2,584 research outputs found
On the Inference of Thermal Inversions in Hot Jupiter Atmospheres
Several studies in the recent past have inferred the existence of thermal
inversions in some transiting hot Jupiter atmospheres. Given the limited data
available, the inference of a thermal inversion depends critically on the
chemical composition assumed for the atmosphere. In this study, we explore the
degeneracies between thermal inversions and molecular abundances in four highly
irradiated hot Jupiter atmospheres, day-side observations of which were
previously reported to be consistent with thermal inversions based on Spitzer
photometry. The four systems are: HD 209458b, HAT-P-7b, TrES-4, and TrES-2. For
each system, we explore the model parameter space with ~ 10^6 models using a
Markov chain Monte Carlo routine. Our results primarily suggest that a thorough
exploration of the model parameter space is necessary to identify thermal
inversions in hot Jupiter atmospheres. We find that existing observations of
TrES-4 and TrES-2 can both be fit very precisely with models with and without
thermal inversions, and with a wide range in chemical composition. On the other
hand, observations of HD 209458b and HAT-P-7b are better fit with thermal
inversions than without, as has been reported before. We do not see a
correlation between irradiation levels and thermal inversions, given current
data. Before JWST becomes available, near-IR observations from ground and with
HST, along with existing Spitzer observations, can potentially resolve thermal
inversions in some systems.Comment: 16 pages in emulate ApJ format, 6 figures, 3 tables (Accepted in ApJ
A Precise Estimate of the Radius of HD 149026b
We present Spitzer 8 micron transit observations of the extrasolar planet
system HD 149026. At this wavelength, transit light curves are weakly affected
by stellar limb-darkening, allowing for a simpler and more accurate
determination of planetary parameters. We measure a planet-star radius ratio of
R_p/R_s = 0.05158 +/- 0.00077, and in combination with ground-based data and
independent constraints on the stellar mass and radius, we derive an orbital
inclination of i = 85.4 +0.9/-0.8 deg. and a planet radius of 0.755 +/- 0.040
Jupiter radii. These measurements further support models in which the planet is
greatly enriched in heavy elements.Comment: To appear in the Proceedings of the 253rd IAU Symposium: "Transiting
Planets", May 2008, Cambridge, M
The Broadband Infrared Emission Spectrum of the Exoplanet HD 189733b
We present Spitzer Space Telescope time series photometry of the exoplanet
system HD 189733 spanning two times of secondary eclipse, when the planet
passes out of view behind the parent star. We estimate the relative eclipse
depth in 5 distinct bands and find the planet-to-star flux ratio to be 0.256
+/- 0.014% (3.6 microns), 0.214 +/- 0.020% (4.5 microns), 0.310 +/- 0.034% (5.8
microns), 0.391 +/- 0.022% (8.0 microns), and 0.598 +/- 0.038% (24 microns).
For consistency, we re-analyze a previously published time series to deduce a
contrast ratio in an additional band, 0.519 +/- 0.020% (16 microns). Our data
are strongly inconsistent with a Planck spectrum, and we clearly detect
emission near 4 microns as predicted by published theoretical models in which
this feature arises from a corresponding opacity window. Unlike recent results
for the exoplanet HD 209458b, we find that the emergent spectrum from HD
189733b is best matched by models that do not include an atmospheric
temperature inversion. Taken together, these two studies provide initial
observational support for the idea that hot Jupiter atmospheres diverge into
two classes, in which a thermal inversion layer is present for the more
strongly irradiated objects.Comment: 20 pages, 3 figures, accepted to the Astrophysical Journal, minor
revision
Off-shell Behavior of the Mixing Amplitude
We extend a recent calculation of the momentum dependence of the
mixing amplitude to the pseudoscalar sector. The
mixing amplitude is calculated in a hadronic model where the mixing is driven
by the neutron-proton mass difference. Closed-form analytic expressions are
presented in terms of a few nucleon-meson parameters. The observed momentum
dependence of the mixing amplitude is strong enough as to question earlier
calculations of charge-symmetry-breaking observables based on the on-shell
assumption. The momentum dependence of the amplitude is,
however, practically identical to the one recently predicted for
mixing. Hence, in this model, the ratio of pseudoscalar to vector mixing
amplitudes is, to a good approximation, a constant solely determined from
nucleon-meson coupling constants. Furthermore, by selecting these parameters in
accordance with charge-symmetry-conserving data and SU(3)-flavor symmetry, we
reproduce the momentum dependence of the mixing amplitude
predicted from chiral perturbation theory. Alternatively, one can use
chiral-perturbation-theory results to set stringent limits on the value of the
coupling constant.Comment: 13 pages, Latex with Revtex, 3 postscript figures (not included)
available on request, SCRI-03089
Quark Coulomb Interactions and the Mass Difference of Mirror Nuclei
We study the Okamoto-Nolen-Schiffer (ONS) anomaly in the binding energy of
mirror nuclei at high density by adding a single neutron or proton to a quark
gluon plasma. In this high-density limit we find an anomaly equal to two-thirds
of the Coulomb exchange energy of a proton. This effect is dominated by quark
electromagnetic interactions---rather than by the up-down quark mass
difference. At normal density we calculate the Coulomb energy of neutron matter
using a string-flip quark model. We find a nonzero Coulomb energy because of
the neutron's charged constituents. This effect could make a significant
contribution to the ONS anomaly.Comment: 4 pages, 2 figs. sub. to Phys. Rev. Let
Detection of Planetary Emission from the Exoplanet TrES-2 using Spitzer /IRAC
We present here the results of our observations of TrES-2 using the Infrared
Array Camera on Spitzer. We monitored this transiting system during two
secondary eclipses, when the planetary emission is blocked by the star. The
resulting decrease in flux is 0.127%+-0.021%, 0.230%+-0.024%, 0.199%+-0.054%,
and 0.359%+-0.060%, at 3.6 microns, 4.5 microns, 5.8 microns, and 8.0 microns,
respectively. We show that three of these flux contrasts are well fit by a
black body spectrum with T_{eff}=1500 K, as well as by a more detailed model
spectrum of a planetary atmosphere. The observed planet-to-star flux ratios in
all four IRAC channels can be explained by models with and without a thermal
inversion in the atmosphere of TrES-2, although with different atmospheric
chemistry. Based on the assumption of thermochemical equilibrium, the chemical
composition of the inversion model seems more plausible, making it a more
favorable scenario. TrES-2 also falls in the category of highly irradiated
planets which have been theoretically predicted to exhibit thermal inversions.
