86 research outputs found
Characterizing Exoplanet Atmospheres: From Light-curve Observations to Radiative-Transfer Modeling
Multi-wavelength transit and secondary-eclipse light-curve observations are some of the most powerful techniques to probe the thermo-chemical properties of exoplanets. Although the large planet-to-star brightness contrast and few available spectral bands produce data with low signal-to-noise ratios, a Bayesian approach can robustly reveal what constraints we can set, without over-interpreting the data. Here I performed an end-to-end analysis of transiting exoplanet data. I analyzed space-telescope data for three planets to characterize their atmospheres and refine their orbits, investigated correlated noise estimators, and contributed to the development of the respective data-analysis pipelines. Chapters 2 and 3 describe the Photometry for Orbits, Eclipses and Transits (POET) pipeline to model Spitzer Space Telescope light curves. I analyzed secondary-eclipse observations of the Jupiter-sized planets WASP-8b and TrES-1, determining their day-side thermal emission in the infrared spectrum. The emission data of WASP-8b indicated no thermal inversion, and an anomalously high 3.6 micron brightness. Standard solar-abundance models, with or without a thermal inversion, can fit the thermal emission from TrES-1 well. Chapter 4 describes the most commonly used correlated-noise estimators for exoplanet light-curve modeling, and assesses their applicability and limitations to estimate parameters uncertainties. I show that the residual-permutation method is unsound for estimating parameter uncertainties. The time-averaging and the wavelet-based likelihood methods improve the uncertainty estimations, being within 20 - 50% of the expected value. Chapter 5 describes the open-source Bayesian Atmospheric Radiative Transfer (BART) code to characterize exoplanet atmospheres. BART combines a thermochemical-equilibrium code, a one-dimensional line-by-line radiative-transfer code, and the Multi-core Markov-chain Monte Carlo statistical module to constrains the atmospheric temperature and chemical-abundance profiles of exoplanets. I applied the BART code to the Hubble and Spitzer Space Telescope transit observations of the Neptune-sized planet HAT-P-11b. BART finds an atmosphere enhanced in heavy elements, constraining the water abundance to ~100 times that of the solar abundance
The Role of N2 as a Geo-Biosignature for the Detection and Characterization of Earth-like Habitats
Since the Archean, N2 has been a major atmospheric constituent in Earth's
atmosphere. Nitrogen is an essential element in the building blocks of life,
therefore the geobiological nitrogen cycle is a fundamental factor in the long
term evolution of both Earth and Earth-like exoplanets. We discuss the
development of the Earth's N2 atmosphere since the planet's formation and its
relation with the geobiological cycle. Then we suggest atmospheric evolution
scenarios and their possible interaction with life forms: firstly, for a
stagnant-lid anoxic world, secondly for a tectonically active anoxic world, and
thirdly for an oxidized tectonically active world. Furthermore, we discuss a
possible demise of present Earth's biosphere and its effects on the atmosphere.
