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Exoplanetary Atmospheres Analysis Through the Transit Spectroscopy Technique

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Exoplanetary Atmospheres Analysis Through the Transit Spectroscopy Technique UNIVERSITÀ DEGLI STUDI DI NAPOLI “FEDERICO II” Scuola Politecnica e delle Scienze di Base Area Didattica di Scienze Matematiche Fisiche e Naturali Dipartimento di Fisica “Ettore Pancini” Laurea Triennale in Fisica Exoplanetary atmospheres analysis through the transit spectroscopy technique Relatori: Candidato: Prof. Giovanni Covone Gallo Francesco Dott. Elisa Quintana Matr. N85/652 Anno Accademico 2020/2021 Contents 1 Introduction to the science of exoplanets 5 1.1 Planetary system formation . 6 1.2 Detection methods . 7 1.2.1 Direct imaging . 7 1.2.2 Gravitational microlensing . 7 1.2.3 Radial velocity . 8 1.2.4 Transit . 8 1.2.5 The limb darkening . 10 1.3 Transiting Exoplanet Survey Satellite . 10 1.4 The Hubble Space Telescope . 11 1.4.1 Slitless spectroscopy . 11 1.4.2 G141 . 12 1.4.3 Wide Field Camera 3 . 13 1.4.4 IR Channel . 14 1.4.5 Spatial scan mode . 15 1.5 Earth-like exoplanets atmospheres: why? . 15 2 Atmospheres of exoplanets 18 2.1 Introduction . 18 2.2 Atmospheres in the Solar system . 19 2.3 Atmospheric general processes . 21 2.4 Composition of an exoplanetary atmosphere . 23 2.5 Transit Spectroscopy . 24 2.5.1 Transmission spectrum . 25 2.5.2 Limits of the transmission spectroscopy . 27 3 Data analysis 29 3.1 Iraclis pipeline . 29 3.2 Reduction steps . 30 3.2.1 Zero read and bias-level corrections . 30 3.2.2 Non-linearity correction . 30 1 3.2.3 Dark current subtraction . 31 3.2.4 Gain variations: flat field correction . 31 3.2.5 Sky background subtraction . 31 3.2.6 Bad pixels and cosmic-rays correction . 31 3.3 Spectrum extraction: geometric distortions and position shifts 32 3.4 Wavelength calibration . 33 3.5 Fitting the white light curve . 35 3.6 Fitting the spectral light curves . 35 3.7 Dataset . 36 3.7.1 HD 209458 b . 37 3.7.2 WASP-121 b . 38 3.7.3 WASP-62 b and WASP-79 b . 40 3.8 Transmission spectrum of L 98-59 b . 43 3.9 Conclusions . 45 Bibliography 46 2 Introduction “Is our planet as unique as it seems? Are we alone in the universe?”. These are questions that humans have been carrying with them for centuries and to which scientists try to answer. The search of life is the ultimate goal of exoplanetary science, which is currently living an unprecedented devel- opment. In the past two decades scientists have discovered exoplanets of various kind, far exceeding the diversity seen in our Solar system. With over four thousand confirmed exoplanets, the research is shifting from discovery to characterisation of exoplanetary systems, thanks to spectroscopic obser- vations. The study of exoplanetary atmospheres is the only way to infer if a planet is habitable or not. Encoded in the spectrum of a planetary at- mosphere there are informations about its chemical and physical properties, as chemical compositions with relative abundances, T-P profiles, dynamic processes, clouds/hazes and insights about the formation and evolution of the planets. The first detection of an exoplanetary atmosphere was on HD 209458 b (Charbonneau, Brown, Noyes & Gilliland, 2002), an hot Jupiter orbiting a sun-like star with a a distance 8 times shorter than that between Mercury and the Sun. Observing the spectra of this planet it was possible to detect the presence of Sodium in his atmosphere, thanks to the characteristic absorption feature at 589nm and later observations showed that the oxygen and carbon found in its atmosphere are evaporating at an immense rate, due to the nearness of his parent star, that a new class of exoplanets has been pro- posed, the ‘chtonian planets’ or ‘dead’ rocky cores of completely evaporated gas giants. Nowadays, several tens of exoplanet spectra have been observed using the transit spectroscopy technique, with the bulk of these observations obtained with HST, Spitzer Space Telescope, and other ground based facil- ities. Thanks to recent space missions like KEPLER and TESS, we have discovered many Earth-like exoplanets in the habitable zone of the parent star. Today the challenge is to understand if the life is possible on these kind of planets, trough the study of their atmosphere, if they have one. This thesis aims to make a panorama on the science of exoplanetary atmospheres. Chap- ter 1 is an introduction to the detection methods, with more informations 3 on the photometric transit technique and the Hubble Space Telescope, which is the main instrument used for the transmission spectroscopy observations of exoplanetary atmospheres. Chapter 2 gives an overview on the atmo- spheric composition of the Solar system’s planets and introduces to transit spectroscopy technique. Chapter 3 contains a brief description about the pipeline we used to analyze our dataset, composed of four hot Jupiters and an Earth-like planet recently discovered (Kostov et al.,, 2019). 