Facultad de Física Instituto de Astrofísica Unveiling Exoplanet Atmospheres with the ACCESS Survey by NÉSTOR ESPINOZA PEREZ Presented to the Instituto de Astrofísica at the Pontificia Universidad Católica de Chile in partial fulfillment of the requirements for the degree of Doctor of Philosophy Thesis Advisor: Dr. Andrés Jordán September, 2017 Santiago, Chile ©2017, Néstor Espinoza Perez Se autoriza la reproducción total o parcial, con fines académicos, por cualquier medio o procedimiento, incluyendo la cita bibliográfica del documento Unveiling exoplanet atmospheres with the ACCESS Survey by Néstor A. Espinoza Perez B.Sc., Pontificia Universidad Católica de Chile (2012) Licenciate in Astronomy, Pontificia Universidad Católica de Chile (2012) Submitted to the Instituto de Astronomía y Astrofísica in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the PONTIFICIA UNIVERSIDAD CATOLICA DE CHILE September 2017 c Pontificia Universidad Catolica de Chile 2017. All rights reserved. ○ Author.............................................................. Instituto de Astronomía y Astrofísica September 22, 2017 Certified by . Andrés Jordán Associate Professor Thesis Supervisor Accepted by. Alejandro Clocchiatti Director de postgrado, Instituto de Astrofísica 2 Unveiling exoplanet atmospheres with the ACCESS Survey by Néstor A. Espinoza Perez Submitted to the Instituto de Astronomía y Astrofísica on September 22, 2017, in partial fulfillment of the requirements for the degree of Doctor of Philosophy Abstract In this thesis, we present the first results of a survey aimed at the detection of exoplanet atmospheres from the ground using the technique of transmission spectroscopy: the Arizona- CfA-Católica Exoplanet Spectroscopy Survey (ACCESS), a multi-institutional effort aimed at the detection of optical features in giant exoplanet atmospheres. The survey is currently being carried out using the Inamori-Magellan Areal Camera and Spectrograph (IMACS) mounted on the Magellan Baade 6.5m telescope at Las Campanas Observatory, Chile, an instrument not designed (or comissioned) for the precise observations requiered to detect these signatures on these distant worlds, this thesis presents the first detailed comissioning and validation of this instrument for such observations, for which a special data reduction pipeline had to be created and which is today used routinely by the group. In this work we also present a detailed analysis of important stellar astrophysical effects that impact on the observed lightcurve of a transiting exoplanet, which is what is used in practice to extract the signatures of atomic and/or molecular features in the atmospheres of exoplanets using the technique of transmission spectroscopy. In particular, we present two detailed works published in the context of this thesis on the effect of limb-darkening, a fundamental effect observed during the transit of an exoplanet in front of a star, in which we detail how this effect impacts on the retrieved transit parameters, how well we actually understand this effect and how to optimally analyze a transit lightcurve using the “best" parametrization for it, avoiding in this way any biases that might arise from an incorrect treatment of the effect. Thesis Supervisor: Andrés Jordán Title: Associate Professor 3 4 Acknowledgments Almost any graduate student knows that getting a PhD is mostly about persistence and perseverance, qualities that are often blurred in the face of failure, a very common end- product during the PhD. I want to start this acknowledgment section by thanking all the people that were there to lift up my spirit during those times, both hearing about the crazy things I was trying out for my research or simply distracting me with the beautiful simple things in life. To all my friends but, especially, to my closest friends Boris Martínez, Rafael Brahm and Diego Sandoval: thank you. To my family, who were always there to support and love me: thank you. I want to especially thank the two most important woman in my life, my wife and my mother, whose love, support and deep insight have not only helped me during my whole career, but have supported me in many ways in all the decisions that I have made in it, including the new European adventure we are about to start with my wife: I have no words to express the gratitude for all of this to you. Thank you very, very much. Every PhD student also knows that the environment where one develops the PhD is also very important. I thus want to thank the Institute of Astrophysics (IA) at the Pontificia Universidad Católica de Chile (PUC) and the Millenium Institute of Astrophysics (MAS) for providing such a rich environment for the development of this thesis, where science from many different areas come together and from which I have learned not only physics and astrophysics, which