254 research outputs found
Interpreting the photometry and spectroscopy of directly imaged planets: a new atmospheric model applied to beta Pictoris b and SPHERE observations
We aim to interpret future photometric and spectral measurements from these
instruments, in terms of physical parameters of the planets, with an
atmospheric model using a minimal number of assumptions and parameters.
We developed Exoplanet Radiative-convective Equilibrium Model (Exo-REM) to
analyze the photometric and spectro- scopic data of directly imaged planets.
The input parameters are a planet's surface gravity (g), effective temperature
(Teff ), and elemental composition. The model predicts the equilibrium
temperature profile and mixing ratio profiles of the most important gases.
Opacity sources include the H2-He collision-induced absorption and molecular
lines from eight compounds (including CH4 updated with the Exomol line list).
Absorption by iron and silicate cloud particles is added above the expected
condensation levels with a fixed scale height and a given optical depth at some
reference wavelength. Scattering was not included at this stage.
We applied Exo-REM to photometric and spectral observations of the planet
beta Pictoris b obtained in a series of near-IR filters. We derived Teff = 1550
+- 150 K, log(g) = 3.5 +- 1, and radius R = 1.76 +- 0.24 RJup (2-{\sigma} error
bars from photometric measurements). These values are comparable to those found
in the literature, although with more conservative error bars, consistent with
the model accuracy. We were able to reproduce, within error bars, the J- and
H-band spectra of beta Pictoris b. We finally investigated the precision to
which the above parameterComment: 15 pages, 14 figures, accepted by A&
Simulated performance of the molecular mapping for young giant exoplanets with the Medium Resolution Spectrometer of JWST/MIRI
Young giant planets are the best targets for characterization with direct
imaging. The Medium Resolution Spectrometer (MRS) of the Mid-Infrared
Instrument (MIRI) of the recently launched James Webb Space Telescope (JWST)
will give access to the first spectroscopic data for direct imaging above 5
m with unprecedented sensitivity at a spectral resolution up to 3700. This
will provide a valuable complement to near-infrared data from ground-based
instruments for characterizing these objects. We aim to evaluate the
performance of MIRI/MRS to detect molecules in the atmosphere of exoplanets and
to constrain atmospheric parameters using Exo-REM atmospheric models. The
molecular mapping technique, based on cross-correlation with synthetic models,
has been introduced recently. This promising detection and characterization
method is tested on simulated MIRI/MRS data. Directly imaged planets can be
detected with MIRI/MRS, and we are able to detect molecules (HO, CO,
NH, CH, HCN, PH, CO) at various angular separation depending on
the strength of the molecular features and brightness of the target. We find
that the stellar spectral type has a weak impact on the detection level. This
method is globally most efficient for planets with temperatures below 1500 K,
for bright targets and angular separation greater than 1. Our parametric
study allows us to anticipate the ability to characterize planets that would be
detected in the future. The MIRI/MRS will give access to molecular species not
yet detected in exoplanetary atmospheres. The detection of molecules as
indicators of the temperature of the planets will make it possible to
discriminate between the various hypotheses of the preceding studies, and the
derived molecular abundance ratios should bring new constraints on planetary
formation scenarios.Comment: 25 pages, 13 figure
Titan's atmosphere as observed by Cassini/VIMS solar occultations: CH, CO and evidence for CH absorption
We present an analysis of the VIMS solar occultations dataset, which allows
us to extract vertically resolved information on the characteristics of Titan's
atmosphere between 100-700 km with a characteristic vertical resolution of 10
km. After a series of data treatment procedures, 4 occultations out of 10 are
retained. This sample covers different seasons and latitudes of Titan. The
transmittances show clearly the evolution of the haze and detect the detached
layer at 310 km in Sept. 2011 at mid-northern latitudes. Through the inversion
of the transmission spectra with a line-by-line radiative transfer code we
retrieve the vertical distribution of CH and CO mixing ratio. The two
methane bands at 1.4 and 1.7 {\mu}m are always in good agreement and yield an
average stratospheric abundance of %. This is significantly less
than the value of 1.48% obtained by the GCMS/Huygens instrument. The analysis
of the residual spectra after the inversion shows that there are additional
absorptions which affect a great part of the VIMS wavelength range. We
attribute many of these additional bands to gaseous ethane, whose near-infrared
spectrum is not well modeled yet. Ethane contributes significantly to the
strong absorption between 3.2-3.5 {\mu}m that was previously attributed only to
C-H stretching bands from aerosols. Ethane bands may affect the surface windows
too, especially at 2.7 {\mu}m. Other residual bands are generated by stretching
modes of C-H, C-C and C-N bonds. In addition to the C-H stretch from aliphatic
hydrocarbons at 3.4 {\mu}m, we detect a strong and narrow absorption at 3.28
{\mu}m which we tentatively attribute to the presence of PAHs in the
stratosphere. C-C and C-N stretching bands are possibly present between 4.3-4.5
{\mu}m. Finally, we obtain the CO mixing ratio between 70-170 km. The average
result of ppm is in good agreement with previous studies.Comment: 51 pages, 28 figure
A Time-Dependent Radiative Model of HD209458b
We present a time-dependent radiative model of the atmosphere of HD209458b
and investigate its thermal structure and chemical composition. In a first
step, the stellar heating profile and radiative timescales were calculated
under planet-averaged insolation conditions. We find that 99.99% of the
incoming stellar flux has been absorbed before reaching the 7 bar level.
