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The Transmission Spectrum of WASP-121B in High Resolution with HARPS

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The Transmission Spectrum of WASP-121B in High Resolution with HARPS The transmission spectrum of WASP-121b in high resolution with HARPS Jan Philip Sindel Space Engineering, master's level (120 credits) 2018 Luleå University of Technology Department of Computer Science, Electrical and Space Engineering The transmission spectrum of WASP-121b in high resolution with HARPS Jan Philip Sindel Supervisors: D. Ehrenreich∗ & J. Hoeijmakersy September 17, 2018 This report was written as part of an internship at the Observatoire de Geneve1 for the M2 ASEP program at Université Toulouse III - Paul Sabatier2 and the Erasmus Mundus SpaceMaster program at Luleå Tekniska Universitet3. The internship was carried out between the 22nd of January and the 29th of June 2018. Abstract Transmission spectroscopy is a powerful tool to analyze the atmospheric composition of exoplanets. In this work we examine data gathered with the HARPS instrument at the 3.6m telescope in La Silla, Chile during three transits of the exoplanet WASP-121b in early 2018. We find evidence for the absorption of sodium at a 5:4σ confidence level. Additionally we investigate the claim of the occurance of VO in this planet by Evans et al.(2017) employing the cross-correlation technique for high resolution but find no evidence. We show that our model transmission spectrum would induce a signal with low significance, concluding that there is neither confirmation, nor disproval of the claim for now. ∗[email protected] [email protected] 151 Chemin des Maillettes, 1290 Versoix, Suisse 2118 route de Narbonne, 31062 TOULOUSE CEDEX 9, France 397187 Luleå, Sweden 1 Contents 1 Introduction 3 1.1 Exoplanets: Historical Context and Motivation....................3 1.2 Detection methods....................................4 1.2.1 Radial Velocity..................................5 1.2.2 Transit photometry...............................6 1.2.3 Other methods..................................7 1.3 Exoplanet atmospheres..................................9 1.3.1 Theory and motivation.............................9 1.4 Transmission spectroscopy................................9 1.4.1 Cross-correlation technique........................... 10 1.4.2 Spectrographs and spectral resolution..................... 11 2 Data analysis 13 2.1 The data-set....................................... 13 2.2 Data reduction...................................... 13 2.2.1 Extraction from .fits files............................ 13 2.2.2 Blaze correction and normalization....................... 15 2.2.3 Removal of hot pixels.............................. 16 2.3 Telluric Correction.................................... 17 2.4 BERV correction..................................... 18 2.5 Removal of the star signal................................ 19 3 Transmission Spectrum of WASP-121b 19 3.1 Retrieval of transmission spectrum........................... 19 3.2 Detection of Sodium................................... 20 4 Cross-correlation 22 4.1 Search for VO....................................... 23 4.2 Injection of an artificial signal.............................. 24 4.2.1 Rotational profile of the planet......................... 24 4.2.2 Movement of the planet through the frame.................. 25 4.2.3 Injection to raw data............................... 25 4.3 Retrievability of model templates............................ 25 5 Discussion and Outlook 26 5.1 Sodium.......................................... 26 5.2 VO............................................. 27 6 Acknowledgements 27 7 List of acronyms 29 2 1 Introduction 1.1 Exoplanets: Historical Context and Motivation Since ancient times humanity has wondered if we are alone in the universe. We developed different methods and techniques to explore the night sky, and found that our rocky sphere wasn’t the only one to orbit its host sphere of plasma. These so-called planets were the first point of attack for our search for alien life. With our scientific knowledge steadily advancing we were able to know ever more about these places that were, on a cosmic scale, so close to us. We landed probes on the rocky planets Mars and Venus, studied gas giants like Jupiter and Saturn as well as their moons from orbit and concluded that life as we know it does not exist in our immediate neighborhood. But our sun was not alone, quite the opposite. There are billions of stars in our galaxy, thousands of which can be seen with the naked eye each night. What if these stars also hosted planets and therefore maybe had the ability to support life? This led to the search for extra-solar planets, or exoplanets for short. The first exoplanet found around a main sequence star was a big surprise, because it is very different from the planets in our solar system. 51 Pegasi b, its discovery attributed to Mayor & Queloz(1995), is a planet with a mass of 0:46MJup (Brogi et al.