1. The experience of BEST Heike Rauer and the BEST Team Institut für Planetenforschung Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR) and Zentrum für Astronomie und Astrophysik Technische Universität Berlin +

2. The experience of BEST Berlin Exoplanet Search Telescope System Goals of BEST: - support for CoRoT - detect large planets - variable stars, additional science

3. Berlin Exoplanet Search Telescope Specifications: TelescopeSchmidt-Cassegrain Aperture20 cm Focal ratiof/2.7 InstrumentAP-10 CCD Size2048 x 2048 pixels Pixel size14 µm Pixel scale5.5 arcsec/pixel Field of view 3.1° x 3.1° 2001 - 2004 Thüringer Landessternwarte Tautenburg (TLS), Germany Since end 2004 Observatoire de Haute Provence (OHP), France

4. BEST I BEST II TEST at TLS

5. Observatorio Cerro Armazones, Chile Instituto de AstronomíaInstituto de Astronomía - Universidad Católica del Norte (UCN) in Antofagasta, Chile Universidad Católica del Norte Astronomisches InstitutAstronomisches Institut- Ruhr- Universität Bochum (RUB), Germany.Ruhr- Universität Bochum

6. BEST II Specifications: TelescopeBRC - 250 Aperture25 cm Focal ratiof/5.0 InstrumentFLI IMG-1680 CCD Size4096 x 4096 pixels Pixel size9 µm Pixel scale1.5 arcsec/pixel Field of view 1.7° x 1.7° Precision < 1% V=15-16 smaller FoV for BEST II is compensated by less stars influenced by crowding

7. Modes of operation BEST I at TLS: - obervations by observer at TLS - data reduction at DLR BEST I at OHP: - observations via remote control from Berlin - data reduction at DLR BEST II at OCA - „robotic“ observations (regular remote monitoring, manual interaction in case of alarm) - basic calibration at OCA, full data reduction at DLR

8. Performance The two critical factors for a transit search system are: 1.High duty cycle: full coverage of planetary orbits by observations. 2. Large number of high quality lightcurves (e.g. rms < 1%).

9. Duty cycle Need: High duty cycle, full coverage of planetary orbits by observations of sufficient quality. BEST experience: duty cycle is the major limiting factor from central Europe (not a surprise ). Next:  place BEST II at OCA, Chile  start building a network (NEST)  participate to ASTEP

10. Performance The two critical factors for a transit search system are: 1.High duty cycle, full coverage of planetary orbits by observations 2. Large number of high quality lightcurves (e.g. rms < 1%) - correction for detector effects (dark, bias, flats, hot/cold/defect pixels,…) - correction of atmosphere (extinction, seeing, scientillation, …) - accurate photometry (aperture/image subtraction/PSF fitting, crowding)

11. For example: a star moves across a hot pixel during the night due to imperfect guiding of the telescope…. Detector effects  Causes transit-like signal which has to be evaluated by comparison with the original data.  adds work-load on transit candidate evaluation

12. Correction for detector effects (dark, bias, flats, hot/cold/defect pixels,…) -Low-quality CCD: need to check transit events for detector effects, check position of the star on CCD -varying bias, dark, etc., adds to systematic noise residuals Recommendation:  buy good h/w  adapt the observing sequence to calibration needs Detector effects

13. Atmosphere effects Correction of atmosphere (extinction, seeing, scintillation, …) - Airmass correction is critical (no filter, large FOV)  restriction in airmass, depending on site and target field  adapt reduction method, e.g. work on sub-fields  implement filter if possible - Effect of seeing variations on crowding

14. Photometry Accurate photometry (aperture/image subtraction/PSF fitting, crowding) - Crowding can be a major problem  improve photometric method (image subtraction ok)  match the pixel scale of h/w

15. B.E.S.T. Candidate 3 BEST Magnitude of host star12.1 Depth [%]2.5 Duration [h]3.0 Orbital period [d] 423.10/n Number of detections2 BESTPOSS-I

16. depth [%]1.0 duration [h]4.5 orbital period [d] > 10 ? semi mayor axis[AU]? number of detections1 target field No.8 host starM dwarf magnitude(BEST)12.56 V magnitude14.73 BEST 18.3‘ x 18.3‘ reference star BEST candidate 5

17. Crowded target fields lead to further reduction of the photometric accuracy The photometric data reduction algorithm needs to be adopted. a)diluted signals - neighboring stars are resolved - but a neighbor contributes flux within the PSF or photometric aperture b) unresolved stars - neighboring stars are not resolved c) a combination of a) and b) - due to varying seeing the resolution of stars changes over the night  a transit signal is weakend  noise is added if the neighbor is variable

18. Comparison of Photometric methods * Source-Extractor, SExtractor (Bertin & Arnouts 1996) Performs different kinds of aperture photometry * Multi Object Multi Frame photometry, MOMF (Kjeldsen & Frandsen 1992) Combination of aperture and PSF photometry * Image Subtraction, ISIS (Alard 2000) Subtraction of a convolved reference frame from all frames.  implementation of parallel approach in data pipeline (SExtractor, ISIS) Karoff et al. 2005

19. TLS SExtractor SExtractor used in less crowded fields

20.  a reduction routine able to deal with very crowded target fields is important Data from 4 nights in spring 2005 obtained at OHP (COROT winter field) Comparison of both methods for the COROT field SExtractor ISIS Karoff et al. 2005

21. Performance of BEST I at OHP after evaluation of its first regular observing season at OHP in summer 2005.

22. 37 nights/142 hours observations from OHP in summer 2005. Search for variable stars and eclipsing binaries: 83 periodic variable stars identified: 76 new discoveries 11 variable stars with period < 120 days known in GCVS, 8 confirmed variables, for 3 stars no variability found 37 of the variables are eclipsing binaries BEST at OHP: Variable stars in the CoRoT center field Karoff et al. 2006

23. Location of the variables in the center field See Karoff et al. 2006 mag#A min - A max [mag] 10 - <1170.02-0.12 11 - <12240.01-0.29 12 - <13270.02-0.20 13 - <14160.05-0.18 14 - <1530.10-0.19 - Periodic variable stars are detected over the whole magnitude range. - There are many more stars with high rms: real but non-periodic variables and distorted lightcurves

24. How complete is our search for variables from OHP?

25. Number of detected eclipsing binaries as a function of period Eclipsing binaries in Hipparcos data Söderhjelm & Dischler 2005 Eclipsing binaries in BEST observations The BEST survey is complete only up to 10 – 20% for periods > 1 day. But: we have indications that the detection algorithm for periodic variables is not perfect… Karoff et al. 2006

26. Strong points:  Simple robust system with good capability for long-term photometric surveys of a substantial number of stars  Well suited to catalog bright stars in the COROT fields  Demonstrated ability to reach the accuracy limit needed to discover Jupiter-sized planets  Cost efficient way to characterize stellar variability over long time periods Weak point:  Transit detections limited by crowding and duty cycle (i.e. to few nights and/or to few stars) BEST – basic lessons from TLS and OHP