1,721,382 research outputs found
Mitigating Stellar Activity in Radial Velocity Analyses to Determine the Physical Parameters of Six Exoplanet Systems
Full text linkLarge transit surveys have shown that small planets are abundant. The compositions of these planets are of particular interest as they span the transition between small rocky planets (super-Earths) and intermediate-sized planets with volatile envelopes (sub-Neptunes). The limiting factor on measuring small exoplanet masses to inform composition or atmospheric models is stellar activity. Gaussian processes are one such way to mitigate stellar activity and achieve precise planet masses. In this dissertation, we find Gaussian processes are an effective way to model solar activity and determine the solar rotation period. Our concurrent solar photometry and radial velocity analyses produce consistent results; stellar photometry may assist radial velocity activity analyses. Subsequently, we perform radial velocity analyses on six exoplanet systems hosting eleven transiting planets: K2-291, GJ 3470, K2-3, HD97658, GJ 9827, and HD 106315. We examine the stellar activity present and use a Gaussian process where relevant. With our measured masses, we comment on the potential compositions and compare these planets with other similar exoplanet systems.The eleven planets measured in this work are consistent with the theory that planets smaller than the radius valley are primarily rocky and those larger than the radius valley contain a volatile envelope
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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Understanding Kepler's Super-Earths and Sub-Neptunes: Insights from Thermal Evolution and Photo-Evaporation
Full text linkExtrasolar planet surveys have identified an abundant new population of highly irradiated planets with sizes that are in between that of the Earth and Neptune. Such planets are unlike anything found in our own Solar System, and many of their basic properties are not understood. As such, these planets provide a fundamental test for models of planets formation and evolution with important implications for the formation of the Earth and planet habitability. In order to understand these new classes of planets, we have developed planetary structure and evolution models that can be used both to answer questions about individual planetary systems and to study populations of planets as a whole. In brief, these models allow usto follow a planet's mass, size, internal structure, and composition as it ages; from the time it finishes formation until it is detected billions of years later.These evolution models are critical because a planet's composition can change substantially over its lifetime. Close-in planets, like most of those found so far, are bombarded by large amounts of ionizing radiation, which over time can completely strip away a planet's atmosphere; even turning a gas-rich Neptune sized planet into a barren rocky super-Earth.Using these models, we explore the structure, composition, and evolution of sub- Neptune sized extrasolar planets found by NASA's Kepler mission. We examine the relationships between planetary masses, radii, and compositions. We show how these compositionshave been sculpted by photo-evaporation, and we examine the interplay between thermal and evaporative evolution
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Bayesian Statistical Inference of Giant Planet Physics
Full text linkThe many exoplanet discoveries of recent years have opened new avenues for studying giant planets and their formation. The giant planets of our solar system have been studied up close and in great detail, and exoplanets can complement this with a rich population to examine statistically. More than just studying their occurrence rates, it is possible to combine physical and statistical models to uncover aspects of their physical processes. I apply this strategy here on a series of related topics. First, I study a set of cool giant exoplanets, infer their bulk compositions, and demonstrate that there is a relationship between a planet's mass and its composition. I further discuss the implications to their formation, and how a planet's bulk composition can usefully complement its observed atmospheric abundances. I also consider hot Jupiters, inferring the amount of internal heating required to explain their anomalously large radii, the cause of which is one of the longest standing open questions in exoplanet science. I show through a careful examination of their radii and parent star evolution that these objects appear to reinflate quickly when their equilibrium temperature is increased. This strongly constrains the physical mechanisms that are causing their inflation. Finally, I outline several immediately relevant areas for future work to better understand these objects
