ASTEP South: an Antarctic Search for Transiting ExoPlanets around the celestial south pole

摘要

Context. The Concordia base in DomeC, Antarctica, isan extremely promising site for photometric astronomy due to the3-month long night during the Antarctic winter, favorable weatherconditions, and low scintillation. Aims. The ASTEP project (Antarctic Search for TransitingExoPlanets) is a pilot project to discover transiting planets andunderstand the limits of visible photometry from the Concordia site. Methods. ASTEP South is the first phase of the ASTEP project. The instrument is a fixed 10cm refractor with a 4k4k CCD camera in a thermalized box, pointing continuously a 2 field of view centered on the celestial south pole. We describe the project and report results of a preliminary data analysis. Results. ASTEP South became fully functional in June 2008 andobtained 1592hours of data during the 2008 Antarctic winter. Thedata are of good quality but the analysis has to account for changes inthe PSF (point spread function) due to rapid ground seeing variationsand instrumental effects. The pointing direction is stable within10arcsec on a daily timescale and drifts by only 34arcsecin 50days. A truly continuous photometry of bright stars ispossible in June (the noon sky background peaks at a magnitude on June22), but becomes challenging in July (the noon sky background magnitude is on July20). The weather conditions are estimated from the numberof stars detected in the field. For the 2008 winter, the statistics arebetween56.3% and68.4% of excellent weather, 17.9%to30% of veiled weather (when the probable presence of thinclouds implies a lower number of detected stars) and13.7% of badweather. Using these results in a probabilistic analysis of transitdetection, we show that the detection efficiency of transitingexoplanets in one given field is improved at DomeC compared to atemperate site such as La Silla. For example we estimate that ayear-long campaign of 10cm refractor could reach an efficiencyof69% at DomeC versus45% at La Silla for detecting2-day period giant planets around target stars from magnitude10to15. The detection efficiency decreases for planets with longerorbital periods, but in relative sense it is even more favorable toDomeC. Conclusions. This shows the high potential of DomeC for photometry and future planet discoveries. Key words: methods: observational - methods: data analysis - site testing - techniques: photometric