125 research outputs found
Limits on Clouds and Hazes for the TRAPPIST-1 Planets
The TRAPPIST-1 planetary system is an excellent candidate for study of the
evolution and habitability of M-dwarf planets. Transmission spectroscopy
observations performed with the Hubble Space Telescope (HST) suggest the
innermost five planets do not possess clear hydrogen atmospheres. Here we
reassess these conclusions with recently updated mass constraints and expand
the analysis to include limits on metallicity, cloud top pressure, and the
strength of haze scattering. We connect recent laboratory results of particle
size and production rate for exoplanet hazes to a one-dimensional atmospheric
model for TRAPPIST-1 transmission spectra. Doing so, we obtain a
physically-based estimate of haze scattering cross sections. We find haze
scattering cross sections on the order of 1e-26 to 1e-19 cm squared are needed
in hydrogen-rich atmospheres for TRAPPIST-1 d, e, and f to match the HST data.
For TRAPPIST-1 g, we cannot rule out a clear hydrogen-rich atmosphere. We also
modeled the effects an opaque cloud deck and substantial heavy element content
have on the transmission spectra. We determine that hydrogen-rich atmospheres
with high altitude clouds, at pressures of 12mbar and lower, are consistent
with the HST observations for TRAPPIST-1 d and e. For TRAPPIST-1 f and g, we
cannot rule out clear hydrogen-rich cases to high confidence. We demonstrate
that metallicities of at least 60xsolar with tropospheric (0.1 bar) clouds
agree with observations. Additionally, we provide estimates of the precision
necessary for future observations to disentangle degeneracies in cloud top
pressure and metallicity. Our results suggest secondary, volatile-rich
atmospheres for the outer TRAPPIST-1 planets d, e, and f.Comment: 15 pages, 3 figures, 2 tables, accepted in the Astronomical Journa
Aerosols are not Spherical Cows: Using Discrete Dipole Approximation to Model the Properties of Fractal Particles
The optical properties of particulate-matter aerosols, within the context of
exoplanet and brown dwarf atmospheres, are compared using three different
models: Mie theory, Modified Mean Field (MMF) Theory, and Discrete Dipole
Approximation (DDA). Previous results have demonstrated that fractal haze
particles (MMF and DDA) absorb much less long-wavelength radiation than their
spherical counterparts (Mie), however it is shown here that the opposite can
also be true if a more varying refractive index profile is used. Additionally,
it is demonstrated that absorption and scattering cross-sections, as well as
the asymmetry parameter, are underestimated if Mie theory is used. Although DDA
can be used to obtain more accurate results, it is known to be much more
computationally intensive; to avoid this, the use of low-resolution aerosol
models is explored, which could dramatically speed up the process of obtaining
accurate computations of optical cross-sections within a certain parameter
space. The validity of DDA is probed for wavelengths of interest for
observations of aerosols within exoplanet and brown dwarf atmospheres (0.2 to
15 micrometres). Finally, novel code is presented to compare the results of
Mie, MMF and DDA theories (CORAL: Comparison Of Radiative AnaLyses), as well as
to increase and decrease the resolution of DDA shape files accordingly
(SPHERIFY). Both codes can be applied to a range of other interesting
astrophysical environments in addition to exoplanet atmospheres, for example
dust grains within protoplanetary disks.Comment: 24 pages, 23 figures, accepted for publication in "Monthly Notices of
the Royal Astronomical Society
High temperature condensate clouds in super-hot Jupiter atmospheres
Deciphering the role of clouds is central to our understanding of exoplanet
atmospheres, as they have a direct impact on the temperature and pressure
structure, and observational properties of the planet. Super-hot Jupiters
occupy a temperature regime similar to low mass M-dwarfs, where minimal cloud
condensation is expected. However, observations of exoplanets such as WASP-12b
(Teq ~ 2500 K) result in a transmission spectrum indicative of a cloudy
atmosphere. We re-examine the temperature and pressure space occupied by these
super-hot Jupiter atmospheres, to explore the role of the initial Al- and
Ti-bearing condensates as the main source of cloud material. Due to the high
temperatures a majority of the more common refractory material is not depleted
into deeper layers and would remain in the vapor phase. The lack of depletion
into deeper layers means that these materials with relatively low cloud masses
can become significant absorbers in the upper atmosphere. We provide
condensation curves for the initial Al- and Ti-bearing condensates that may be
used to provide quantitative estimates of the effect of metallicity on cloud
masses, as planets with metal-rich hosts potentially form more opaque clouds
because more mass is available for condensation. Increased metallicity also
