I. The Berlin Exoplanet Search Telescope (BEST) system consisting of a telescope, a CCD camera, a mount with a computer-controllable drive system, a autoguiding system and a control PC was set up at the Thüringer Landessternwarte (TLS) Tautenburg. The components were tested and successfully implemented building up a system capable of detecting transits of Hot Jupiter planets.
II. An automated data reduction and analysis pipeline was developed with the capabilities of calibrating the image data, extracting the light curves, applying an extinction correction and analyzing the resulting light curves for transit-like events in near real time. The data pipeline was optimized to deliver light curves with high photometric precision on a nightly basis.
III. A follow-up strategy was developed allowing the identification of real planetary transit events and excluding possible false alarm cases. The strategy includes the analyses of data in online data bases (Aladin, ViZier) and catalogues (2MASS, USNO B1.0, UCAC) as well as spectroscopic and photometric follow-up observations (2m TLS, 2.7m HJST, 9.2m HET, 0.9m Westerlund telescope). Based on a precise knowledge of the host star parameters a transit model can be fitted to the observed signal to identify if the transit-like signal could be caused by a planet-sized object. Radial velocity measurement are used to determine the mass of the secondary object to discriminate between giant planets, brown dwarfs and M dwarfs (all Jupiter-sized companions).
IV. An observational campaign lasting from 2001 to 2003 was conducted at the TLS monitoring three target fields during 90 nights with a total observing time of 438 hours. Target field F15 was observed for 163 hours, target field F2 for 105 hours and field F8 for 170 hours. The target fields F15 and F2 are located in the Galactic plane showing a high stellar density (F15: 37,000 detectable stars, F2: 32,000; both at a 1.5σ level). Target field F8 with 6,000 detectable stars is located above the Galactic plane thus it contains less stars. The target fields were observed with three exposure times (15 sec, 40 sec, 240 sec) to cover a large magnitude range (8 < R < 14) with a photometric precision better than 1% thereby sufficient for the detection of transits of Jupiter-sized planets orbiting Solar-like stars.
Additional observations were performed to observe two transits of the planet orbiting the star HD 209458. Both transits were successfully detected demonstrating the capabilities and the feasibility of the BEST system for monitoring of planetary transit events.
V. The collected data were calibrated and the resulting light curves were analyzed for transit-like events on a nightly basis. A box search routine was applied to the data searching for single transit events allowing to detect these events in near real time after the observations. A photometric precision better than 1% was obtained for up to 4,000 light curves per night monitoring target fields in the Galactic plane sufficient to detect transiting planets of the size of HD 209458 b orbiting Solar-like stars. For light curves with the best precision of 0.2% planets with radii larger than 0.5 Rjup could
Chapter 9: Summary and main results
individual nights were combined to composite light curves covering all observations. These combined light curves were phase-folded with trial periods to be searched for low-amplitude transit signals with the box search routine. Transits of planets with Jupiter size orbiting Solar-sized stars could have been detected for 1% precision light curves if the detection consists of three single transit signals in the folded data.
VI. Five transit candidates were identified in the BEST data acquired at TLS. Four of the candidates were identified as false alarms. The majority of the false alarms (three) are due to eclipsing binaries with a small secondary component including one blending case. One of these secondaries (BEST C 1) was characterized as being among the smallest M dwarfs with a determined radius. The observations and analysis of this system will continue to allow a more precise determination of the physical parameters of the M dwarf. These parameters are of great importance for the improvement of models of low mass stars. Candidate BEST C5 is still under investigation since a potential planet around this M star causing the signal could have a radius of 0.3 Rjup. Further photometric monitoring is ongoing to divulge the true nature behind the detected transit-like signal.
VII. In an analysis it was investigated how to optimize BEST-like transit survey considering aspects like optimal target field selection, pixelscale and crowding, noise terms of the photometry and orbital phase coverage. The analysis showed that the BEST system allows higher detection rates for transits of exoplanets compared to other ongoing transit search systems. Nevertheless, a new optimized transit search system is proposed. Based on present technological limits an optimal transit search system was defined with a maximal detection probability for transits of planets orbiting stars with R < 15. This magnitude limit was proposed to allow precise spectroscopic follow-up measurements. The proposed system consists of a F/1 Schmidt telescope with an aperture of 45cm combined with a CCD with 4096 x 4096 pixels and a pixelsize of 9 µm. As a new telescope design the manufacture of such a kind of telescopes would be, however, cost-intensive. This system could monitor a FOV of 4.7°x 4.7° with a pixelscale of 4.1 arcsec and Nyquist-sampled stellar PSFs with a FWHM of 2 pixels. The expected yield is a detection rate of up to four transiting exoplanets per observed target field in the galactic plane. The detection probability of this system was compared with other typical wide-angle transit surveys, which all show lower detection probabilities for transits of exoplanets. For these systems an optimized target field selection is proposed to get higher detection probabilities. Systems with an aperture less than 20cm should preferably observe target fields moderately above the Galactic plane to reduce degradation of the photometry by crowding. Larger systems can observe both target fields centered in the Galactic plane and above. Generally the pixelscale of the instruments has to be adopted to the stellar density in the observed fields. For BEST an upgrade of the system with a 4k CCD and an automatic focusing unit is recommended to reach an optimal pixelscale without defocusing. This allows to double the detections rates for transiting planets and to improve the quality of the data due to more controlled focusing. Finally, the orbital phase coverage has to be improved by observing from a site with better photometrical conditions. At the new observing site of BEST, Observatoire de Haute-Provence (OHP), the duty cycle was doubled during the first season. Other strategies to improve the orbital phase coverage are to observe the same target field for a second season or the introduction of telescope networks enabling
VIII. The data set of BEST observations from TLS was analyzed for variable stars. In total 83 stars were identified as variable stars showing typical light curves, all of them are new discoveries. For 53 of these variables the periodicity was determined. Among the variables 38 objects were classified as eclipsing binaries. The total number of light curves that show some kind of variability with the BEST duty cycle is estimated to be 10-20%.
Appendix A:
Overview of ground-based transit search systems
and strategies
Following the successful detection of the first transiting extrasolar planet HD 209458b in 1999 several other ground-based transit searches were developed worldwide. Two main directions of transit searches have been developed. Several meter-class telescopes were used for temporary deep transit searches in small field of views and several smaller wide-angle search systems were established dedicated to long-term transit search only. In this chapter an overview of both kinds of ongoing and future ground-based transit search projects and their observing strategies starting with wide-field systems will be given.