158 research outputs found
ExoTiC-ISM: A Python package for marginalised exoplanet transit parameters across a grid of systematic instrument models
To address the the problem of calibration of instrument systematics in
transit light curves, we present the Python package ExoTiC-ISM. Transit
spectroscopy can reveal many different chemical components in exoplanet
atmospheres, but such results depend on well-calibrated transit light curve
observations. Each transit data set will contain instrument systematics that
depend on the instrument used and will need to be calibrated out with an
instrument systematic model. The proposed solution in Wakeford et al. (2016)
(arXiv:1601.02587 [astro-ph.EP]) is to use a marginalisation across a grid of
systematic models in order to retrieve marginalised transit parameters. Doing
this over observations in multiple wavelengths yields a robust transmission
spectrum of an exoplanet. ExoTiC-ISM provides tools to perform this analysis,
and its current capability contains a systematic grid that is applicable to the
Wide Field Camera 3 (WFC3) detector on the Hubble Space Telescope (HST),
particularly for the two infrared grisms G141 and G102. By modularisation of
the code and implementation of more systematic grids, ExoTiC-ISM can be used
for other instruments, and an implementation for select detectors on the James
Webb Space Telescope (JWST) will provide robust transit spectra in the future.Comment: 5 pages, published in JOS
Cloudy with a chance of water: Investigating hot Jupiter exoplanet atmospheres through observation and analysis
Since the discovery of the first exoplanet orbiting a sun-like star in 1995, the fundamental questions as to the formation of our Solar System have met a paradigm shift. The presence of hot Jupiter exoplanets, Jupiter sized worlds rapidly orbiting their host stars, was unlike anything previously seen or predicted. The later discovery of these strange new worlds transiting their stars opened up a new realm of studies into their atmospheres using transit spectroscopy to separate the signals between the star and planetary atmosphere. This thesis investigates the transmission spectral properties of hot Jupiter exoplanets through observations and theoretical analysis from the search for H2O in the near-IR to the signatures of cloud condensates in the IR.
Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) observations of transiting hot Jupiters were used to investigate the atmospheric composition over water bands in the near infrared. We put forward a new analysis method to treat the varying systematics seen across transit datasets in a consistent and robust way, in which we marginalise over a grid of possible systematic models used to correct the lightcurves, with each model contributing to the extracted spectrum based on its statistical likelihood. We apply this new method to five previously studied hot Jupiter exoplanet transmission spectra and make direct comparisons between the planetary atmospheres. An apparent dichotomy emerges between two possible sub-classes of hot Jupiter atmospheres with clouds and hazes playing a key role. WFC3 appears to cover a critical wavelength range in exoplanet atmospheres where clouds and hazes potentially obscure the expected molecular signatures in systems where they are found to be obscured in the optical.
Using analytical models following Mie theory, we explore the potential atmospheric transmission spectral signatures that would be caused by a variety of cloud condensates in hot Jupiter atmospheres. We find that the observed optical slope representing Rayleigh scattering at high altitudes can constrain the cloud condensate particle size and can be used as a diagnostic for potential condensate features in the IR where almost all condensate absorption features occur. We find that the major transmission spectral absorption features are generated by the vibrational modes of the major diatomic bond pair in each condensate species, which is often seen in the IR at 5-25 microns, and explore the potential for future JWST investigations using MIRI.Science and Technology Funding Council (STFC
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
Quantifying the Transit Light Source Effect: Measurements of Spot Temperature and Coverage on the Photosphere of AU Microscopii with High-Resolution Spectroscopy and Multi-Color Photometry
AU Mic is an active 24 Myr pre-main sequence M dwarf in the stellar
neighborhood (d9.7 pc) with a rotation period of 4.86 days. The two
transiting planets orbiting AU Mic, AU Mic b and c, are warm sub-Neptunes on
8.5 and 18.9 day periods and are targets of interest for atmospheric
observations of young planets. Here we study AU Mic's unocculted starspots
using ground-based photometry and spectra in order to complement current and
future transmission spectroscopy of its planets. We gathered multi-color LCO
0.4m SBIG photometry to study the star's rotational modulations and LCO NRES
high-resolution spectra to measure the different spectral components within the
integrated spectrum of the star, parameterized by 3 spectral components and
their coverage fractions. We find AU Mic's surface has at least 2 spectral
components, a K ambient photosphere with cool spots that have a
temperature of K and cover percent of the surface,
increasing and decreasing by 5 from the average throughout a rotation. We
also detect a third flux component with a filling factor less than 0.5 and
a largely uncertain temperature that we attribute to flare flux not entirely
omitted in the time-averaged spectra. We include measurements of spot
temperature and coverage fraction from both 2- and 3- temperature models, which
we find agree with each other strongly. Our expanded use of various techniques
to study starspots will help us better understand this system and may have
applications for interpreting the transmission spectra for exoplanets
transiting stars of a wide range of activity levels.Comment: 25 pages, 13 figures, Accepted to 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
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
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