93 research outputs found
Thermal Phase Variations of WASP-12b: Defying Predictions
[Abridged] We report Warm Spitzer full-orbit phase observations of WASP-12b
at 3.6 and 4.5 micron. We are able to measure the transit depths, eclipse
depths, thermal and ellipsoidal phase variations at both wavelengths. The large
amplitude phase variations, combined with the planet's previously-measured
day-side spectral energy distribution, is indicative of non-zero Bond albedo
and very poor day-night heat redistribution. The transit depths in the
mid-infrared indicate that the atmospheric opacity is greater at 3.6 than at
4.5 micron, in disagreement with model predictions, irrespective of C/O ratio.
The secondary eclipse depths are consistent with previous studies. We do not
detect ellipsoidal variations at 3.6 micron, but our parameter uncertainties
-estimated via prayer-bead Monte Carlo- keep this non-detection consistent with
model predictions. At 4.5 micron, on the other hand, we detect ellipsoidal
variations that are much stronger than predicted. If interpreted as a geometric
effect due to the planet's elongated shape, these variations imply a 3:2 ratio
for the planet's longest:shortest axes and a relatively bright day-night
terminator. If we instead presume that the 4.5 micron ellipsoidal variations
are due to uncorrected systematic noise and we fix the amplitude of the
variations to zero, the best fit 4.5 micron transit depth becomes commensurate
with the 3.6 micron depth, within the uncertainties. The relative transit
depths are then consistent with a Solar composition and short scale height at
the terminator. Assuming zero ellipsoidal variations also yields a much deeper
4.5 micron eclipse depth, consistent with a Solar composition and modest
temperature inversion. We suggest future observations that could distinguish
between these two scenarios.Comment: 19 pages, 10 figures, ApJ in press. Improved discussion of gravity
brightenin
XO-5b: A Transiting Jupiter-sized Planet With A Four Day Period
The star XO-5 (GSC 02959-00729, V=12.1, G8V) hosts a Jupiter-sized,
Rp=1.15+/-0.12 Rjup, transiting extrasolar planet, XO-5b, with an orbital
period of P=4.187732+/-0.00002 days. The planet mass (Mp=1.15+/-0.08 Mjup) and
surface gravity (gp=22+/-5 m/s^2) are significantly larger than expected by
empirical Mp-P and Mp-P-[Fe/H] relationships. However, the deviation from the
Mp-P relationship for XO-5b is not large enough to suggest a distinct type of
planet as is suggested for GJ 436b, HAT-P-2b, and XO-3b. By coincidence XO-5
overlies the extreme H I plume that emanates from the interacting galaxy pair
NGC 2444/NGC 2445 (Arp 143).Comment: 10 pages, 9 Figures, Submitted to Ap
Secondary Eclipse Photometry of WASP-4b with Warm Spitzer
We present photometry of the giant extrasolar planet WASP-4b at 3.6 and 4.5
micron taken with the Infrared Array Camera on board the Spitzer Space
Telescope as part of Spitzer's extended warm mission. We find secondary eclipse
depths of 0.319+/-0.031% and 0.343+/-0.027% for the 3.6 and 4.5 micron bands,
respectively and show model emission spectra and pressure-temperature profiles
for the planetary atmosphere. These eclipse depths are well fit by model
emission spectra with water and other molecules in absorption, similar to those
used for TrES-3 and HD 189733b. Depending on our choice of model, these results
indicate that this planet has either a weak dayside temperature inversion or no
inversion at all. The absence of a strong thermal inversion on this highly
irradiated planet is contrary to the idea that highly irradiated planets are
expected to have inversions, perhaps due the presence of an unknown absorber in
the upper atmosphere. This result might be explained by the modestly enhanced
activity level of WASP-4b's G7V host star, which could increase the amount of
UV flux received by the planet, therefore reducing the abundance of the unknown
stratospheric absorber in the planetary atmosphere as suggested in Knutson et
al. (2010). We also find no evidence for an offset in the timing of the
secondary eclipse and place a 2 sigma upper limit on |ecos(omega)| of 0.0024,
which constrains the range of tidal heating models that could explain this
planet's inflated radius.Comment: 8 pages, 7 figures (some in color), accepted for publication in Ap
NICMOS Observations of the Transiting Hot Jupiter XO-1b
We refine the physical parameters of the transiting hot Jupiter planet XO-1b
and its stellar host XO-1 using HST NICMOS observations. XO-1b has a radius
Rp=1.21+/-0.03 RJup, and XO-1 has a radius Rs=0.94+/-0.02 RSun, where the
uncertainty in the mass of XO-1 dominates the uncertainty of Rp and Rs. There
are no significant differences in the XO-1 system properties between these
broad-band NIR observations and previous determinations based upon ground-based
optical observations. We measure two transit timings from these observations
with 9 s and 15 s precision. As a residual to a linear ephemeris model, there
is a 2.0 sigma timing difference between the two HST visits that are separated
by 3 transit events (11.8 days). These two transit timings and additional
timings from the literature are sufficient to rule out the presence of an Earth
mass planet orbiting in 2:1 mean motion resonance coplanar with XO-1b. We
identify and correct for poorly understood gain-like variations present in
NICMOS time series data. This correction reduces the effective noise in time
series photometry by a factor of two, for the case of XO-1.Comment: 13 pages, 8 figures, submitted to Ap
Disequilibrium Carbon, Oxygen, and Nitrogen Chemistry in the Atmospheres of HD 189733b and HD 209458b
We have developed 1-D photochemical and thermochemical kinetics and diffusion
models for the transiting exoplanets HD 189733b and HD 209458b to study the
effects of disequilibrium chemistry on the atmospheric composition of "hot
Jupiters." Here we investigate the coupled chemistry of neutral carbon,
hydrogen, oxygen, and nitrogen species, and we compare the model results with
existing transit and eclipse observations. We find that the vertical profiles
of molecular constituents are significantly affected by transport-induced
quenching and photochemistry, particularly on cooler HD 189733b; however, the
warmer stratospheric temperatures on HD 209458b can help maintain
thermochemical equilibrium and reduce the effects of disequilibrium chemistry.
