Ezequiel Gonzalez

Slide 1: Extrasolar planets are planets located beyond our solar system, orbiting around other stars. In the last decade, there has been a discovery boost with ~4,200 exoplanets confirmed so far. Yet, a detailed characterization of exoplanets’ atmospheres remains challenging. There are four main types of exoplanets: Gas Giant (the biggest type), Neptune-like, Super-Earth, and Terrestrial. The project goal is to explore the atmospheres of gas giant exoplanets using the transit spectroscopy technique, focusing on HAT-P-1b, a gas giant exoplanet that orbits around a G-type star with a period of 4.465 days at a distance of ~139 pc. (a so-called “hot Jupiter"). Lastly, spectroscopy refers to the study of the interaction between matter and electromagnetic radiation as a function of wavelength.
Slide 2: The transit method is a photometric technique employed in the search for exoplanets. Transiting exoplanets can be detected by the decrease in the flux of the host star when the planet passes in front of it. Figure 1 shows the Flux vs. Time and illustrates the depth in the light curve when the planet passes in front of the host star. The Transit spectroscopy method is the most powerful to characterize exoplanets’ atmospheres. The transit spectroscopy method will be used by JWST and ARIEL missions. The concept is as follows. When a transit occurs, the starlight passes through the planet’s atmosphere and interacts with atoms, molecules, or clouds. Figure 2 illustrates this for three cases of atmospheres: (1) rich in H and extended in height, (2) with low proportion of H and compact, and (3) with clouds blocking the starlight and masking absorption signatures. Absorption by the atmosphere makes the planet appear bigger or smaller as a function of wavelength. By measuring the transit depth as a function of wavelength, the composition of the atmosphere can be derived.
Slide 3: The steps of the data reduction pipeline are depicted in the slide, including 7 steps: 1) Obtain key parameters of the exoplanet system, 2) Define regions of Spectral Traces and Sky Background, 3) Build Transit White Light Curve, 4) Divide Spectral Traces by Wavelengths (bins), 5) Build Transit Spectroscopic Light Curves, 6) Perform Wavelength Calibration, 7) Build Transmission Spectrum. Three figures are shown. Figure 3 shows an example of a the low-resolution spectra for HAT-P-1b dataset, acquired by the Nordic Optical Telescope with the ALFOSC instrument. Two spectral traces can be observed: target and reference star. The reference star is used to correct for the variations induced by the Earth’s atmosphere. Figure 4 presents the spectroscopic light curves of bins 10, 20, and 30, including the fit, and the residuals. Finally figure 5 is the transmission spectrum of HAT-P-1b, and shows the ratio of the planet radius to the star radius, as a function of wavelength.
Slide 4: The transmission spectrum of HAT-P-1b is compared with four atmosphere models: (1) Rayleigh scattering, (2) Clear with Sodium and Potassium, (3) Clear with Sodium, Potassium and Titanium oxide, and (4) Cloudy atmosphere. All these models and the data points corresponding to the observed spectrum, are plotted together in Figure 6. The Chi-squared has been computed for the four of them, with its lowest value for the cloudy atmosphere. The conclusions of the project are the following: (A) HAT-P-1b's spectrum is consistent with a cloudy
atmosphere, (B) There is no clear detection of the Rayleigh scattering slope, (B) There is a hint of Na detection at ~6000 Angstrom. Further observations with larger facilities would be needed for confirmation, (D) More precise data is necessary to better distinguish between the models and reach a more robust conclusion.