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

+

The experience of BEST

Berlin Exoplanet Search Telescope SystemBerlin Exoplanet Search Telescope System

Goals of BEST: - support for CoRoT

- detect large planets

- variable stars, additional science

Berlin Exoplanet Search Telescope

Specifications:

Telescope Schmidt-CassegrainAperture 20 cmFocal ratio f/2.7InstrumentAP-10 CCD Size 2048 x 2048 pixelsPixel size 14 µmPixel scale 5.5 arcsec/pixelField of view 3.1° x 3.1°

2001 - 2004 Thüringer LandessternwarteTautenburg (TLS), Germany

Since end 2004 Observatoire de Haute Provence (OHP), France

BEST I

BEST II

TEST at TLS

Observatorio Cerro Armazones, Chile

Instituto de Astronomía - Universidad Católica del Norte (UCN) in Antofagasta, Chile

Astronomisches Institut- Ruhr-Universität Bochum (RUB), Germany.

BEST II

Specifications:Telescope BRC - 250Aperture 25 cmFocal ratio f/5.0InstrumentFLI IMG-1680 CCD Size 4096 x 4096 pixelsPixel size 9 µmPixel scale 1.5 arcsec/pixelField 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

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

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%).

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

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)

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

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

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

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

B.E.S.T. Candidate 3

BEST Magnitude of host star 12.1Depth [%] 2.5Duration [h] 3.0Orbital period [d] 423.10/nNumber of detections 2

BEST POSS-I

depth [%] 1.0duration [h] 4.5 orbital period [d] > 10 ?semi mayor axis[AU] ?number of detections 1target field No. 8host star M dwarfmagnitude(BEST)12.56V magnitude 14.73

BEST 18.3‘ x 18.3‘

reference star

BEST candidate 5

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

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

TLS

SExtractor

SExtractor used in less crowded fields

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

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

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

Location of the variables in the center field

See Karoff et al. 2006

mag # Amin- Amax

[mag]

10 - <11 7 0.02-0.12

11 - <12 24 0.01-0.29

12 - <13 27 0.02-0.20

13 - <14 16 0.05-0.18

14 - <15 3 0.10-0.19

- 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

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

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

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