For both planets, the methane and ammonia mole fractions are found to be
enhanced over their equilibrium values at pressures of a few bar to less than a
mbar due to transport-induced quenching, but CH4 and NH3 are photochemically
removed at higher altitudes. Atomic species, unsaturated hydrocarbons
(particularly C2H2), some nitriles (particularly HCN), and radicals like OH,
CH3, and NH2 are enhanced overequilibrium predictions because of quenching and
photochemistry. In contrast, CO, H2O, N2, and CO2 more closely follow their
equilibrium profiles, except at pressures < 1 microbar, where CO, H2O, and N2
are photochemically destroyed and CO2 is produced before its eventual
high-altitude destruction. The enhanced abundances of HCN, CH4, and NH3 in
particular are expected to affect the spectral signatures and thermal profiles
HD 189733b and other, relatively cool, close-in transiting exoplanets. We
examine the sensitivity of our results to the assumed temperature structure and
eddy diffusion coefficientss and discuss further observational consequences of
these models.Comment: 40 pages, 16 figures, accepted for publication in Astrophysical
Journa
The thermal emission of the young and massive planet CoRoT-2b at 4.5 and 8 microns
We report measurements of the thermal emission of the young and massive
planet CoRoT-2b at 4.5 and 8 microns with the Spitzer Infrared Array Camera
(IRAC). Our measured occultation depths are 0.510 +- 0.042 % and 0.41 +- 0.11 %
at 4.5 and 8 microns, respectively. In addition to the CoRoT optical
measurements, these planet/star flux ratios indicate a poor heat distribution
to the night side of the planet and are in better agreement with an atmosphere
free of temperature inversion layer. Still, the presence of such an inversion
is not definitely ruled out by the observations and a larger wavelength
coverage is required to remove the current ambiguity. Our global analysis of
CoRoT, Spitzer and ground-based data confirms the large mass and size of the
planet with slightly revised values (Mp = 3.47 +- 0.22 Mjup, Rp = 1.466 +-
0.044 Rjup). We find a small but significant offset in the timing of the
occultation when compared to a purely circular orbital solution, leading to e
cos(omega) = -0.00291 +- 0.00063 where e is the orbital eccentricity and omega
is the argument of periastron. Constraining the age of the system to be at most
of a few hundreds of Myr and assuming that the non-zero orbital eccentricity is
not due to a third undetected body, we model the coupled orbital-tidal
evolution of the system with various tidal Q values, core sizes and initial
orbital parameters. For log(Q_s') = 5 - 6, our modelling is able to explain the
large radius of CoRoT-2b if log(Q_p') <= 5.5 through a transient tidal
circularization and corresponding planet tidal heating event. Under this model,
the planet will reach its Roche limit within 20 Myr at most.Comment: 13 pages, 2 tables, 11 figures. Accepted for publication in Astronomy
and Astrophysic
Transmission Spectra of Transiting Planet Atmospheres: Model Validation and Simulations of the Hot Neptune GJ 436b for JWST
We explore the transmission spectrum of the Neptune-class exoplanet GJ 436b,
including the possibility that its atmospheric opacity is dominated by a
variety of non- equilibrium chemical products. We also validate our
transmission code by demonstrating close agreement with analytic models that
use only Rayleigh scattering or water vapor opacity. We find broad disagreement
with radius variations predicted by another published model. For GJ 436b, the
relative coolness of the planet's atmosphere, along with its implied high
metallicity, may make it dissimilar in character compared to "hot Jupiters."
