A. Agabi

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

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 × 4k CCD camera in a thermalized box, pointing continuously a 3.88 × 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.

Seeing, outer scale of optical turbulence, and coherence outer scale at different astronomical sites using instruments on meteorological balloons

Here we analyze 168 optical turbulence profiles made at nine different locations worldwide by means of free flight balloons equipped with instrumentation. Seeing eFWHM, optical turbulence outer scales Lo, and wavefront outer scales £o are derived for the different locations and taking into account the contribution of the surface layer (0, 50 m), the boundary layer (0, 1 km) and the free atmosphere (1 km, 30 km). Noticeable changes are found between the different locations, mainly due to the boundary layer contribution as well as the surface layer contribution. The free atmosphere contribution seems almost the same for each site. There is a very good coherence between the outer scale derived by us and that measured using the Generalized Seeing Monitor (GSM) technique.

First statistics of the turbulence outer scale at Dome C

Context. The outer scale of wavefronts is of interest for the dimensioning and the optimisation of the High Angular Resolution techniques such as Interferometry and Adaptive Optics, particularly for the new generation of telescopes such as the Extremely Large Telescopes (ELT). Aims. We aim to provide the first statistics of the outer scale at Dome C site in Antarctica to finalize the characterization of this site. Methods. A new version of the Generalized Seeing Monitor has been developed for extremely cold conditions. Two DIMMs (Differential Image Motion Monitor) were coupled to extract Angle-of-Arrival (AA) fluctuations using CCD detectors. Correlations of these AA fluctuations for different baselines lead to outer scale estimations. Results. For the first time, statistics of the outer scale at Dome C are provided leading to small values in the surface layer at Dome C. These small outer scale values compared to temperate sites considerably reduce the fringe excursion of interferometers and the low orders of the normalized amplitude of atmospheric Zernike coefficients, particularly the tip-tilt. Conclusions. The Dome C small outer scale combined with the large coherence time and large isoplanatic angle are very useful for the development of Adaptive Optics systems and long-baseline interferometers.

Optical turbulence modeling in the boundary layer and free atmosphere using instrumented meteorological balloons

In this paper, we study the behavior of atmospheric turbulence in the boundary layer and free atmosphere using balloon-born radiosondes. The data measurements were carried out with 168 balloons launched from 9 sites. Profiles of various parameters (C 2 N, M 2 and L o ) characterizing atmospheric turbulence are established. We work out statistical mode atmospheric turbulence in the boundary layer, and we checked their coherence by comparing them with other models. In the free atmosphere, the C 2 N profile is coherent with those of the models of Hufnagel and of Brown-Beland. A complete behavior of the outer scale L o is given from the ground up to 30 km which fits with partial models of Coulman-Vernin and Beland-Brown.

First Single Star Scidar measurements at Dome C, Antarctica

Aims. First operational running of the Single Star Scidar (SSS) under harsh weather conditions at Dome C in Antarctica. Continuous monitoring of the optical turbulence and wind speed profiles throughout the whole atmosphere. Methods. SSS is mainly composed of “off the shelf” light-weight components, a 16 inches telescope installed on an equatorial mount. Scintillation patterns are computed (auto and cross-corre lations) in real time and analyzed off line to retrieve continuously vertical profiles of optical turbulence C 2 (h) and wind speed V(h), from ground up to 20 km . Results. Using a simulated annealing method, we have analyzed about 6.5 hours of observations, putting into evidence the strong s urface layer contribution to seeing degradation. SSS results show a good agreement with simultaneous measurements with a Differential Image Motion Monitor as well as weather archive from NOAA. Conclusions. SSS has shown its usefullness for site characterization sin ce it measures simultaneously C 2 N and V profiles, from which most of adaptative optic parameters are deduced, such as isoplanatic angle and coherence time of the wavefront. Due to its small size, it is well adapted to site characterization, even when low infrastructure is available.

Observations of optically active turbulence in the planetary boundary layer by sodar at the Concordia astronomical observatory, Dome C, Antarctica

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.

