to two mid-L dwarfs is able to reproduce their near-infrared spectra far more closely than grid models. Our retrieved thermal, chemical, and cloud profiles allow us to estimate K and for 2MASS J05002100+0330501 and for 2MASSW J2224438-015852 we find K and , in close agreement with previous empirical estimates. Our best model for both objects includes an optically thick cloud deck which passes (looking down) at a pressure of around 5 bar. The temperature at this pressure is too high for silicate species to condense, and we argue that corundum and/or iron clouds are responsible for this cloud opacity. Our retrieved profiles are cooler at depth, and warmer at altitude than the forward grid models that we compare, and we argue that some form of heating mechanism may be at work in the upper atmospheres of these L dwarfs. We also identify anomalously high CO abundance in both targets, which does not correlate with the warmth of our upper atmospheres or our choice of cloud model, and find similarly anomalous alkali abundance for one of our targets. These anomalies may reflect unrecognised shortcomings in our retrieval model, or inaccuracies in our gas phase opacities.Peer reviewedFinal Accepted Versio
Atmospheric Circulation of Eccentric Hot Neptune GJ436b
GJ436b is a unique member of the transiting extrasolar planet population
being one of the smallest and least irradiated and possessing an eccentric
orbit. Because of its size, mass and density, GJ436b could plausibly have an
atmospheric metallicity similar to Neptune (20-60 times solar abundances),
which makes it an ideal target to study the effects of atmospheric metallicity
on dynamics and radiative transfer in an extrasolar planetary atmosphere. We
present three-dimensional atmospheric circulation models that include realistic
non-gray radiative transfer for 1, 3, 10, 30, and 50 times solar atmospheric
metallicity cases of GJ436b. Low metallicity models (1 and 3 times solar) show
little day/night temperature variation and strong high-latitude jets. In
contrast, higher metallicity models (30 and 50 times solar) exhibit day/night
temperature variations and a strong equatorial jet. Spectra and light curves
produced from these simulations show strong orbital phase dependencies in the
50 times solar case and negligible variations with orbital phase in the 1 times
solar case. Comparisons between the predicted planet/star flux ratio from these
models and current secondary eclipse measurements support a high metallicity
atmosphere (30-50 times solar abundances) with disequilibrium carbon chemistry
at play for GJ436b. Regardless of the actual atmospheric composition of GJ436b,
our models serve to illuminate how metallicity influences the atmospheric
circulation for a broad range of warm extrasolar planets.Comment: 25 pages, 13 figure
Reflected Spectra and Albedos of Extrasolar Giant Planets I: Clear and Cloudy Atmospheres
The reflected spectra of extrasolar giant planets are primarily influenced by
Rayleigh scattering, molecular absorption, and atmospheric condensates. We
present model geometric albedo and phase integral spectra and Bond albedos for
planets and brown dwarfs with masses between 0.8 and 70 Jupiter masses.
Rayleigh scattering predominates in the blue while molecular absorption removes
most red and infrared photons. Thus cloud-free atmospheres, found on giant
planets with effective temperatures exceeding about 400 K, are quite dark in
reflected light beyond 0.6 microns. In cooler atmospheres first water clouds
and then other condensates provide a bright reflecting layer. Only planets with
cloudy atmospheres will be detectable in reflected light beyond 1 micron.
Thermal emission dominates the near-infrared for warm objects with clear
atmospheres. However the presence of other condensates, not considered here,
may brighten some planets in reflected near-infrared light and darken them in
the blue and UV. Bond albedos, the ratio of the total reflected to incident
power, are sensitive to the spectral type of the primary. Most incident photons
from early type stars will be Rayleigh scattered, while most incident photons
from late type stars will be absorbed. The Bond albedo of a given planet thus
may range from 0.4 to 0.05, depending on the primary type. Condensation of a
water cloud increases the Bond albedo of a given planet by up to a factor of
two. The spectra of cloudy planets are strongly influenced by poorly
constrained cloud microphysical properties, particularly particle size and
supersaturation. Both Bond and geometric albedos are comparatively less
sensitive to variations in planet mass and effective temperature.Comment: AASTeX; 23 pages, 2 tables, 18 figures; ApJ in press; typo fixe
New H2 collision-induced absorption and NH3 opacity and the spectra of the coolest brown dwarfs
We present new cloudy and cloudless model atmospheres for brown dwarfs using
recent ab initio calculations of the line list of ammonia (NH3) and of the
collision-induced absorption of molecular hydrogen (H2). We compare the new
synthetic spectra with models based on an earlier description of the H2 and NH3
opacities. We find a significant improvement in fitting the nearly complete
spectral energy distribution of the T7p dwarf Gliese 570D and in near infrared
color-magnitude diagrams of field brown dwarfs. We apply these new models to
the identification of NH3 absorption in the H band peak of very late T dwarfs
and the new Y dwarfs and discuss the observed trend in the NH3-H spectral
index. The new NH3 line list also allows a detailed study of the medium
resolution spectrum of the T9/T10 dwarf UGPS J072227.51-054031.2 where we
identify several specific features caused by NH3.Comment: 37 pages, 13 figures. Accepted for publication in the Astrophysical
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