(2013)) that orbits its host star at distance of 0:0527AU. This means it’s a gas giant type planet that has an orbital period of only a few days and is very close to its host star. Since then, more planets like it around different stars have been discovered and have been dubbed hot Jupiters. A lot of different surveys for exoplanets have since been conducted with varying degrees of success. At the date of this work, there are 3725 confirmed exoplanets4, out of which 1559 reside in multi- planet systems. This provides a great statistical sample that allows us to investigate the population of exoplanets.5 One of the first things we realized was that hot Jupiters, even though they were the most common type of exoplanet found early on, were not nearly as prevalent throughout our galaxy as thought and merely a statistical artifact because of how easy they were to detect. One of the main goals of exoplanet science to this day is to find a planet that is as similar to Earth as possible, and can therefore be able to support life as we know it. The most important quality of such a planet is that its surface temperature allows for water to exist in its liquid form, since that is where life formed on Earth. Since stars vary wildly in the amount of energy they give off to their environment each star has its own habitable zone, defined by the distance to the star where the incident flux onto a planet would allow it to have a surface temperature that can support liquid water. This habitable zone can be extended in both direction by introducing atmospheres and thereby positive or negative temperature feedback cycles, extending the borders of the classical habitable zone (Kasting et al.(1993)). These limits for several different stars and discovered sys- tems are pictured in figure3. The search for earth like planets in the habitable zones of foreign starts had already yielded a couple of candidates, for example the second and third planet in the TRAPPIST-1 planetary system (Gillon et al.(2017)). In a statistical analysis Petigura et al.(2013) found that 11 ˘ 4% of sun-like stars have an earth sized planet in their that receives between 1 and 4 earth-equivalent amounts of energy from their host star. Ever since the apparent prevalence of exoplanets in our galaxy has become known, efforts have been made to find them and analyse them in depth. During the time of this work the TESS (Terrestrial exoplanet survey satellite, Ricker et al.(2015)) spacecraft was launched and is expected to further advance the statistical data-set of exoplanets available to us. The search for a second earth is in full effect and it might not be long until we find it. 4https://exoplanetarchive.ipac.caltech.edu/index.html, visited on 21/05/18 5A database for all known exoplanet can be found on http://exoplanet.eu/ 3 Figure 3: The habitable zone for the solar system and several other discovered planetary systems. Image Credit: Chester Harman / NASA 1.2 Detection methods In the search for planetary mass objects outside of our solar system several methods have been proposed and used in order to infer their existence from observational data. The first successful method was the observation of the timing of Pulsar name, leading to the discovery of a planetary system around the Pulsar PSR B1257+12 Wolszczan & Frail(1992). The method used during the detection of the first exoplanet orbiting a main-sequence star was the radial velocity method, utilizing the shifts in the star spectrum induced by the motion of the star around the common barycenter with its planet. The most successful method to this day is the transit-method, observing a dip in brightness of the host star when the planet passes in front of it, which has lead to the discovery of 2911 exoplanets. Other methods include direct imaging, microlensing and astrometry. Figure 4 The evolution of the set of known exoplanets over the years, separated in color by detection technique. The large increase in transit discoveries after 2013 is due to NASAs Kepler mission.6 4 1.2.1 Radial Velocity By measuring the position of spectral lines in the spectrum of a star it is possible to infer its velocity in our line of sight. If there is a planet orbiting said star, the latter will also orbit the common barycenter and therefore change its velocity as measured from our line of sight. The amount of displacement ∆λ is dependent of the wavelength of the observed line λ0 and the velocity in our line of sight v˚ sin i by the doppler relation: ∆λ v “ ˚ sin i (1) λ0 c Observing the spectrum of a star that is orbited by a planet produces a quasi-sinusodial signal in the observed radial velocity of the star, whose semi-amplitude K˚ and period are directly related to the mass ratio and orbital properties of the system by: 1 2πG 3 m sin i 1 K “ p ? (2) ˚ 2 2 P 3 1 ´ e ˆ ˙ m˚ This relation can be used to then obtain the period and minimum mass of the system. Since the inclination is generally unknown and cannot be inferred through spectroscopic measurements, there is a degeneracy between it and the planet mass.
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