A Search for Transiting Extrasolar Planets with the Automated Patrol Telescope by M´arton Gergely Hidas A thesis submitted in satisfaction of the requirements for the degree of Doctor of Philosophy in the Faculty of Science. 11 November 2005 Abstract In the past decade some 150 planets have been detected outside our Solar System, mostly via precise radial-velocity measurements of their host stars. Using an alter- native method, transit searches have recently added 6 planets to the tally, and are expected to make more significant contributions in the future. The transit method is based on the detection of the tiny, periodic dip in the apparent brightness of a star when an orbiting planet passes in front of it. It requires intensive photometric monitoring of ∼ 104 stars, with a precision better than ∼ 1%. The 0.5 m Automated Patrol Telescope (APT) at Siding Spring Observatory, Australia, with its wide field of view and large aperture, is ideal for this task. This combination is also somewhat unique among telescopes used in transit searches. Since 2001, the APT has been semi-dedicated to a search for extrasolar planets. In this thesis work, observing, data reduction and analysis procedures were de- veloped for the project. A significant fraction of the initial effort was focused on reaching the required photometric precision. This was achieved by implementing a new observing technique, and robust data reduction software. In the first two years of regular observations (starting in August 2002), 8 crowded Galactic fields were monitored, with photometric precision reaching 0.2% for the brightest stars. We searched the lightcurves of the brightest stars (V<13) and selected 5 planet candi- dates. Follow-up photometry and spectroscopy revealed all of these to be eclipsing binary stars. To date, no planets have been detected by this project. A detailed Monte Carlo simulation of the observations, using the currently known frequency and properties of extrasolar planets, resulted in a low calculated detection rate, consistent with the lack of detections. Using this simulation, we have investigated the observational and target star/planet parameters that determine the sensitivity of transit searches. The results highlighted the factors limiting our detection rate, and allowed us to sig- nificantly improve our observing strategy. According to the simulations, we should now detect ∼ 2 planets per year. This will increase by a factor of a few when a new camera, currently under construction, is installed on the APT in early 2006. Contents Abstract ..................................... i List of tables .................................. vii List of figures .................................. ix Acknowledgments ................................ x Preface ...................................... xii 1 Introduction 1 1-1 What is a planet? ............................. 2 1-2 Extrasolar planet search techniques ................... 3 1-2.1 Radial velocities .......................... 3 1-2.2 Pulsar timing ........................... 6 1-2.3 Astrometry ............................ 7 1-2.4 Microlensing ............................ 8 1-2.5 Direct imaging .......................... 10 1-2.6 Reflected light ........................... 11 1-2.7 Other exoplanet-related observations .............. 12 1-3 Properties of known extrasolar planetary systems ........... 12 1-3.1 Frequency of planetary systems around Sun-like stars ..... 13 1-3.2 Orbital characteristics ...................... 14 1-3.3 Physical properties of the planets ................ 16 1-3.4 Properties of host stars ...................... 19 1-3.5 Multiple planetary systems .................... 20 1-3.6 Planets in stellar multiple systems ................ 21 1-4 Theories of planet formation and evolution ............... 22 Contents iii 1-4.1 The planetesimal hypothesis and core accretion ........ 22 1-4.2 Gravitational instability ..................... 24 1-4.3 Other formation hypotheses ................... 25 1-4.4 Orbital migration in a protoplanetary disk ........... 25 1-4.5 Interactions between planets ................... 27 1-4.6 Interactions between planet and host star ............ 27 1-4.7 Planets in stellar multiple systems ................ 29 1-5 What we don’t yet know about planets ................. 29 1-6 Transit searches .............................. 31 1-6.1 Why transits? ........................... 32 1-6.2 Transit search projects ...................... 35 1-6.2.1 Shallow, wide-field ................... 36 1-6.2.2 Deep, narrow-angle ................... 36 1-6.2.3 Open clusters ...................... 38 1-6.2.4 Globular clusters .................... 38 1-6.2.5 Binary stars ....................... 38 1-6.2.6 Transits from space ................... 39 1-6.3 Algorithms for detecting transits in lightcurves ......... 