have been (and will be) very useful in my research, but where I also have received huge support from the administrative side both from the IA (from Carmen Gloria, Lilena, Giselle, Mauricio, Cynthia, Mariela, Juan) and MAS (Denisse, Manuela, María José, Makarena, Claudia). All of this was fundamental for this thesis and, without all them, it would not have been the same. This thesis is in many ways also of that people behind it so thank you very, very much. I also want to take the opportunity to thank all the staff at Las Campanas Observatories (LCO), in which I spent many nights both observing and learning about the instruments that provided the rich datasets presented in this thesis: to the administrative staff, especially to Roberto Bermudez, to the telescope operators, especially to Mauricio Martínez, Mauricio Navarrete and Jorge Araya, and all the staff, especially to Dave Osip from which I have learned so much, thank you very, very much. 5 Working on a great team, as any PhD student also knows, makes everything easier and richer. For this, first and foremost, I want to thank my thesis advisor, Andrés Jordán, for his infinite support not only on this thesis, but on my career in general. Andrés has and will be an integral part of how I approach science because from day one he treated me not as a PhD student, but as a colleague, always asking my opinion on everything, genuinely hearing my ideas and giving his advice more as an opinion than as an order. I think this is the optimal way of training a scientist, because our work is based on peer review, which is exactly how Andrés approached his position as my PhD advisor. I also want to thank all the people in the local IA group that took the time to teach not only me, but also other students about important details such as observing and reducing data. Hats off go especially to Markus Rabus, Vincent Suc and my good friend Rafael Brahm, whose infinite knowledge about the know-how of astronomy forms a fundamental part of the researcher I have become. I cannot end this paragraph without acknowledging all the support of the ACCESS team, especially that of Daniel Apai, Mercedes Lopez-Morales and David Osip, whose guidance and trust on the research I was performing became a motivation to keep performing the work I did in this thesis and that of Benjamin Rackham, which is not only one of the most insightful researchers I know, but also has become a very good friend. To all of you: thank you very, very much. Finally, I want to thank the funding agencies that allowed me to perform the work I did. First to the COmisión Nacional de Investigación Científica Y Tecnológica (CONICYT), which provided me with the PhD fellowship through the CONICYT-PCHA/Doctorado Nacional fellowship that formed the basis for the funding of my PhD. To the BASAL CATA PFB-06, which helped me during the later stages of the writing of the PhD thesis, and to the MAS institute, which provided additional and fundamental financial support for the development of this thesis through the Ministry for the Economy, Development, and Tourism Programa Iniciativa Científica Milenio through grant IC 120009. Being paid for doing what you love is, definitely, the best payment one can receive. 6 Contents 1 Introduction 31 1.1 Exoplanet atmospheres . 37 1.1.1 The Solar System perspective . 37 1.1.2 Observing exoplanet atmospheres . 38 1.1.3 C/O ratios and exoplanet atmospheres . 42 1.1.4 Clouds, hazes and optical absorbers . 45 1.2 Transmission spectroscopy . 47 1.2.1 Observability . 47 1.2.2 Theory . 50 1.3 The ACCESS Survey . 52 2 Transmission spectroscopy: stellar astrophysical challenges 55 2.1 The transit lightcurve . 56 2.2 The impact of stellar heterogeneities . 60 2.2.1 Occulted stellar heterogeneities . 60 2.2.2 Unnoculted stellar heterogeneities . 64 2.3 The impact of limb-darkening . 68 2.3.1 Fitting limb-darkening models . 70 2.3.2 Measuring the limb-darkening effect from transit lightcurves . 82 2.3.3 The effect of using fixed LDCs in transit fitting . 95 2.3.4 Lessons learned and best-practices . 102 2.4 Optimal limb-darkening laws for transit applications . 105 7 2.4.1 Efficient sampling of coefficients from the logarithmic and expo- nential laws revisited . 107 2.4.2 Optimal selection of limb-darkening laws . 115 Appendices 129 2.A Least-squares fits to limb-darkening laws . 129 2.B Limiting cases for known target limb-darkening laws . 131 2.B.1 Limiting coefficient a for the linear law when sampling from the non-linear law . 131 2.B.2 Limiting coefficients u1 and u2 for the quadratic law when sampling from the non-linear law . 133 2.C Fitting and sampling from skew-normal distributions given parameter esti- mates with asymmetrical errorbars . 135 2.C.1 Fitting a skew-normal distribution to observed estimates . 136 2.C.2 Sampling from a skew-normal with known parameters .