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E Ects of Stellar Activity on the Measurement of Precise Radial Joao˜ Gomes da Silva E↵ects of stellar activity on the measurement of precise radial velocity Departamento de F´ısica e Astronomia Faculdade de Cienciasˆ da Universidade do Porto Maio de 2014 Joao˜ Gomes da Silva E↵ects of stellar activity on the measurement of precise radial velocity Tese submetida `aFaculdade de Ciˆenciasda Universidade do Porto para obten¸c˜aodo grau de Doutor em Astronomia Departamento de F´ısica e Astronomia Faculdade de Cienciasˆ da Universidade do Porto Maio de 2014 Acknowledgments I would like to thank my supervisor, Nuno, the motivation, patience, guidance, and scientific expertise he shared with me, and for all the work he had to make this thesis a reality. I would also like to thank all my co-workers and staff at CAUP for helping me with the scientific and bureaucratic aspects of the PhD, but also for the fun we had in innumerable occasions during these last four years. And of course to all my friends and family. This research was funded by Fundac¸ao˜ para a Cienciaˆ e Tecnologia, Portugal (grant ref- erence SFRH/BD/64722/2009) as well as by the European Research Council/European Community under the FP7 through Starting Grant agreement number 239953. 3 Abstract The radial velocity (RV) method is one of the most prolific techniques in detecting and confirming extrasolar planets. However, due to its indirect nature, it is also sensitive to other sources of RV signals. One of the most important limiting factors of using the RV method to discover low-mass or long-period extrasolar planets is stellar activity. The phenomena that comprises activity is capable of inducing ”artificial” RV variations that will interfere with planetary detections, by adding noise to the data or producing periodic modulations that might be confused with the ones originating from the pull of planetary companions. These phenomena have a large range of timescales, from stellar oscillations and flares that can last for minutes, to magnetic cycles that last for decades. When searching for planets with various orbital periods, all these timescales need to be taken into account. Therefore, it is very important to understand the activity diagnostic tools and how activity interferes with the measured RV. In this thesis my focus is on the long-term interference of activity on the measured RV signals and how to diagnose them. A large part of the work involved studying activity cycles of M dwarfs, due to their increasing importance in planetary searches, mainly because of their low-mass, which maximises the detection of planets by using methods such as RV or transits. I compared various activity indicators measured over timescales of years to try to understand how they relate to each other and select the most appropriate for these kind of stars. The flux in the Na i D lines was found to follow very well the activity measured by the Ca ii H & K lines. Furthermore their use is most appropriate for M dwarfs due to the higher signal-to-noise at the Na i D wavelengths. I also found that the flux in the Ca ii and H↵ lines is correlated for the highest activity stars but uncorrelated or anti-correlated for he most quiet M dwarfs. Indications that activity cycles are present in early-M dwarfs were also detected. In a following study with one more year of extended data, the flux on the Na i lines was used to detect activity cycles and compare those variations with the simultaneous RV signals. This was done with the aim of finding correlations between activity and RV and 4 to understand to which order those cycles could be One of the results of this work was that around 2/3 of early-M dwarfs present long-term variability. This fraction of stars with long-term variability is comparable to that of FGK stars. However, only 19% of early-M dwarfs show the presence of activity cycles (cf. 60% for FGKs). This might be a signal ⇠ of departure from an ↵⌦-dynamo to an ↵2-dynamo were no activity cycles are expected. I also found that long term activity variations are capable of producing RV signals with 1 amplitudes that can reach the 5ms− level. This is enough to hide the signal of a ⇠ low-mass planet or to simulate that of a long-period planet. In the last part of this thesis I studied the long-term correlation between two of the most widely used optical activity indicators, the Ca ii H & K and the H↵ lines, for a sample of FGK stars. These two indices are known to have a wide range of correlations but this behaviour is not well studied. The correlations between these indices was found to be depend on the activity level of the stars (as was found also for M dwarfs) and the stellar metal content might be also having an effect on the correlations between these two activity indices. 