596 research outputs found
The Influence of Non-Uniform Cloud Cover on Transit Transmission Spectra
We model the impact of non-uniform cloud cover on transit transmission
spectra. Patchy clouds exist in nearly every solar system atmosphere, brown
dwarfs, and transiting exoplanets. Our major findings suggest that fractional
cloud coverage can exactly mimic high mean molecular weight atmospheres and
vice-versa over certain wavelength regions, in particular, over the Hubble
Space Telescope (HST) Wide Field Camera 3 (WFC3) bandpass (1.1-1.7 m). We
also find that patchy cloud coverage exhibits a signature that is different
from uniform global clouds. Furthermore, we explain analytically why the
"patchy cloud-high mean molecular weight" degeneracy exists. We also explore
the degeneracy of non-uniform cloud coverage in atmospheric retrievals on both
synthetic and real planets. We find from retrievals on a synthetic solar
composition hot Jupiter with patchy clouds and a cloud free high mean molecular
weight warm Neptune, that both cloud free high mean molecular weight
atmospheres and partially cloudy atmospheres can explain the data equally well.
Another key find is that the HST WFC3 transit transmission spectra of two well
observed objects, the hot Jupiter HD189733b and the warm Neptune HAT-P-11b, can
be explained well by solar composition atmospheres with patchy clouds without
the need to invoke high mean molecular weight or global clouds. The degeneracy
between high molecular weight and solar composition partially cloudy
atmospheres can be broken by observing the molecular Rayleigh scattering
differences between the two. Furthermore, the signature of partially cloudy
limbs also appears as a 100 ppm residual in the ingress and egress of the
transit light curves, provided the transit timing is known to seconds.Comment: Accepted to ApJ Feb. 8, 201
Retrieving Temperatures and Abundances of Exoplanet Atmospheres with High-Resolution Cross-Correlation Spectroscopy
Hi-resolution spectroscopy (R > 25,000) has recently emerged as one of the
leading methods to detect atomic and molecular species in the atmospheres of
exoplanets. However, it has so far been lacking in a robust method to extract
quantitative constraints on temperature structure and molecular/atomic
abundances. In this work we present a novel Bayesian atmospheric retrieval
framework applicable to high resolution cross-correlation spectroscopy (HRCCS)
that relies upon the cross-correlation between data and models to extract the
planetary spectral signal. We successfully test the framework on simulated data
and show that it can correctly determine Bayesian credibility intervals on
atmospheric temperatures and abundances allowing for a quantitative exploration
of the inherent degeneracies. Furthermore, our new framework permits us to
trivially combine and explore the synergies between HRCCS and low-resolution
spectroscopy (LRS) to provide maximal leverage on the information contained
within each. This framework also allows us to quantitatively assess the impact
of molecular line opacities at high resolution. We apply the framework to VLT
CRIRES K-band spectra of HD 209458 b and HD 189733 b and retrieve abundant
carbon monoxide but sub-solar abundances for water, largely invariant under
different model assumptions. This confirms previous analysis of these datasets,
but is possibly at odds with detections of water at different wavelengths and
spectral resolutions. The framework presented here is the first step towards a
true synergy between space observatories and ground-based hi-resolution
observations.Comment: Accepted Version (01/16/19
A Systematic Retrieval Analysis of Secondary Eclipse Spectra III: Diagnosing Chemical Disequilibrium in Planetary Atmospheres
Chemical disequilibrium has recently become a relevant topic in the study of the atmospheres of of transiting extrasolar planets, brown dwarfs, and directly imaged exoplanets. We present a new way of assessing whether or not a Jovian-like atmosphere is in chemical disequilibrium from observations of detectable or inferred gases such as H_2 O, CH_4, CO, and H _2. Our hypothesis, based on previous kinetic modeling studies, is that cooler atmospheres will show stronger signs of disequilibrium than hotter atmospheres. We verify this with chemistry-transport models and show that planets with temperatures less than ~ 1200 K are likely to show the strongest signs of disequilibrium due to the vertical quenching of CO, and that our new approach is able to capture this process. We also find that in certain instances a planetary composition may appear in equilibrium when it actually is not due to the degeneracy in the shape of the vertical mixing ratio profiles. We determine the state of disequilibrium in eight exoplanets using the results from secondary eclipse temperature and abundance retrievals. We find that all of the planets in our sample are consistent with thermochemical equilibrium to within 3-sigma. Future observations are needed to further constrain the abundances in order to definitively identify disequilibrium in exoplanet atmospheres
New Analysis Indicates No Thermal Inversion in the Atmosphere of HD 209458b
An important focus of exoplanet research is the determination of the
atmospheric temperature structure of strongly irradiated gas giant planets, or
hot Jupiters. HD 209458b is the prototypical exoplanet for atmospheric thermal
inversions, but this assertion does not take into account recently obtained
data or newer data reduction techniques. We re-examine this claim by
investigating all publicly available Spitzer Space Telescope secondary-eclipse
photometric data of HD 209458b and performing a self-consistent analysis. We
employ data reduction techniques that minimize stellar centroid variations,
apply sophisticated models to known Spitzer systematics, and account for
time-correlated noise in the data. We derive new secondary-eclipse depths of
0.119 +/- 0.007%, 0.123 +/- 0.006%, 0.134 +/- 0.035%, and 0.215 +/- 0.008% in
the 3.6, 4.5, 5.8, and 8.0 micron bandpasses, respectively. We feed these
results into a Bayesian atmospheric retrieval analysis and determine that it is
unnecessary to invoke a thermal inversion to explain our secondary-eclipse
depths. The data are well-fitted by a temperature model that decreases
monotonically between pressure levels of 1 and 0.01 bars. We conclude that
there is no evidence for a thermal inversion in the atmosphere of HD 209458b.Comment: 8 pages, 5 figures; accepted for publication in 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
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