Since life forms are the most efficient means for recycling deposited nitrogen
back into the atmosphere nowadays, they sustain its surface partial pressure at
high levels. Also, the simultaneous presence of significant N2 and O2 is
chemically incompatible in an atmosphere over geological timescales. Thus, we
argue that an N2-dominated atmosphere in combination with O2 on Earth-like
planets within circumstellar habitable zones can be considered as a
geo-biosignature. Terrestrial planets with such atmospheres will have an
operating tectonic regime connected with an aerobe biosphere, whereas other
scenarios in most cases end up with a CO2-dominated atmosphere. We conclude
with implications for the search for life on Earth-like exoplanets inside the
habitable zones of M to K-stars
A Spitzer Five-Band Analysis of the Jupiter-Sized Planet TrES-1
With an equilibrium temperature of 1200 K, TrES-1 is one of the coolest hot
Jupiters observed by {\Spitzer}. It was also the first planet discovered by any
transit survey and one of the first exoplanets from which thermal emission was
directly observed. We analyzed all {\Spitzer} eclipse and transit data for
TrES-1 and obtained its eclipse depths and brightness temperatures in the 3.6
{\micron} (0.083 % {\pm} 0.024 %, 1270 {\pm} 110 K), 4.5 {\micron} (0.094 %
{\pm} 0.024 %, 1126 {\pm} 90 K), 5.8 {\micron} (0.162 % {\pm} 0.042 %, 1205
{\pm} 130 K), 8.0 {\micron} (0.213 % {\pm} 0.042 %, 1190 {\pm} 130 K), and 16
{\micron} (0.33 % {\pm} 0.12 %, 1270 {\pm} 310 K) bands. The eclipse depths can
be explained, within 1 errors, by a standard atmospheric model with
solar abundance composition in chemical equilibrium, with or without a thermal
inversion. The combined analysis of the transit, eclipse, and radial-velocity
ephemerides gives an eccentricity , consistent
with a circular orbit. Since TrES-1's eclipses have low signal-to-noise ratios,
we implemented optimal photometry and differential-evolution Markov-chain Monte
Carlo (MCMC) algorithms in our Photometry for Orbits, Eclipses, and Transits
(POET) pipeline. Benefits include higher photometric precision and \sim10 times
faster MCMC convergence, with better exploration of the phase space and no
manual parameter tuning.Comment: 17 pages, Accepted for publication in Ap
Non-Local Thermodynamic Equilibrium Transmission Spectrum Modelling of HD209458b
Context - Exoplanetary upper atmospheres are low density environments where
radiative processes can compete with collisional ones and introduce non-local
thermodynamic equilibrium (NLTE) effects into transmission spectra.
Aims - We develop a NLTE radiative transfer framework capable of modelling
exoplanetary transmission spectra over a wide range of planetary properties.
Methods - We adapt the NLTE spectral synthesis code Cloudy to produce an
atmospheric structure and atomic transmission spectrum in both NLTE and local
thermodynamic equilibrium (LTE) for the hot Jupiter HD209458b, given a
published T-P profile and assuming solar metallicity. Selected spectral
features, including H, Na I D, He I 10830, Fe I & II
ultra-violet (UV) bands, and C, O and Si UV lines, are compared with literature
observations and models where available. The strength of NLTE effects are
measured for individual spectral lines to identify which features are most
strongly affected.
Results - The developed modelling framework computing NLTE synthetic spectra
reproduces literature results for the He I 10830 triplet, the Na I D
lines, and the forest of Fe I lines in the optical. Individual spectral lines
in the NLTE spectrum exhibit up to 40 % stronger absorption relative to the LTE
spectrum.Comment: Accepted for publication in A&A, 15 pages, 13 figure
The Hubble/STIS Near-ultraviolet Transmission Spectrum of HD 189733b
The benchmark hot Jupiter HD 189733b has been a key target to lay out the
foundations of comparative planetology for giant exoplanets. As such, HD
189733b has been extensively studied across the electromagnetic spectrum. Here,
we report the observation and analysis of three transit light curves of HD
189733b obtained with {\Hubble}/STIS in the near ultraviolet, the last
remaining unexplored spectral window to be probed with present-day
instrumentation for this planet. The NUV is a unique window for atmospheric
mass-loss studies owing to the strong resonance lines and large photospheric
flux. Overall, from a low-resolution analysis () we found that the
planet's near-ultraviolet spectrum is well characterized by a relatively flat
baseline, consistent with the optical-infrared transmission, plus two regions
at 2350 and 2600 {\AA} that exhibit a broad and significant excess
absorption above the continuum. From an analysis at a higher resolution
(), we found that the transit depths at the core of the magnesium
resonance lines are consistent with the surrounding continuum. We discarded the
presence of \ion{Mg}{ii} absorption in the upper atmosphere at a
2--4 confidence level, whereas we could place no significant
constraint for \ion{Mg}{i} absorption. These broad absorption features coincide
with the expected location of \ion{Fe}{ii} bands; however, solar-abundance
hydrodynamic models of the upper atmosphere are not able to reproduce the
amplitude of these features with iron absorption. Such scenario would require a
combination of little to no iron condensation in the lower-atmosphere,
super-solar metallicities, and a mechanism to enhance the absorption features
(such as zonal wind broadening). The true nature of this feature remains to be
confirmed.Comment: Accepted for publication at Astronomy and Astrophysic
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