5 Contents Acknowledgments 3 Abstract 4 List of Tables 8 List of Figures 9 1 Introduction 10 1.1 Detecting exoplanets using radial velocity . 12 1.1.1 General properties of exoplanets . 13 1.1.2 The radial velocity method . 16 1.1.3 Limitations of the radial velocity method . 18 1.2 Stellar activity . 19 1.2.1 Stellar chromospheric activity . 20 1.2.2 Activity proxies . 25 1.2.3 Mean activity level of stars . 30 1.3 Stellar activity at different time scales and its influence on RV . 32 1.3.1 Oscillations and Granulation . 32 1.3.2 Rotationally modulated active regions . 36 1.3.3 Long-term activity: magnetic cycles . 38 6 1.4 Motivation . 43 2 Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program I. Comparison of activity indices 45 3 Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program II. Activity and radial velocity 63 4 On the long-term correlation between the flux in the Ca ii H & K and Halpha lines of FGK stars 89 5 Conclusions 108 5.1 Activity indices for M dwarfs . 108 5.2 Long-term activity variability and cycles of M dwarfs . 110 5.3 Influence of long-term activity on the RV of M dwarfs . 111 5.4 The long-term correlation between Ca II and Halpha for FGK stars . 112 5.5 Future prospects and things to be done . 113 5.5.1 Activity cycles in M dwarfs . 114 5.5.2 Activity indices . 115 References 116 7 List of Tables 1.1 Stellar activity at different timescales and correction of RV. 33 8 List of Figures 1.1 Mass distribution of close-in, low-mass planets . 14 1.2 Radius distribution of low-mass planets . 15 1.3 Solar flare . 21 1.4 Temperature structure of the solar chromosphere . 23 1.5 RV induced by stellar oscillations . 34 1.6 Effect of granulation on line bisector . 35 1.7 Effect of spots on line profile and RV . 37 1.8 The Sun’s butterfly diagram and average sunspot area . 39 1.9 MWO long-term activity classification . 40 1.10 Long-term correlations between RV and activity proxies . 42 9 Chapter 1 Introduction During the last 15 years, astronomers have been discovering dozens of planets orbiting other stars. This quest is, however, not an easy task. The difficulty of detecting extra- solar planets arises from the fact that most of the optical radiation coming from a planet is simply reflected starlight. Because of this, exoplanets will be billions of times fainter than their host stars. And since they generally orbit at extremely small angular separations, their direct detection is incredibly hard. One way to circumvent this problem is to use indirect methods such as detecting the dynamical perturbations provoked in the host star by a planetary companion. Several indirect methods exists now that are able to detect these small bodies in other stellar systems. The most successful to date are the radial velocity (RV) method, one of the focus of this thesis, which measures the line-of-sight velocity of star as it moves around of the star-planet centre of mass (e.g. Mayor & Queloz 1995), and transit pho- tometry which detects the shallow brightness decrease of a star as a planet passes in front of its disk (e.g. Henry et al. 1999; Charbonneau et al. 2000). Other indirect methods currently in use include astrometry, which measures the apparent movements of the parent star by measuring it’s position with time (e.g. Benedict et al. 2006), gravitational microlensing events, which detects the huge jump in the brightness of a planet as a lens star passes in front of its host star (this method is however not easily repeatable) (e.g. Bond et al. 2004). Transit timing variations, which measure the variations in the transit time of a planet produced by another perturbing body (e.g. Ballard et al. 2011), is recent new method for confirming or detecting exoplanets. Another example of an indirect planet search technique is pulsar timing, which measures the tiny anomalies in the timing of its observed radio pulses and can be used to track the pulsar’s motion, and therefore to detect a perturbing companion (e.g. Wolszczan & Frail 1992). This last 10 CHAPTER 1. INTRODUCTION 11 one was the technique that first detected planet mass objects outside the solar system Wolszczan & Frail (1992). Due to the indirect nature of these methods, they will become sensitive to other sources of signals, similar to the ones they are supposed to detect, which can be produced by the parent star.
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