However, more observations at infrared and visible wavelengths would be needed
to confirm a thermal inversion in this system. Furthermore, we find that the
times of the secondary eclipses are consistent with previously published times
of transit and the expectation from a circular orbit. This implies that TrES-2
most likely has a circular orbit, and thus does not obtain additional thermal
energy from tidal dissipation of a non-zero orbital eccentricity, a proposed
explanation for the large radius of this planet.Comment: 8 pages, 4 figures, 2 tables. Accepted for publication in the
Astrophysical Journal. V2: New figure added ; other minor changes throughou
The Orbit of WASP-12b Is Decaying
WASP-12b is a transiting hot Jupiter on a 1.09 day orbit around a late-F star. Since the planet's discovery in 2008, the time interval between transits has been decreasing by 29 ± 2 ms yr⁻¹. This is a possible sign of orbital decay, although the previously available data left open the possibility that the planet's orbit is slightly eccentric and is undergoing apsidal precession. Here, we present new transit and occultation observations that provide more decisive evidence for orbital decay, which is favored over apsidal precession by a ΔBIC of 22.3 or Bayes factor of 70,000. We also present new radial-velocity data that rule out the Rømer effect as the cause of the period change. This makes WASP-12 the first planetary system for which we can be confident that the orbit is decaying. The decay timescale for the orbit is P/P˙=3.25±0.23. Interpreting the decay as the result of tidal dissipation, the modified stellar tidal quality factor is Q′⋆=1.8×10⁵
Searching for Far-Ultraviolet Auroral/Dayglow Emission from HD209458b
We present recent observations from the HST-Cosmic Origins Spectrograph aimed
at characterizing the auroral emission from the extrasolar planet HD209458b. We
obtained medium-resolution (R~18-20,000) far-ultraviolet (1150-1700A) spectra
at both the Phase 0.25 and Phase 0.75 quadrature positions as well as a stellar
baseline measurement at secondary eclipse. This analysis includes a catalog of
stellar emission lines and a star-subtracted spectrum of the planet. We present
an emission model for planetary H2 emission, and compare this model to the
planetary spectrum. No unambiguously identifiable atomic or molecular features
are detected, and upper limits are presented for auroral/dayglow line
strengths. An orbital velocity cross-correlation analysis finds a statistically
significant (3.8 sigma) feature at +15 (+/- 20) km/s in the rest frame of the
planet, at 1582 A. This feature is consistent with emission from H2 B-X (2-9)
P(4) (lambda_{rest} = 1581.11 A), however the physical mechanism required to
excite this transition is unclear. We compare limits on relative line strengths
seen in the exoplanet spectrum with models of ultraviolet fluorescence to
constrain the atmospheric column density of neutral hydrogen between the star
and the planetary surface. These results support models of short period
extrasolar giant planets with weak magnetic fields and extended atomic
atmospheres.Comment: Accepted to ApJ. 12 pages, 5 figures, 4 table
Large-Scale Circulation and Climate Variability
The causes of regional climate trends cannot be understood without considering the impact of variations in large-scale atmospheric circulation and an assessment of the role of internally generated climate variability. There are contributions to regional climate trends from changes in large-scale latitudinal circulation, which is generally organized into three cells in each hemisphere-Hadley cell, Ferrell cell and Polar cell-and which determines the location of subtropical dry zones and midlatitude jet streams. These circulation cells are expected to shift poleward during warmer periods, which could result in poleward shifts in precipitation patterns, affecting natural ecosystems, agriculture, and water resources. In addition, regional climate can be strongly affected by non-local responses to recurring patterns (or modes) of variability of the atmospheric circulation or the coupled atmosphere-ocean system. These modes of variability represent preferred spatial patterns and their temporal variation. They account for gross features in variance and for teleconnections which describe climate links between geographically separated regions. Modes of variability are often described as a product of a spatial climate pattern and an associated climate index time series that are identified based on statistical methods like Principal Component Analysis (PC analysis), which is also called Empirical Orthogonal Function Analysis (EOF analysis), and cluster analysis
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