Aims. The Antarctica Search for Transiting ExoPlanets (ASTEP) program was originally aimed at probing the quality of the Dome C, Antarctica for the discovery and characterization of exoplanets by ...photometry. In the first year of operation of the 40 cm ASTEP 400 telescope (austral winter 2010), we targeted the known transiting planet WASP-19b in order to try to detect its secondary transits in the visible. This is made possible by the excellent sub-millimagnitude precision of the binned data. Methods. The WASP-19 system was observed during 24 nights in May 2010. Once brought back from Antarctica, the data were processed using various methods, and the best results were with an implementation of the optimal image subtraction (OIS) algorithm. Results. The photometric variability level due to starspots is about 1.8% (peak-to-peak), in line with the SuperWASP data from 2007 (1.4%) and higher than in 2008 (0.07%). We find a rotation period of WASP-19 of 10.7 ± 0.5 days, in agreement with the SuperWASP determination of 10.5 ± 0.2 days. Theoretical models show that this can only be explained if tidal dissipation in the star is weak, i.e. the tidal dissipation factor Q'★ > 3×107. Separately, we find evidence of a secondary eclipse of depth 390 ± 190 ppm with a 2.0σ significance, a phase that is consistent with a circular orbit and a 3% false positive probability. Given the wavelength range of the observations (420 to 950 nm), the secondary transit depth translates into a day-side brightness temperature of 2690-220+150 K, in line with measurements in the z′ and K bands. The day-side emission observed in the visible could be due either to thermal emission of an extremely hot day side with very little redistribution of heat to the night side or to direct reflection of stellar light with a maximum geometrical albedo Ag = 0.27 ± 0.13. We also report a low-frequency oscillation in phase at the planet orbital period, but with a lower limit amplitude that could not be attributed to the planet phase alone and that was possibly contaminated with residual lightcurve trends. Conclusions. This first evidence of a secondary eclipse in the visible from the ground demonstrates the high potential of Dome C, Antarctica, for continuous photometric observations of stars with exoplanets. These continuous observations are required to understand star-planet interactions and the dynamical properties of exoplanetary atmospheres.
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Context. Dome C in Antarctica is a promising site for photometric observations thanks to the continuous night during the Antarctic winter and favorable weather conditions. Aims.We developed ...instruments to assess the quality of this site for photometry in the visible and to detect and characterize variable objects through the Antarctic Search for Transiting ExoPlanets (ASTEP) project. Methods. Here, we present the full analysis of four winters of data collected with ASTEP South, a 10 cm refractor pointing continuously toward the celestial south pole. We improved the instrument over the years and developed specific data reduction methods. Results. We achieved nearly continuous observations over the winters. We measure an average sky background of 20 mag arcsec−2 in the 579–642 nm bandpass. We built the lightcurves of 6000 stars and developed a model to infer the photometric quality of Dome C from the lightcurves themselves. The weather is photometric 67.1 ± 4.2% of the time and veiled 21.8 ± 2.0% of the time. The remaining time corresponds to poor quality data or winter storms. We analyzed the lightcurves of σ Oct and HD 184465 and find that the amplitude of their main frequency varies by a factor of 3.5 and 6.7 over the four years, respectively. We also identify 34 new variable stars and eight new eclipsing binaries with periods ranging from 0.17 to 81 days. Conclusion. The phase coverage that we achieved with ASTEP South is exceptional for a ground-based instrument and the data quality enables the detection and study of variable objects. These results demonstrate the high quality of Dome C for photometry in the visible and for time series observations in general.
