549 research outputs found
A precise optical transmission spectrum of the inflated exoplanet WASP-52b
We have measured a precise optical transmission spectrum for WASP-52b, a
highly inflated hot Jupiter with an equilibrium temperature of 1300 K. Two
transits of the planet were observed spectroscopically at low resolution with
the auxiliary-port camera (ACAM) on the William Herschel Telescope (WHT),
covering a wide range of 4000-8750 \AA. We use a Gaussian process approach to
model the correlated noise in the multi-wavelength light curves, resulting in a
high precision relative transmission spectrum with errors on the order of a
pressure scale height. We attempted to fit a variety of different
representative model atmospheres to the transmission spectrum, but did not find
a satisfactory match to the entire spectral range. For the majority of the
covered wavelength range (4000-7750 \AA) the spectrum is flat, and can be
explained by an optically thick and grey cloud layer at 0.1 mbar, but this is
inconsistent with a slightly deeper transit at wavelengths \AA. We
were not able to find an obvious systematic source for this feature, so this
opacity may be the result of an additional unknown absorber.Comment: Submitted to MNRAS 17 Jan 2017, revised version after comments from
reviewer, 12 pages, 10 figure
Constraining the Atmospheric Composition of the Day-Night Terminators of HD 189733b : Atmospheric Retrieval with Aerosols
A number of observations have shown that Rayleigh scattering by aerosols
dominates the transmission spectrum of HD 189733b at wavelengths shortward of 1
m. In this study, we retrieve a range of aerosol distributions consistent
with transmission spectroscopy between 0.3-24 m that were recently
re-analyzed by Pont et al. (2013). To constrain the particle size and the
optical depth of the aerosol layer, we investigate the degeneracies between
aerosol composition, temperature, planetary radius, and molecular abundances
that prevent unique solutions for transit spectroscopy. Assuming that the
aerosol is composed of MgSiO, we suggest that a vertically uniform aerosol
layer over all pressures with a monodisperse particle size smaller than about
0.1 m and an optical depth in the range 0.002-0.02 at 1 m provides
statistically meaningful solutions for the day/night terminator regions of HD
189733b. Generally, we find that a uniform aerosol layer provide adequate fits
to the data if the optical depth is less than 0.1 and the particle size is
smaller than 0.1 m, irrespective of the atmospheric temperature, planetary
radius, aerosol composition, and gaseous molecules. Strong constraints on the
aerosol properties are provided by spectra at wavelengths shortward of 1 m
as well as longward of 8 m, if the aerosol material has absorption
features in this region. We show that these are the optimal wavelengths for
quantifying the effects of aerosols, which may guide the design of future space
observations. The present investigation indicates that the current data offer
sufficient information to constrain some of the aerosol properties of
HD189733b, but the chemistry in the terminator regions remains uncertain.Comment: Transferred to ApJ and accepted. 11 pages, 10 figures, 1 tabl
Seasonal Variability of Saturn's Tropospheric Temperatures, Winds and Para-H from Cassini Far-IR Spectroscopy
Far-IR 16-1000 m spectra of Saturn's hydrogen-helium continuum measured
by Cassini's Composite Infrared Spectrometer (CIRS) are inverted to construct a
near-continuous record of upper tropospheric (70-700 mbar) temperatures and
para-H fraction as a function of latitude, pressure and time for a third of
a Saturnian year (2004-2014, from northern winter to northern spring). The
thermal field reveals evidence of reversing summertime asymmetries superimposed
onto the belt/zone structure. The temperature structure that is almost
symmetric about the equator by 2014, with seasonal lag times that increase with
depth and are qualitatively consistent with radiative climate models. Localised
heating of the tropospheric hazes (100-250 mbar) create a distinct perturbation
to the temperature profile that shifts in magnitude and location, declining in
the autumn hemisphere and growing in the spring. Changes in the para-H
() distribution are subtle, with a 0.02-0.03 rise over the spring
hemisphere (200-500 mbar) perturbed by (i) low- air advected by both the
springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of
high- air at northern high latitudes, responsible for a developing
north-south asymmetry in . Conversely, the shifting asymmetry in the
para-H disequilibrium primarily reflects the changing temperature structure
(and the equilibrium distribution of ), rather than actual changes in
induced by chemical conversion or transport. CIRS results interpolated to
the same point in the seasonal cycle as re-analysed Voyager-1 observations show
