462 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
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
A Search for Water in the Atmosphere of HAT-P-26b Using LDSS-3C
The characterization of a physically-diverse set of transiting exoplanets is
an important and necessary step towards establishing the physical properties
linked to the production of obscuring clouds or hazes. It is those planets with
identifiable spectroscopic features that can most effectively enhance our
understanding of atmospheric chemistry and metallicity. The newly-commissioned
LDSS-3C instrument on Magellan provides enhanced sensitivity and suppressed
fringing in the red optical, thus advancing the search for the spectroscopic
signature of water in exoplanetary atmospheres from the ground. Using data
acquired by LDSS-3C and the Spitzer Space Telescope, we search for evidence of
water vapor in the transmission spectrum of the Neptune-mass planet HAT-P-26b.
Our measured spectrum is best explained by the presence of water vapor, a lack
of potassium, and either a high-metallicity, cloud-free atmosphere or a
solar-metallicity atmosphere with a cloud deck at ~10 mbar. The emergence of
multi-scale-height spectral features in our data suggests that future
observations at higher precision could break this degeneracy and reveal the
planet's atmospheric chemical abundances. We also update HAT-P-26b's transit
ephemeris, t_0 = 2455304.65218(25) BJD_TDB, and orbital period, p =
4.2345023(7) days.Comment: 9 pages, 8 figures, Accepted for publication in Ap
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