Some recent observational and modeling efforts suggest low relative abundances
of H2O and CH4 present in GJ 436b's atmosphere, compared to calculations from
equilibrium chemistry. We include these characteristics in our models and
examine the effects of absorption from methane-derived higher order
hydrocarbons. Significant absorption from HCN and C2H2 are found throughout the
infrared, while C2H4 and C2H6 are less easily seen. We perform detailed
simulations of JWST observations, including all likely noise sources, and find
that we will be able to constrain chemical abundance regimes from this planet's
transmission spectrum. For instance, the width of the features at 1.5, 3.3, and
7 microns indicates the amount of HCN versus C2H2 present. The NIRSpec prism
mode will be useful due to its large spectral range and the relatively large
number of photo-electrons recorded per spectral resolution element. However,
extremely bright host stars like GJ 436 may be better observed with a higher
spectroscopic resolution mode in order to avoid detector saturation. We find
that observations with the MIRI low resolution spectrograph should also have
high signal-to-noise in the 5 - 10 micron range due to the brightness of the
star and the relatively low spectral resolution (R ~ 100) of this mode.Comment: 33 pages, 12 figures, Accepted to Ap
Atmospheric circulation of hot Jupiters: Coupled radiative-dynamical general circulation model simulations of HD 189733b and HD 209458b
We present global, three-dimensional numerical simulations of HD 189733b and
HD 209458b that couple the atmospheric dynamics to a realistic representation
of non-gray cloud-free radiative transfer. The model, which we call the
Substellar and Planetary Atmospheric Radiation and Circulation (SPARC) model,
adopts the MITgcm for the dynamics and uses the radiative model of McKay,
Marley, Fortney, and collaborators for the radiation. Like earlier work with
simplified forcing, our simulations develop a broad eastward equatorial jet,
mean westward flow at higher latitudes, and substantial flow over the poles at
low pressure. For HD 189733b, our simulations without TiO and VO opacity can
explain the broad features of the observed 8 and 24-micron light curves,
including the modest day-night flux variation and the fact that the planet/star
flux ratio peaks before the secondary eclipse. Our simulations also provide
reasonable matches to the Spitzer secondary-eclipse depths at 4.5, 5.8, 8, 16,
and 24 microns and the groundbased upper limit at 2.2 microns. However, we
substantially underpredict the 3.6-micron secondary-eclipse depth, suggesting
that our simulations are too cold in the 0.1-1 bar region. Predicted temporal
variability in secondary-eclipse depths is ~1% at Spitzer bandpasses,
consistent with recent observational upper limits at 8 microns. We also show
that nonsynchronous rotation can significantly alter the jet structure. For HD
209458b, we include TiO and VO opacity; these simulations develop a hot (>2000
K) dayside stratosphere. Despite this stratosphere, we do not reproduce current
Spitzer photometry of this planet. Light curves in Spitzer bandpasses show
modest phase variation and satisfy the observational upper limit on day-night
phase variation at 8 microns. (abridged)Comment: 20 pages (emulate-apj format), 21 figures, final version now
published in ApJ. Includes expanded discussion of radiative-transfer methods
and two new figure
Thermal emission from WASP-24b at 3.6 and 4.5 {\mu}m
Aims. We observe occultations of WASP-24b to measure brightness temperatures
and to determine whether or not its atmosphere exhibits a thermal inversion
(stratosphere). Methods. We observed occultations of WASP-24b at 3.6 and 4.5
{\mu}m using the Spitzer Space Telescope. It has been suggested that there is a
correlation between stellar activity and the presence of inversions, so we
analysed existing HARPS spectra in order to calculate log R'HK for WASP-24 and
thus determine whether or not the star is chromospherically active. We also
observed a transit of WASP-24b in the Str\"{o}mgren u and y bands, with the
CAHA 2.2-m telescope. Results. We measure occultation depths of 0.159 \pm 0.013
per cent at 3.6 {\mu}m and 0.202 \pm 0.018 per cent at 4.5 {\mu}m. The
corresponding planetary brightness temperatures are 1974 \pm 71 K and 1944 \pm
85 K respectively. Atmosphere models with and without a thermal inversion fit
the data equally well; we are unable to constrain the presence of an inversion
without additional occultation measurements in the near-IR. We find log R'HK =
-4.98 \pm 0.12, indicating that WASP-24 is not a chromospherically active star.
Our global analysis of new and previously-published data has refined the system
parameters, and we find no evidence that the orbit of WASP-24b is non-circular.
Conclusions. These results emphasise the importance of complementing Spitzer
measurements with observations at shorter wavelengths to gain a full
understanding of hot Jupiter atmospheres.Comment: 7 pages, 4 figures, 3 tables. Accepted for publication in A&
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