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This paper analyses 3$\frac{1}{2}$ years of site testing data obtained at Dome C, Antarctica, based on measurements obtained with three DIMMs located at three different elevations. Basic statistics ...of the seeing and the isoplanatic angle are given, as well as the characteristic time of temporal fluctuations of these two parameters, which we found to around 30 min at 8 m. The 3 DIMMs are exploited as a profiler of the surface layer, and provide a robust estimation of its statistical properties. It appears to have a very sharp upper limit (less than 1 m). The fraction of time spent by each telescope above the top of the surface layer permits us to deduce a median height of between 23 m and 27 m. The comparison of the different data sets led us to infer the statistical properties of the free atmosphere seeing, with a median value of 0.36 arcsec. The $C_n^2$ profile inside the surface layer is also deduced from the seeing data obtained during the fraction of time spent by the 3 telescopes inside this turbulence. Statistically, the surface layer, except during the 3-month summer season, contributes to 95 percent of the total turbulence from the surface level, thus confirming the exceptional quality of the site above it.
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Aims. An experiment was set up at the Concordia station in Antarctica during the winter-over period in 2012 to determine the behaviour of atmospheric optical turbulence in the lower part of the ...atmospheric boundary layer. The aim of the experiment was to study the influence of turbulence and weather conditions on the quality of astronomical observations. The Concordia station is characterised by the high quality of astronomical images thanks to very low seeing values. The surface layer in the interior of Antarctica during the winter is very stably stratified with the differences of temperature between the surface and the top of the inversion, which reach 20−35°C. In spite of this strong static stability, considerable thermal optically active turbulence sometimes occurs and extends to several tens of metres above the surface, depending on weather conditions. It is important to know the meteorological characteristics that favour good astronomical observations. Methods. The optical measurements of the seeing made by differential image motion monitors installed at two levels of 8 and 20 m were accompanied by observations of turbulence in the lowest one hundred meters. Turbulence was detected and evaluated using a high-resolution sodar developed specially for this purpose. The statistics of some relevant meteorological variables including the long-wave downward radiation, which indicates cloudiness, were determined. Results. Typical patterns of the vertical and temporal structure of turbulence shown by sodar echograms were identified, analysed, and classified. The statistics of the depth of the surface-based turbulent layer and the turbulent optical factor for different height layers are presented together with the seeing statistics. We analysed the dependence of both seeing and integral turbulence intensity within the first 100 m on temperature and wind speed. Conclusions. Seeing and turbulence intensity in the atmospheric boundary layer appear to be correlated. The best values of the seeing (<1 arcsec) are observed when the sodar shows very low turbulence intensity. The main contribution to the image distortion is due to turbulence generated within the lowest 30−50 m near the surface. The presented statistics of the vertical distribution of the atmospheric optical turbulence can be used to determine the optimal location for astronomical instruments.
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The optical turbulence above Dome C in winter is mainly concentrated in the first tens of metres above the ground. Properties of this so-called surface layer (SL) were investigated during the period ...2007–2012 by a set of sonic anemometers placed on a 45 m high tower. We present the results of this long-term monitoring of the refractive index structure constant
$C_n^2$
within the SL, and confirm its thickness of 35 m. We give statistics of the contribution of the SL to the seeing and coherence time. We also investigate properties of large-scale structure functions of the temperature and show evidence of a second inertial zone at kilometric spatial scales.
Aims. Future extremely large telescopes will certainly be equipped with wide-field adaptive optics systems. The optimization of the performances of these techniques requires a precise specification ...of the different components of these AO systems. Most of these technical specifications are related to the atmospheric turbulence parameters, particularly the profile of the refractive index structure constant CN2(h). A new monitor called Profiler of Moon Limb (PML) for the extraction of the CN2(h) profile with high vertical resolution and its first results are presented. Methods. The PML instrument uses an optical method based on the observation of the Moon limb through two subapertures. The use of the lunar limb leads to a continuum of double stars allowing a scan of the whole atmosphere with high resolution in altitude. Results. The first prototype of the PML has been installed at Dome C in Antarctica and the first results of the PML are presented and compared to radio-sounding balloon profiles. In addition to the CN2(h) profile obtained with high vertical resolution, PML is also able to provide other atmospheric turbulence parameters such as the outer scale profile, the total seeing, and the isoplanatic and isopistonic angles.