qualitative consistency, with the exception of the tropical tropopause near the
equatorial zones and belts, where downward propagation of a cool temperature
anomaly associated with Saturn's stratospheric oscillation could potentially
perturb tropopause temperatures, para-H and winds. [ABRIDGED]Comment: Preprint accepted for publication in Icarus, 29 pages, 18 figure
Upper limits for PH3 and H2S in Titan's Atmosphere from Cassini CIRS
We have searched for the presence of simple P and S-bearing molecules in
Titan's atmosphere, by looking for the characteristic signatures of phosphine
and hydrogen sulfide in infrared spectra obtained by Cassini CIRS. As a result
we have placed the first upper limits on the stratospheric abundances, which
are 1 ppb (PH3) and 330 ppb (H2S), at the 2-sigma significance level.Comment: 12 pages text, 1 table, 2 figure
On the potential of the EChO mission to characterise gas giant atmospheres
Space telescopes such as EChO (Exoplanet Characterisation Observatory) and
JWST (James Webb Space Telescope) will be important for the future study of
extrasolar planet atmospheres. Both of these missions are capable of performing
high sensitivity spectroscopic measurements at moderate resolutions in the
visible and infrared, which will allow the characterisation of atmospheric
properties using primary and secondary transit spectroscopy. We use the NEMESIS
radiative transfer and retrieval tool (Irwin et al. 2008, Lee et al. 2012) to
explore the potential of the proposed EChO mission to solve the retrieval
problem for a range of H2-He planets orbiting different stars. We find that
EChO should be capable of retrieving temperature structure to ~200 K precision
and detecting H2O, CO2 and CH4 from a single eclipse measurement for a hot
Jupiter orbiting a Sun-like star and a hot Neptune orbiting an M star, also
providing upper limits on CO and NH3. We provide a table of retrieval
precisions for these quantities in each test case. We expect around 30
Jupiter-sized planets to be observable by EChO; hot Neptunes orbiting M dwarfs
are rarer, but we anticipate observations of at least one similar planet.Comment: 22 pages, 30 figures, 4 tables. Accepted for publication in MNRA
Testing 2D temperature models in Bayesian retrievals of atmospheric properties from hot Jupiter phase curves
Spectroscopic phase curves of transiting hot Jupiters are spectral
measurements at multiple orbital phases, giving a set of disc-averaged spectra
that probe multiple hemispheres. By fitting model phase curves to observations,
we can constrain the atmospheric properties of hot Jupiters such as molecular
abundance, aerosol distribution and thermal structure, which offer insights
into their dynamics, chemistry, and formation. In this work, we propose a novel
2D temperature scheme consisting of a dayside and a nightside to retrieve
information from near-infrared phase curves, and apply the scheme to phase
curves of WASP-43b observed by HST/WFC3 and Spitzer/IRAC. In our scheme,
temperature is constant on isobars on the nightside and varies with
cos(longitude/) on isobars on the dayside, where and
are free parameters. We fit all orbital phases simultaneously using
the radiative transfer package NEMESISPY coupled to a Bayesian inference code.
We first validate the performance of our retrieval scheme with synthetic phase
curves generated from a GCM, and find our 2D scheme can accurately retrieve the
latitudinally-averaged thermal structure and constrain the abundance of HO
and CH. We then apply our 2D scheme to the observed phase curves of
WASP-43b and find: (1) the dayside temperature-pressure profiles do not vary
strongly with longitude and are non-inverted; (2) the retrieved nightside
temperatures are extremely low, suggesting significant nightside cloud
coverage; (3) the HO volume mixing ratio is constrained to
--, and we retrieve an upper bound for
CH at 10.Comment: 23 pages, 20 figures, 3 tables, accepted for publication in MNRA
Measurement of CHD on Titan at Submillimeter Wavelengths
We present the first radio/submillimeter detection of monodeuterated methane
(CHD) in Titan's atmosphere, using archival data from of the Atacama Large
Millimeter/submillimeter Array (ALMA). The and
transitions at 465.235 and 465.250 GHz ( mm) were measured at
significance levels of and , respectively. These two
lines were modeled using the Non-linear optimal Estimator for MultivariatE
spectral analySIS (NEMESIS) radiative transfer code to determine the
disk-averaged CHD volume mixing ratio = in Titan's
stratosphere (at altitudes km). By comparison with the CH vertical
abundance profile measured by Cassini-Huygens mass spectrometry, the resulting
value for D/H in CH is . This is consistent
with previous ground-based and in-situ measurements from the Cassini-Huygens
mission, though slightly lower than the average of the previous values.