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Understanding Atmospheres of Exoplanets and Brown Dwarfs with JWST
Full text linkWe have entered a new era of detailed exoplanet and brown dwarf atmospheric characterization with the James Webb Space Telescope (JWST), enabling very high signal-to-noise spectroscopic observations. This has allowed us to probe their atmospheres and interiors with unprecedented detail. This thesis introduces new constraints on previously uncertain processes such as atmospheric dynamics, cloud physics, and interior heat flux of transiting and self-luminous giant exoplanets, exo-Neptunes, and brown dwarfs, using newly developed advanced theoretical models and JWST observations.Constraints on Aerosols– Using 3D general circulation models, we examine whether the unknown 3D cloud distribution in exoplanets and brown dwarfs can be probed by their disk-integrated polarized thermal radiation. Our findings show that the clouds predicted in 3D GCMs can replicate the observed thermal polarization of brown dwarfs Luhman 16 A and B. However, we also show that small-scale vortices and cloud particle radii can sufficiently alter the disk-integrated polarized signals from such objects. To probe the 3D distribution pattern of aerosols in giant transiting exoplanet atmospheres, we present limb-resolved transmission spectra of the morning and evening limbs of hot Jupiter WASP-94A b, obtained using JWST. We find the colder morning limb of WASP-94A b is completely enshrouded in aerosols whereas the hotter evening limb is clear, showing prominent gas absorption features. We leverage these observations to show that aerosols in hot Jupiters can be primarily composed of condensate clouds instead of photochemical hazes. We also show that ignoring such aerosol-driven limb-to-limb differences can lead to severe (∼5σ) bias in their inferred composition, not only for hot Jupiters, but for the broader class of transiting exoplanets including smaller planets like sub-Neptunes.Constraints on Atmospheric Dynamics– Atmospheric dynamics is one of the least understood and most poorly constrained aspects of substellar atmospheres. Atmospheric dynamics along the radial direction, or vertical mixing, is often represented by the Kzz diffusion parameter. Theoretical and empirical uncertainty on Kzz spans several orders of magnitude. To constrain Kzz, we introduce PICASO 3.0, an opensource Python-based 1D radiative-convective equilibrium model that self-consistently treats disequilibrium chemistry from vertical mixing. Using PICASO 3.0, we demonstrate that JWST data can precisely constrain Kzz in both radiative and convective atmospheric regions. Our models predict brown dwarfs with higher gravities and temperatures of 500-900 K should have a second “detached” radiative region, which makes them ideal targets for constraining the very uncertain Kzz in their radiative atmospheres. To explore how metallicity, C/O ratio, and Kzz together shape the spectra of directly imaged planets and brown dwarfs, we present the Sonora Elf Owl model grid, which has been calculated with the PICASO model. We show that metallicity and Kzz driven vertical mixing can have degenerate effects on their spectra at various wavelength ranges. We use this model grid along with AKARI and Spitzer observations to show that free-floating brown dwarfs with temperatures between 500-900 K indeed have very low Kzz values, which agrees with our theoretical predictions of detached radiative regions in their deep atmospheres.Constraints on Interior and Atmospheric Properties– To assess whether JWST can constrain vertical mixing and interior properties of transiting planets, including warm-to-hot sub-Neptunes and gas giants, we integrate photochemistry into PICASO using the Photochem model. We show that CH4 is an ideal probe of Kzz in warm giants, while CO is better for warm sub-Neptunes. We also find that SO2 is no longer the main S- bearing gas in planets with Teq≤700 K or Teq ≥1100 K, with CS and CS2 becoming dominant. Using this model and JWST data, we constrain the atmospheric and interior properties of Neptune-class exoplanet GJ 436 b. We present the panchromatic JWST eclipse spectrum of GJ 436 b, showing it differs significantly from previously published Spitzer photometry in the 3.6 µm band. We find weak evidence of CO2 absorption in its spectrum. Using PICASO modeling, we find the planet’s day-side is either covered with a thick layer of aerosols with a metallicity ≥300×solar, or its interior is colder (Tint ∼ 60 K) with a metallicity ≥80×solar. Future observations of the planet, preferably in transmission, are required to distinguish between these scenarios and constrain its interior properties more precisely
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Terrestrial Exoplanet Atmospheres: From Primordial Compositions to Likely Observable Biosignatures