pushes the point of condensation to hotter, deeper layers in the planetary
atmosphere further increasing the density of the cloud. We suggest that planets
around metal-rich hosts are more likely to have thick refractory clouds, and
discuss the implication on the observed spectra of WASP-12b.Comment: Accepted for publication in MNRAS, 10 pages, 1 table, 5 figure
A comparative study of WASP-67b and HAT-P-38b from WFC3 data
Atmospheric temperature and planetary gravity are thought to be the main
parameters affecting cloud formation in giant exoplanet atmospheres. Recent
attempts to understand cloud formation have explored wide regions of the
equilibrium temperature-gravity parameter space. In this study, we instead
compare the case of two giant planets with nearly identical equilibrium
temperature ( ) and gravity (. During Cycle 23, we collected WFC3/G141
observations of the two planets, WASP-67 b and HAT-P-38 b. HAT-P-38 b, with
mass 0.42 M and radius 1.4 , exhibits a relatively
clear atmosphere with a clear detection of water. We refine the orbital period
of this planet with new observations, obtaining . WASP-67 b, with mass 0.27 M and radius 0.83
, shows a more muted water absorption feature than that of
HAT-P-38 b, indicating either a higher cloud deck in the atmosphere or a more
metal-rich composition. The difference in the spectra supports the hypothesis
that giant exoplanet atmospheres carry traces of their formation history.
Future observations in the visible and mid-infrared are needed to probe the
aerosol properties and constrain the evolutionary scenario of these planets.Comment: 16 pages, 17 figures, 8 tables, accepted for publication in The
Astronomical Journa
Importance of Sample Selection in Exoplanet Atmosphere Population Studies
Understanding planet formation requires robust population studies, which are
designed to reveal trends in planet properties. In this work, we aim to
determine if different methods for selecting populations of exoplanets for
atmospheric characterization with JWST could influence population-level
inferences. We generate three hypothetical surveys of
super-Earths/sub-Neptunes, each spanning a similar radius-insolation flux
space. The survey samples are constructed based on three different selection
criteria (evenly-spaced-by-eye, binned, and a quantitative selection function).
Using an injection-recovery technique, we test how robustly individual-planet
atmospheric parameters and population-level parameters can be retrieved. We
find that all three survey designs result in equally suitable targets for
individual atmospheric characterization, but not equally suitable targets for
constraining population parameters. Only samples constructed with a
quantitative method or that are sufficiently evenly-spaced-by-eye result in
robust population parameter constraints. Furthermore, we find that the sample
with the best targets for individual atmospheric study does not necessarily
result in the best constrained population parameters. The method of sample
selection must be considered. We also find that there may be large variability
in population-level results with a sample that is small enough to fit in a
single JWST cycle (12 planets), suggesting that the most successful
population-level analyses will be multi-cycle. Lastly, we infer that our
exploration of sample selection is limited by the small number of transiting
planets with measured masses around bright stars. Our results can guide future
development of programs that aim to determine underlying trends in exoplanet
atmospheric properties and, by extension, formation and evolution processes.Comment: 16 pages, 7 figures, accepted Ap
Optical to near-infrared transmission spectrum of the warm sub-Saturn HAT-P-12b
We present the transmission spectrum of HAT-P-12b through a joint analysis of
data obtained from the Hubble Space Telescope Space Telescope Imaging
Spectrograph (STIS) and Wide Field Camera 3 (WFC3) and Spitzer, covering the
wavelength range 0.3-5.0 m. We detect a muted water vapor absorption
feature at 1.4 m attenuated by clouds, as well as a Rayleigh scattering
slope in the optical indicative of small particles. We interpret the
transmission spectrum using both the state-of-the-art atmospheric retrieval
code SCARLET and the aerosol microphysics model CARMA. These models indicate
that the atmosphere of HAT-P-12b is consistent with a broad range of
metallicities between several tens to a few hundred times solar, a roughly
solar C/O ratio, and moderately efficient vertical mixing. Cloud models that
include condensate clouds do not readily generate the sub-micron particles
necessary to reproduce the observed Rayleigh scattering slope, while models
that incorporate photochemical hazes composed of soot or tholins are able to
match the full transmission spectrum. From a complementary analysis of
secondary eclipses by Spitzer, we obtain measured depths of
and at 3.6 and 4.5 m, respectively, which are
