3,764 research outputs found
Identification of Absorption Features in an Extrasolar Planet Atmosphere
Water absorption is identified in the atmosphere of HD209458b by comparing
models for the planet's transmitted spectrum to recent, multi-wavelength,
eclipse-depth measurements (from 0.3 to 1 microns) published by Knutson et al.
(2007). A cloud-free model which includes solar abundances, rainout of
condensates, and photoionization of sodium and potassium is in good agreement
with the entire set of eclipse-depth measurements from the ultraviolet to
near-infrared. Constraints are placed on condensate removal by gravitational
settling, the bulk metallicity, and the redistribution of absorbed stellar
flux. Comparisons are also made to the Charbonneau et al. (2002) sodium
measurements.Comment: Accepted for publication in ApJL., in emulate ApJ forma
On the Presence of Water and Global Circulation in the Transiting Planet HD 189733b
Detailed models are compared to recent infrared observations of the nearby
extrasolar planet, HD 189733b. It is demonstrated that atmospheric water is
present and that the planet's day side has a non-isothermal structure down to
gas pressures of ~ 0.1 bars. Furthermore, model spectra with different amounts
of CO are compared to the observations and an atmosphere absent of CO is
excluded at roughly 2-sigma. Constraining the CO concentration beyond that is
unfortunately not possible with the current Spitzer photometry. However,
radically enhanced (or depleted) metal abundances are unlikely and the basic
composition of this planet is probably similar to that of its host star. When
combined with Spitzer observations, a recent ground-based upper limit for the
K-band day side flux allows one to estimate the day-to-night energy
redistribution efficiency to be ~ 43%.Comment: accepted (2008 Feb. 5), ApJ Letter
Two Classes of Hot Jupiters
We identify two classes of transiting planet, based on their equilibrium
temperatures and Safronov numbers. We examine various possible explanations for
the dichotomy. It may reflect the influence of planet or planetesimal
scattering in determining when planetary migration stops. Another possibility
is that some planets lose more mass to evaporation than others. If this
evaporation process preferentially removes Helium from the planet, the
consequent reduction in the mean molecular weight may explain why some planets
have anomalously large radii.Comment: 35 pages, 16 figures in Preprint format. Submitted to Ap
The Influence of Host Star Spectral Type on Ultra-Hot Jupiter Atmospheres
Ultra-hot Jupiters are the most highly irradiated gas giant planets, with
equilibrium temperatures from 2000 to over 4000 K. Ultra-hot Jupiters are
amenable to characterization due to their high temperatures, inflated radii,
and short periods, but their atmospheres are atypical for planets in that the
photosphere possesses large concentrations of atoms and ions relative to
molecules. Here we evaluate how the atmospheres of these planets respond to
irradiation by stars of different spectral type. We find that ultra-hot
Jupiters exhibit temperature inversions that are sensitive to the spectral type
of the host star. The slope and temperature range across the inversion both
increase as the host star effective temperature increases due to enhanced
absorption at short wavelengths and low pressures. The steep temperature
inversions in ultra-hot Jupiters around hot stars result in increased thermal
dissociation and ionization compared to similar planets around cooler stars.