Full text linkExoplanet science is now focusing on characterizing the physics and chemistry of exoplanet atmospheres, including those of terrestrial-class, potentially habitable planets. In this thesis, I use a combination of laboratory experiments and theoretical modeling to understand two themes related to these atmospheres: (1) their primordial outgassing compositions from an experimental cosmochemistry approach, and (2) the planetary context for observable biosignature gases using modeling tools. There is no first-principles understanding of how to connect a planet’s bulk composition to its initial atmospheric properties. Since terrestrial exoplanets likely form their atmospheres through outgassing, an important step towards establishing this connection is to assay meteorites, remnants of planetary building blocks, by heating and measuring their outgassed volatiles. In the first theme, I use multiple experimental techniques to determine meteorites’ outgassing compositions over a range of temperatures and pressures. I describe the results of heating carbonaceous chondrite samples and measuring their abundances of released volatiles as a function of temperature in a high-vacuum environment. I find that these meteorites outgas significant amounts of H2O, CO, CO2 and smaller quantities of H2 and H2S. I also discuss a complementary bulk element analysis to monitor outgassing of heavier elements (e.g., sulfur, iron, zinc). I compare these experimental results to thermochemical equilibrium models of chondrite outgassing and determine how these experiments can improve atmospheric models and inform the connection between bulk composition and early atmospheres.For the second theme, I perform a comprehensive analysis of the necessary planetary conditions for atmospheric methane to be a compelling biosignature gas. Methane is one of the only biosignatures that JWST can readily detect in terrestrial atmospheres. Therefore, it is essential to understand methane biosignatures to contextualize these imminent observations. Using a combination of multiphase thermodynamic and atmospheric chemistry models, I investigate abiotic sources of methane and determine the planetary conditions for which these sources could be enhanced on terrestrial planets so as to result in false positives. I determine that known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO due to the strong redox disequilibrium between CH4 and CO2, providing the first tentative framework for assessing methane biosignatures
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Clouds and Hazes in Exoplanets and Brown Dwarfs
Full text linkThe formation of clouds significantly alters the spectra of cool substellar atmospheres from terrestrial planets to brown dwarfs. In cool planets like Earth and Jupiter, volatile species like water and ammonia condense to form ice clouds. In hot planets and brown dwarfs, iron and silicates instead condense, forming dusty clouds. Irradiated methane-rich planets may have substantial hydrocarbon hazes. During my dissertation, I have studied the impact of clouds and hazes in a variety of substellar objects. First, I present results for cool brown dwarfs in- cluding clouds previously neglected in model atmospheres. Model spectra that include sulfide and salt clouds can match the spectra of T dwarf atmospheres; water ice clouds will alter the spectra of the newest and coldest brown dwarfs, the Y dwarfs. These sulfide/salt and ice clouds potentially drive spectroscopic variability in these cool objects, and this variability should be distinguishable from variability caused by hot spots.Next, I present results for small, cool exoplanets between the size of Earth and Neptune. They likely have sulfide and salt clouds and also have photochemical hazes caused by stellar irradiation. Vast resources have been dedicated to characterizing the handful of super Earths and Neptunes accessible to current telescopes, yet of the planets smaller than Neptune studied to date, all have radii in the near-infrared consistent with being constant in wavelength, likely showing that these small planets are consistently enshrouded in thick hazes and clouds. For the super Earth GJ 1214b, very thick, lofted clouds of salts or sulfides in high metallicity (1000× solar) atmospheres create featureless transmission spectra in the near-infrared. Photochemical hazes also create featureless transmission spectra at lower metallicities. For the Neptune-sized GJ 436b, its thermal emission and transmission spectra combine indicate a high metallicity atmosphere, potentially heated by tides and affected by disequilibrium chemistry. I show that despite the challenges, there are promising avenues for understanding small planets: by observing thermal emission and reflected light, we can break the degeneracies and con- strain the atmospheric compositions. These future observations will provide rich diagnostics of molecules and clouds in small planets
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