4 Chapter 1 Introduction to the science of exoplanets The International Astronomical Union defines a planet as follow:" a celes- tial body that (a) is in orbit around a star, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. Moreover, it is important to distinguish a planet from a brown dwarf, due to the discovery of many massive exoplanets. A brown dwarf is a star with a mass between 13MJ < Mbd < 72MJ , hence that burns some deuterium to produce small amount of energy and luminosity, but that never reaches stable nuclear-burning phase to self-stabilize. An exoplanet is a planet outside our Solar system, hence which orbits around a star differ- ent from the Sun. At date, more than four thousand exoplanets have been discovered, with a wide range of types, far different from the planets of our Solar system, that can be classified into different families, depending on the masses and radii: • Eart-like, planets with masses similar to that of the Earth and a radii of 0:8 − 1:25REarth with a composition prevalently rocky • Super Earths, planets with masses of 2 − 10MEarth and radii of 1:25 − 2REarth with a composition prevalently rocky • Mini-Neptunes, planets less massive than Neptune with a radii of 2 − 4REarth, with a rocky core and a thick layer of hydrogen and helium. • Gaseous giants, planets with masses and radii similar to Jupiter and that have a thick layer of hydrogen and helium. These family also contains some strange or "exotic" planets, but that are those discovered 5 more frequently, the hot Jupiters and warm Neptunes, planets that have migrated from their original orbits and that now are very close to their host stars. 1.1 Planetary system formation The formation process, favored by many astronomers, is that planets grow in a process of accretion of smaller building blocks. After the formation of the protostar, the nebular disk’s temperature decreases, allowing the formation of small dust grains with icy mantles, that collide and stick together randomly. They grow in mass and dimension, beginning to gravitational influence other material in their areas, and forming the so-called planetesimals. The distance from the planetesimal where a particle can be gravitationally influenced is called Hill Radius, defined as the distance where the particle around the planetesimal has an orbital period equal to that the planetesimal around the star, for example for the Sun: M 1=3 R = a (1.1) H M where M and a are respectively the mass of the Sun and the distance from it for a planetesimal with mass M. The accretion disk can be divided virtually in two regions: one closer to the star where the temperature is too high to let volatiles to condense and the other beyond the so-called "snow line". Gas and ice giants can form only beyond the snow line, were the limited action of the radiation pressure and the lower temperatures allow volatile materials to condense, in particular with the formation of water-ice. This kind of planets are made of a rocky core with the addiction of water-ice, which can reaches a mass of 10 − 15MEarth, surrounded by a thick atmosphere of hydrogen and helium swept away from the inner regions of the nebula, with the addiction of heavier elements for the ice giants, like methane-ice, ammonia,etc. In the inner region of the nebula, due to the higher temperatures, only heavier elements are present, and they can’t be swept away by the radiation pressure of the star. This region is full of CAI’s, silicates and other refractory elements that sticking together can form rocky planetesimals, like the four we see today in our Solar system. With the entry of the star in the T-Tauri phase, all the material of the disk that hadn’t yet collected into planetesimals, are swept away. 6 Figure 1.1: Protoplanetary disk around the young star AB Aurigae, obtained with the SPHERE VLT’s instrument. In the inner region of the disk we can see the ’twist’, in bright yellow, where probably a planet is forming. CREDITS: ESO/ (Boccaletti et al., 2020) 1.2 Detection methods The first discovered exoplanet orbiting a Sun-like star was 51 Pegasi b (Mayor & Queloz, 1995), trough the radial velocity method. At date, more than four thousand exoplanets have been found, with the bulk of the discoveries made by the NASA’s missions TESS and KEPLER with the transit method. Here is a brief description of the modern available detection methods. 1.2.1 Direct imaging Trough this method, a star is directly observed by a telescope, trying to obtain a direct image of a planet orbiting around it.
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