consistent with a blackbody temperature of K and indicate
efficient day-night heat recirculation. HAT-P-12b joins the growing number of
well-characterized warm planets that underscore the importance of clouds and
hazes in our understanding of exoplanet atmospheres.Comment: 25 pages, 19 figures, accepted for publication in AJ, updated with
proof correction
A Library of Self-Consistent Simulated Exoplanet Atmospheres
We present a publicly available library of model atmospheres with
radiative-convective equilibrium Pressure-Temperature (-) profiles fully
consistent with equilibrium chemical abundances, and the corresponding emission
and transmission spectrum with R5000 at 0.2 m decreasing to
R35 at 30 m, for 89 hot Jupiter exoplanets, for four re-circulation
factors, six metallicities and six C/O ratios. We find the choice of
condensation process (local/rainout) alters the - profile and thereby the
spectrum substantially, potentially detectable by JWST. We find H opacity
can contribute to form a strong temperature inversion in ultra-hot Jupiters for
C/O ratios 1 and can make transmission spectra features flat in the
optical, alongside altering the entire emission spectra. We highlight how
adopting different model choices such as thermal ionisation, opacities,
line-wing profiles and the methodology of varying the C/O ratio, effects the
- structure and the spectrum. We show the role of Fe opacity to form
primary/secondary inversion in the atmosphere. We use WASP-17b and WASP-121b as
test cases to demonstrate the effect of grid parameters across their full
range, while highlighting some important findings, concerning the overall
atmospheric structure, chemical transition regimes and their observables.
Finally, we apply this library to the current transmission and emission spectra
observations of WASP-121b, which shows HO and tentative evidence for VO at
the limb, and HO emission feature indicative of inversion on the dayside,
with very low energy redistribution, thereby demonstrating the applicability of
library for planning and interpreting observations of transmission and emission
spectrum.Comment: 26 pages, 19 figures in the main paper. 13 pages, 6 figures, 3 tables
in the supplementary material attached with the main paper here. Accepted for
Publication in MNRAS. Full grid of model P-T profiles, chemical abundances,
transmission and emission spectra, contribution functions are available here,
https://drive.google.com/drive/folders/1zCCe6HICuK2nLgnYJFal7W4lyunjU4J
Understanding Exoplanet Atmospheres with UV Observations I: NUV and Blue/Optical
Much of the focus of exoplanet atmosphere analysis in the coming decade will be atinfrared wavelengths, with the planned launches of the James Webb Space Telescope (JWST) and the Wide-Field Infrared Survey Telescope (WFIRST). However, without being placed in the context of broader wavelength coverage, especially in the optical and ultraviolet, infrared observations produce an incomplete picture of exoplanet atmospheres. Scattering information encoded in blue optical and near-UV observations can help determine whether muted spectral features observed in the infrared are due to a hazy/cloudy atmosphere, or a clear atmosphere with a higher mean molecular weight. UV observations can identify atmospheric escape and mass loss from exoplanet atmospheres, providing a greater understanding of the atmospheric evolution of exoplanets, along with composition information from above the cloud deck. In this white paper we focus on the science case for exoplanet observations in the near-UV; an accompanying white paper led by Eric Lopez will focus on the science case in the far-UV
Starspot occultations in infrared transit spectroscopy: the case of WASP-52b
Stellar activity is one of the main obstacles to high-precision exoplanet
observations and has motivated extensive studies in detection and
characterization problems. Most efforts focused on unocculted starspots in
optical transit spectrophotometry, while the impact of starspot crossings is
assumed to be negligible in the near-infrared. Here, we present
\textit{HST}/WFC3 transit observations of the active star WASP-52, hosting an
inflated hot Jupiter, which present a possible starspot occultation signal. By
using this data set as a benchmark, we investigated whether the masking of the
transit profile distortion or modeling it with both a starspot model and a
Gaussian process affects the shape of the transmission spectrum. Different
methods produced spectra with the same shape and a robust detection of water
vapor, and with different reference radii for the planet.
The solutions of all methods are in agreement and reached a similar level of
precision. Our WFC3 light curve of WASP-52b hints that starspot crossings might
become more problematic with \textit{JWST}'s higher sensitivity and complete
coverage of the transit profile.Comment: 15 pages, 15 figures, 5 tables, data available onlin
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