39 1-6.4 Follow-up techniques for transit candidates ........... 41 1-6.5 Biases in the sensitivity of transit searches ........... 44 1-7 Motivation for the UNSW transit search ................ 45 2 Methods 47 2-1 The Automated Patrol Telescope .................... 47 2-1.1 Operation of the telescope .................... 49 2-2 Factors limiting photometric precision with the APT ......... 51 2-2.1 Poisson noise ........................... 52 2-2.2 Atmospheric scintillation ..................... 52 2-2.3 Atmospheric seeing ........................ 53 2-2.4 Sky transparency variations ................... 54 2-2.5 Colour-dependence of extinction ................. 54 Contents iv 2-2.6 Differential refraction and image rotation ............ 55 2-2.7 Cosmic rays ............................ 55 2-2.8 Image undersampling ....................... 56 2-2.9 Spatial variation of the PSF across the field .......... 56 2-2.10 Tracking errors .......................... 58 2-2.11 Flatfielding ............................ 58 2-2.12 Intra-pixel sensitivity variations ................. 59 2-2.13 Non-linearity of CCD response .................. 60 2-2.14 Saturation ............................. 60 2-2.15 The “ghost effect” ........................ 61 2-2.16 Readout noise and dark current ................. 62 2-2.17 Crowding ............................. 62 2-2.18 Photometry aperture positioning ................ 63 2-2.19 Other processing errors ...................... 63 2-2.20 Other effects ............................ 64 2-3 Early attempts at high precision photometry in this thesis work ... 65 2-3.1 Simple aperture photometry in IRAF .............. 65 2-3.2 Modelling the effect of intra-pixel sensitivity variations .... 65 2-3.3 Fitting an effective PSF ..................... 67 2-4 Observing technique ........................... 69 2-4.1 Raster scan ............................ 69 2-4.2 Nightly routine .......................... 70 2-5 Data reduction .............................. 72 2-5.1 Image processing ......................... 73 2-5.2 Object detection ......................... 73 2-5.3 Coordinate transformations ................... 74 2-5.4 Sky subtraction .......................... 75 2-5.5 Aperture photometry ....................... 75 2-5.6 Selecting photometry aperture sizes ............... 76 2-5.7 Photometric calibration ..................... 77 Contents v 2-5.8 The pipeline ............................ 78 2-6 Field selection and observing strategy .................. 79 3 Data and photometry obtained 81 3-1 Fields observed in 2001 .......................... 81 3-2 Observations of the transiting planet around HD 209458 ....... 81 3-3 Fields targeted in the planet search ................... 82 3-3.1 Summary of observations in 2002–2004 ............. 83 3-3.2 Latest fields ............................ 86 3-4 Photometric precision ........................... 86 3-5 Systematic errors ............................. 88 3-5.1 Gradual changes ......................... 88 3-5.2 Sudden changes .......................... 91 3-5.3 Removing systematic trends ................... 91 4 Analysis & Results 94 4-1 Candidate selection ............................ 94 4-1.1 Visual inspection ......................... 94 4-1.2 Software detection ........................ 95 4-1.3 A trend filtering algorithm .................... 97 4-1.4 A possible improvement to the detection algorithm ...... 98 4-2 Eliminating false positives ........................100 4-3 Transit candidates in the NGC 6633 field ................101 4-4 Variable stars in the NGC 6633 field ..................106 4-5 Transit candidates in other fields ....................106 5 Follow-up Observations 110 5-1 Photometry at high spatial resolution ..................110 5-2 Medium-resolution spectroscopy .....................112 5-2.1 Spectra obtained .........................112 5-2.2 Spectral types ...........................113 5-2.3 Radial velocities ..........................114 5-3 A K7V eclipsing binary system .....................116 6 Search Sensitivity and Detection Rate 121 6-1 A rough estimate .............................121 6-2 Monte Carlo simulation ..........................124 6-3 Validating the simulation .........................128 6-4 Results for the NGC 6633 field ......................128 6-5 Increasing the detection rate .......................131 6-5.1 Minimum detectable transit depth ................133 6-5.2 Observing schedule ........................134 6-5.3 Choice of filter ..........................137 6-5.4 Galactic latitude .........................139 6-6 Discussion .................................142 7 Conclusions & Future Work 146 7-1 Summary of results so far ........................146 7-2 The way ahead ..............................148