Resumen de Characterization of exo-atmospheres with transmission spectroscopy
Alejandro Sánchez López
In this thesis, we have aimed at characterizing the atmosphere of hot Jupiters during their primary transit using transmission spectroscopy. To that end, we have adapted the tools available to our group for the solving of the radiative transfer equation in planetary atmospheres (i.e., the KOPRA algorithm) to the typical exoplanetary geometries. This step allowed us to compute synthetic spectra of the exo-atmospheres both in transmission and in emission, assuming different pressure-temperature and abundance profiles in the exo-atmosphere. In order to validate our simulations, we compared the spectra produced by KOPRA with those obtained by other well-tested algorithms used by other groups and found them to be in very good agreement.
Next, we learned and optimized the cross-correlation technique applied to high resolution data of the exo-atmosphere. This technique is based on cross-correlating the measurements with a model of the absorption of the exo-atmosphere, which we computed using the algorithm KOPRA. This process provided us with a powerful tool for the identification of atmospheric species in exoplanets and for obtaining basic information about the exo-atmospheric dynamics. Consecutively, we applied this technique to high-resolution data recorded with the spectrograph CARMENES, mounted in the 3.5m telescope at the Calar Alto Observatory (Almería, Spain).
In this thesis, we were able to detect the presence of water vapor in the atmospheres of the hot Jupiters HD 189733 b and HD 209458 b. In particular, we succeeded in detecting water vapor by using each of its absorption bands covered by the instrument in the near infrared separately. This study was only possible for space telescopes until now. Also, we obtained basic information about the atmospheric dynamics of both exoplanets in the form of strong winds in their terminators. Moreover, we were able to perform sensitivity studies so as to determine the capabilities of this technique to distinguish between different abundances of the compounds and temperature profiles of the exo-atmosphere. Unfortunately, we show that this technique is not able to provide us with such important information. This means that the cross-correlation technique used here is only sensitive to the relative depth of the molecular absorption lines with respect to the continuum of the spectra.
Furthermore, the comparison of the cross-correlation signals obtained in both hot Jupiters by using the water vapor band at around 1 micrometer reveals that HD 189733 b, when compared to HD 209458 b, is a rather hazy exoplanet. This is because this absorption band is clearly detected in the latter, whereas it is completely obscured in the former, likely due to a strong extinction by atmospheric aerosols. This result is in good agreement with previous observations of these hot Jupiters by using space telescopes.
Lastly, in this thesis we have aimed at the detection of water vapor in the two canonical hot Jupiters, HD 189733 b and HD 209458 b, by analyzing spectra observed at optical wavelengths with CARMENES. Although preliminary, our analyses point to a preliminary detection of water vapor in HD 209458 b in the visible. If confirmed in future studies, this could represent the first detection of water vapor in the visible in any exoplanet so far. In addition, the water vapor cross-correlation signal is found to not be significant in HD 189733 b at these optical wavelengths, which is consistent with our previous results and the literature.
Overall, in this thesis we have contributed to better understand the atmospheres of hot Jupiters, which is an essential step for future studies targeting smaller and colder exoplanets. By refining our tools in these high signal-to-noise ratio targets, we can steadily and consistently expand the boundaries of our knowledge on this field so as to, one day, characterize an Earth-like exoplanet with the required precision.