The resulting increase in H opacity leads to a transit spectrum that has
muted absorption features. The emission spectrum, however, exhibits a large
contrast in brightness temperature, a signature that will be detectable with
both secondary eclipse observations and high-dispersion spectroscopy. We also
find that the departures from local thermodynamic equilibrium in the stellar
atmosphere can affect the degree of heating caused by atomic metals in the
planet's upper atmosphere. Additionally, we further quantify the significance
of heating by different opacity sources in ultra-hot Jupiter atmospheres.Comment: 13 pages, 9 figures, 2 tables. Accepted for publication in Ap
The PHOENIX Exoplanet Retrieval Algorithm and Using H Opacity as a Probe in Ultra-hot Jupiters
Atmospheric retrievals are now a standard tool to analyze observations of
exoplanet atmospheres. This data-driven approach quantitatively compares
atmospheric models to observations in order to estimate atmospheric properties
and their uncertainties. In this paper, we introduce a new retrieval package,
the PHOENIX Exoplanet Retrieval Analysis (PETRA). PETRA places the PHOENIX
atmosphere model in a retrieval framework, allowing us to combine the strengths
of a well-tested and widely-used atmosphere model with the advantages of
retrieval algorithms. We validate PETRA by retrieving on simulated data for
which the true atmospheric state is known. We also show that PETRA can
successfully reproduce results from previously published retrievals of WASP-43b
and HD 209458b. For the WASP-43b results, we show the effect that different
line lists and line profile treatments have on the retrieved atmospheric
properties. Lastly, we describe a novel technique for retrieving the
temperature structure and density in ultra-hot Jupiters using H
opacity, allowing us to probe atmospheres devoid of most molecular features
with JWST.Comment: 17 pages, 18 figures. Accepted for publication in A
Large negative magnetoresistance in a ferromagnetic shape memory alloy : Ni_{2+x}Mn_{1-x}Ga
5% negative magnetoresistance (MR) at room temperature has been observed in
bulk Ni_{2+x}Mn_{1-x}Ga. This indicates the possibility of using
Ni_{2+x}Mn_{1-x}Ga as magnetic sensors. We have measured MR in the
ferromagnetic state for different compositions (x=0-0.2) in the austenitic,
pre-martensitic and martensitic phases. MR is found to increase with x. While
MR for x=0 varies almost linearly in the austenitic and pre-martensitic phases,
in the martensitic phase it shows a cusp-like shape. This has been explained by
the changes in twin and domain structures in the martensitic phase. In the
austenitic phase, which does not have twin structure, MR agrees with theory
based on s-d scattering model.Comment: 3 pages, 3 figures, Appl. Phys. Lett 86, 202508 (2005
Optimal operating conditions and characteristics of acetone/CaF_2 detector for inverse photoemission spectroscopy
Performance and characteristics of a band-pass photon detector using acetone
gas and CaF_2 window (acetone/CaF_2) have been studied and compared with an
ethanol/MgF_2 detector. The optimal operating conditions are found to be 4 mbar
acetone pressure and 745+/-20 V anode voltage. The count rate obtained by us is
about a factor of 3 higher than what has been reported earlier for the acetone
detector. Unlike other gas filled detectors, this detector works in the
proportional region with very small dead time (4 micro sec). A detector
band-pass of 0.48+/-0.01 eV FWHM is obtained.Comment: Review of Scientific Instruments 76, 066102 (2005
Evolution of photoemission spectral functions in doped transition metal oxides
We discuss the experimental photoemission and inverse photoemission of early
transition metal oxides, in the light of the dynamical mean field theory of
correlated electrons which becomes exact in the limit of infinite dimensions.
We argue that a comprehensive description of the experimental data requires
spatial inhomogeneities and present a calculation of the evolution of the
spectral function in an inhomogenous system with various degrees of
inhomogeneity. We also point out that comparaison of experimental results and
large d calculations require that the degree of correlation and disorder is
larger in the surface than in the bulk
Spectral functions in doped transition metal oxides
We present experimental photoemission and inverse photoemission spectra of
SrTiO representing electron doped systems. Photoemission
spectra in presence of electron doping exhibit prominent features arising from
electron correlation effects, while the inverse photoemssion spectra are
dominated by spectral features explainable within single-particle approaches.
We show that such a spectral evolution in chemically doped correlated systems
is not compatible with expectations based on Hubbard or any other similar
model. We present a new theoretical approach taking into account the
inhomogeneity of the `real' system which gives qualitatively different results
compared to standard `homogeneous' models and is in quantitative agreement with
experiments.Comment: 10 pages; 1 tex file+4 postscript files (to appear in Europhysics
Letters
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