Stellar photons cannot therefore penetrate deeply enough to explain the large
radius of the planet. We derive a radiative time constant which increases with
depth and reaches about 8 hr at 0.1 bar and 2.3 days at 1 bar. Time-dependent
temperature profiles were also calculated, in the limit of a zonal wind that is
independent on height (i.e. solid-body rotation) and constant absorption
coefficients. We predict day-night variations of the effective temperature of
\~600 K, for an equatorial rotation rate of 1 km/s, in good agreement with the
predictions by Showman &Guillot (2002). This rotation rate yields day-to-night
temperature variations in excess of 600 K above the 0.1-bar level. These
variations rapidly decrease with depth below the 1-bar level and become
negligible below the ~5--bar level for rotation rates of at least 0.5 km/s. At
high altitudes (mbar pressures or less), the night temperatures are low enough
to allow sodium to condense into Na2S. Synthetic transit spectra of the visible
Na doublet show a much weaker sodium absorption on the morning limb than on the
evening limb. The calculated dimming of the sodium feature during planetary
transites agrees with the value reported by Charbonneau et al. (2002).Comment: 9 pages, 8 figures, replaced with the revised versio
Titan's Prolific Propane: The Cassini CIRS Perspective
In this paper we select large spectral averages of data from the Cassini
Composite Infrared Spectrometer (CIRS) obtained in limb-viewing mode at low
latitudes (30S--30N), greatly increasing the path length and hence
signal-to-noise ratio for optically thin trace species such as propane. By
modeling and subtracting the emissions of other gas species, we demonstrate
that at least six infrared bands of propane are detected by CIRS, including two
not previously identified in Titan spectra. Using a new line list for the range
1300-1400cm -1, along with an existing GEISA list, we retrieve propane
abundances from two bands at 748 and 1376 cm-1. At 748 cm-1 we retrieve 4.2 +/-
0.5 x 10(-7) (1-sigma error) at 2 mbar, in good agreement with previous
studies, although lack of hotbands in the present spectral atlas remains a
problem. We also determine 5.7 +/- 0.8 x 10(-7) at 2 mbar from the 1376 cm-1
band - a value that is probably affected by systematic errors including
continuum gradients due to haze and also an imperfect model of the n6 band of
ethane. This study clearly shows for the first time the ubiquity of propane's
emission bands across the thermal infrared spectrum of Titan, and points to an
urgent need for further laboratory spectroscopy work, both to provide the line
positions and intensities needed to model these bands, and also to further
characterize haze spectral opacity. The present lack of accurate modeling
capability for propane is an impediment not only for the measurement of propane
itself, but also for the search for the emissions of new molecules in many
spectral regions.Comment: 7 Figures, 3 Tables. Typeset in Latex with elsart.cls. In press for
Planetary and Space Scienc
Evidence for carbonyl sulfide (OCS) conversion to CO in the lower atmosphere of Venus
The chemical regimes in the atmosphere of Venus vary from photochemistry in the middle atmosphere to thermal equilibrium chemistry in the lower atmosphere and the surface. Many chemical cycles have been proposed, but few details about these cycles are fully verified by comparison between observations and modeling. Recent high-quality data of carbonyl sulfide (OCS) and CO from ground-based and Venus Express observations provide a unique opportunity to test our understanding of chemistry and transport in the lower atmosphere of Venus. The spatial distributions of OCS and CO in the atmosphere reflect a sensitive balance between chemistry and transport. On the basis of our updated photochemical model and winds from Lee et al.'s (2007) general circulation model, we study the chemistry and transport in a simplified two-dimensional chemistry-transport model. OCS is produced by heterogeneous reactions on the surface; the middle atmosphere is a net sink for OCS. The combination of data and modeling provides strong evidence for the loss of OCS by conversion to CO. The detailed chemical mechanism is currently unknown, although a number of speculations have been proposed. The sensitivity of the distributions of OCS and CO to model parameters is reported
Spatial Variations in the Altitude of the CH4 Homopause at Jupiter's Mid-to-high Latitudes, as Constrained from IRTF-TEXES Spectra
Peer reviewedPublisher PD
The formation and evolution of Titan's winter polar vortex
The polar hot-spot appeared in Titan after equinox in 2010 suddenly cooled in early 2012, which wasnât predicted by models. Here the authors use observations to show that the increase in trace gases during the hot-spot resulted in radiative cooling feedback
A chemical survey of exoplanets with ARIEL
Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planetâs birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25â7.8 ÎŒm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10â100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed â using conservative estimates of mission performance and a full model of all significant noise sources in the measurement â using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL â in line with the stated mission objectives â will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio
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