Typical duration of good seeing sequences at Concordia

Context. The winter seeing at Concordia is essentially bimodal, excellent or quite poor, with relative proportions that depend on altitude above the snow surface. This paper studies the temporal behavior of the good seeing sequences. Aims. An e cient exploitation of extremely good seeing with an adaptive optics system needs long integrations. It is then important to explore the temporal distribution of the fraction of time providing excellent seeing. Methods. Temporal windows of good seeing are created by a simple binary process. Good or bad. Their autocorrelations are corrected for those of the existing data sets, since these are not continuous, being often interrupted by technical problems in addition to the adverse weather gaps. At the end these corrected autocorrelations provide the typical duration of good seeing sequences. This study has to be a little detailed as its results depend on the season, summer or winter. Results. Using a threshold of 0.5 arcsec to define the “good seeing”, three characteristic numbers are found to describe the temporal evolution of the good seeing windows. The first number is the mean duration of an uninterrupted good seeing sequence: it is 0 = 7:5 hours at 8 m above the ground (15 hours at 20 m). These sequences are randomly distributed in time, with a negative exponential law of damping time 1 = 29 hours (at elevation 8 m and 20 m). The third number is the mean time between two 29 hours episodes. It is T = 10 days at 8 m high (5 days at 20 m). Conclusions. There is certainly no other site on Earth, except for the few other high altitude Domes on the Antarctic plateau, that can get close to these really peculiar seeing conditions.

San Pedro Mártir: astronomical site evaluation

The Observatorio Astronomico Nacional at San Pedro Martir is situated on the summit of the San Pedro Martir Sierra in the Baja California peninsula of Mexico, at 2800m above sea level. For as long as three decades, a number of groups and individuals have gathered extremely valuable data leading to the site characterization for astronomical observations. Here we present a summary of the most important results obtained so far. The aspects covered are: weather, cloud coverage, local meteorology, atmospheric optical extinction, millimetric opacity, geotechnical studies, seeing, optical turbulence profiles, wind profiles and 3D simulations of atmospheric turbulence. The results place San Pedro Martir among the most favorable sites in the world for astronomical observations. It seems to be particularly well-suited for extremely large telescopes because of the excellent turbulence and local wind conditions, to mention but two characteristics. Long-term monitoring of some parameters still have to be undertaken. The National University of Mexico (UNAM) and other international institutions are putting a considerable effort in that sense.

Thermalizing a telescope in Antarctica – analysis of ASTEP observations

The installation and operation of a telescope in Antarctica represent particular challenges, in particular the requirement to operate at extremely cold temperatures, to cope with rapid temperature fluctuations and to prevent frosting. Heating of electronic subsystems is a necessity, but solutions must be found to avoid the turbulence induced by temperature fluctuations on the optical paths. ASTEP 400 is a 40cm Newton telescope installed at the Concordia station, Dome C since 2010 for photometric observations of fields of stars and their exoplanets. While the telescope is designed to spread star light on several pixels to maximize photometric stability, we show that it is nonetheless sensitive to the extreme variations of the seeing at the ground level (between about 0′′.1 and 5′′) and to temperature fluctuations between –30°C and –80 °C. We analyze both day-time and night-time observations and obtain the magnitude of the seeing caused by the mirrors, dome and camera. The most important effect arises from the heating of the primary mirror which gives rise to a mirror seeing of 0′′.23 K–1. We propose solutions to mitigate these effects. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Two years of polar winter observations with the ASTEP400 telescope

The ASTEP program is dedicated to exo-planet transit search from the Concordia Station located at Dome C, Antarctica. It comprises two instruments: a fixed 10cm refractor pointed toward the celestial South Pole, and a 400mm Newton telescope with a 1x1 degree field of view. This work focuses on the latter instrument. It has been installed in November 2009, and has been observing since then during the two polar winters 2010 and 2011. After presenting the main science observing programs, we review the telescope installation, performance, and describe its operating conditions as well as the data reduction and handling strategy. The resulting lightcurves are generally very stable and of excellent quality, as shown by continuous observations of WASP-19 that we present here.