Additional CHD observations at higher spatial resolution will be required
to determine a value truly comparable with the Cassini-Huygens CH
measurements, by measuring CHD with ALMA close to Titan's equator. In the
post-Cassini era, spatially resolved observations of CHD with ALMA will
enable the latitudinal distribution of methane to be determined, making this an
important molecule for further studies.Comment: 9 pages, 4 figure
Exoplanet atmospheres with EChO: spectral retrievals using EChOSim
We demonstrate the effectiveness of the Exoplanet Characterisation
Observatory mission concept for constraining the atmospheric properties of hot
and warm gas giants and super Earths. Synthetic primary and secondary transit
spectra for a range of planets are passed through EChOSim (Waldmann & Pascale
2014) to obtain the expected level of noise for different observational
scenarios; these are then used as inputs for the NEMESIS atmospheric retrieval
code and the retrieved atmospheric properties (temperature structure,
composition and cloud properties) compared with the known input values,
following the method of Barstow et al. (2013a). To correctly retrieve the
temperature structure and composition of the atmosphere to within 2 {\sigma},
we find that we require: a single transit or eclipse of a hot Jupiter orbiting
a sun-like (G2) star at 35 pc to constrain the terminator and dayside
atmospheres; 20 transits or eclipses of a warm Jupiter orbiting a similar star;
10 transits/eclipses of a hot Neptune orbiting an M dwarf at 6 pc; and 30
transits or eclipses of a GJ1214b-like planet.Comment: 13 pages, 15 figures, 1 table. Accepted by Experimental Astronomy.
The final publication will shortly be available at Springer via
http://dx.doi.org/10.1007/s10686-014-9397-
Optimal Estimation Retrievals of the Atmospheric Structure and Composition of HD 189733b from Secondary Eclipse Spectroscopy
Recent spectroscopic observations of transiting hot Jupiters have permitted
the derivation of the thermal structure and molecular abundances of H2O, CO2,
CO, and CH4 in these extreme atmospheres. Here, for the first time, we apply
the technique of optimal estimation to determine the thermal structure and
composition of an exoplanet by solving the inverse problem. The development of
a suite of radiative transfer and retrieval tools for exoplanet atmospheres is
described, building upon a retrieval algorithm which is extensively used in the
study of our own solar system. First, we discuss the plausibility of detection
of different molecules in the dayside atmosphere of HD 189733b and the
best-fitting spectrum retrieved from all publicly available sets of secondary
eclipse observations between 1.45 and 24 {\mu}m. Additionally, we use
contribution functions to assess the vertical sensitivity of the emission
spectrum to temperatures and molecular composition. Over the altitudes probed
by the contribution functions, the retrieved thermal structure shows an
isothermal upper atmosphere overlying a deeper adiabatic layer (temperature
decreasing with altitude), which is consistent with previously-reported
dynamical and observational results. The formal uncertainties on retrieved
parameters are estimated conservatively using an analysis of the
cross-correlation functions and the degeneracy between different atmospheric
properties. The formal solution of the inverse problem suggests that the
uncertainties on retrieved parameters are larger than suggested in previous
studies, and that the presence of CO and CH4 is only marginally supported by
the available data. Nevertheless, by including as broad a wavelength range as
possible in the retrieval, we demonstrate that available spectra of HD 189733b
can constrain a family of potential solutions for the atmospheric structure.Comment: 13 pages, 10 figures, 1 table, Accepted for publication in MNRA
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