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Context. The Concordia base in Dome C, Antarctica, is an extremely promising site for photometric astronomy due to the 3-month long night during the Antarctic winter, favorable weather conditions, ...and low scintillation. Aims. The ASTEP project (Antarctic Search for Transiting ExoPlanets) is a pilot project to discover transiting planets and understand 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 10 cm refractor with a 4k $\times$ 4k CCD camera in a thermalized box, pointing continuously a 3.88 $\times$ 3.88°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 and obtained 1592 hours of data during the 2008 Antarctic winter. The data are of good quality but the analysis has to account for changes in the PSF (point spread function) due to rapid ground seeing variations and instrumental effects. The pointing direction is stable within 10 arcsec on a daily timescale and drifts by only 34 arcsec in 50 days. A truly continuous photometry of bright stars is possible in June (the noon sky background peaks at a magnitude R ≈ 15 arcsec-2 on June 22), but becomes challenging in July (the noon sky background magnitude is R ≈ 12.5 arcsec-2 on July 20). The weather conditions are estimated from the number of stars detected in the field. For the 2008 winter, the statistics are between 56.3% and 68.4% of excellent weather, 17.9% to 30% of veiled weather (when the probable presence of thin clouds implies a lower number of detected stars) and 13.7% of bad weather. Using these results in a probabilistic analysis of transit detection, we show that the detection efficiency of transiting exoplanets in one given field is improved at Dome C compared to a temperate site such as La Silla. For example we estimate that a year-long campaign of 10 cm refractor could reach an efficiency of 69% at Dome C versus 45% at La Silla for detecting 2-day period giant planets around target stars from magnitude 10 to 15. The detection efficiency decreases for planets with longer orbital periods, but in relative sense it is even more favorable to Dome C. Conclusions. This shows the high potential of Dome C for photometry and future planet discoveries.
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We present long term site testing statistics based on DIMM and GSM data obtained at Dome C, Antarctica. These data have been collected on the bright star Canopus since the end of 2003. We give values ...of the integrated turbulence parameters in the visible (wavelength 500 nm). The median value we obtained for the seeing are 1.2 arcsec, 2.0 arcsec and 0.8 arcsec at respective elevations of 8m, 3m and 20m above the ground. The isoplanatic angle median value is 4.0 arcsec and the median outer scale is 7.5m. We found that both the seeing and the isoplanatic angle exhibit a strong dependence with the season (the seeing is larger in winter while the isoplanatic angle is smaller).
ASTEP South: a first photometric analysis Crouzet, N.; Guillot, T.; Mékarnia, D. ...
Proceedings of the International Astronomical Union,
08/2012, Volume:
8, Issue:
S288
Journal Article
Peer reviewed
Open access
The ASTEP project aims at detecting and characterizing transiting planets from Dome C, Antarctica, and qualifying this site for photometry in the visible. The first phase of the project, ASTEP South, ...is a fixed 10 cm diameter instrument pointing continuously towards the celestial South Pole. Observations were made almost continuously during 4 winters, from 2008 to 2011. The point-to-point RMS of 1-day photometric lightcurves can be explained by a combination of expected statistical noises, dominated by the photon noise up to magnitude 14. This RMS is large, from 2.5 mmag at R = 8 to 6% at R = 14, because of the small size of ASTEP South and the short exposure time (30 s). Statistical noises should be considerably reduced using the large amount of collected data. A 9.9-day period eclipsing binary is detected, with a magnitude R = 9.85. The 2-season lightcurve folded in phase and binned into 1,000 points has a RMS of 1.09 mmag, for an expected photon noise of 0.29 mmag. The use of the 4 seasons of data with a better detrending algorithm should yield a sub-millimagnitude precision for this folded lightcurve. Radial velocity follow-up observations reveal a F-M binary system. The detection of this 9.9-day period system with a small instrument such as ASTEP South and the precision of the folded lightcurve show the quality of Dome C for continuous photometric observations, and its potential for the detection of planets with orbital periods longer than those usually detected from the ground.
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