During the austral winter 2005, the first astronomical site testing campaign were performed at Dome C, in Antarctica. Thirty-five meteorological balloons equipped with microthermal sensors were used to sense the vertical profile of the optical turbulence intensity C 2 N above Dome C up to 20 km. All the profiles of the 2005 campaign are statistically analyzed. We provide the median C 2N profiles and the mean potential temperature, mean horizontal wind speed, and mean direction profiles for the three seasons covered by this campaign (autumn, winter, and beginning of the spring). The structure of the optical turbulence in the atmosphere above Dome C is analyzed and compared with the well-known median C 2 N profiles of midlatitude sites. Of the whole optical turbulence, 80% lies within the first 33 m above the ground and 9% in the upper part of the boundary layer, between 33 m and 1 km above the ground. The remaining 11% are in the free atmosphere. This is an extreme situation when compared with "classical" midlatitude sites where the surface layer extends up to 200 m. This strong and thin surface layer is the result of the kinetic turbulent mixing of air combined with a strong potential temperature gradient. The site is characterized from the adaptive optics point of view. Seeing, isoplanatic angle, and coherence time are estimated for each considered seasons. A four-layer decomposition for each season is provided for adaptive optics simulations. For high angular astronomy, a telescope at Dome C needs to be elevated over this surface layer, or a specific GLAO needs to be designed. Combined with the unique possibility of performing continuous observation from Antarctica, scientific programs such as microlensing, pulsating stars, and asteroseismology become feasible.
Context. The CoRoT -Convection Rotation and planetary Transits -space mission is a great opportunity for monitoring stars with excellent time-sampling and unprecedented photometric precision for up to 150 days. As an important benefit, high-quality RR Lyrae light curves are obtained with a quasi-uninterrupted coverage over several pulsation and Blazhko cycles. Aims. The Blazhko effect in RR Lyrae stars is an unsolved problem of astrophysics. We used the high-precision space data to contribute more precise knowledge to explain the possible physical processes behind the phenomenon. Methods. We applied different period-finding techniques including Period04, MuFrAn, PDM and SigSpec. Amplitude and phase modulation were investigated by an analytical function method as well as with the traditional O-C diagrams. Results. The Blazhko modulation frequency is directly detected in the spectrum, as well as its first and second harmonics. It shows the non-linear nature of the Blazhko modulation. Besides the triplets, further higher-order modulation side peaks appear around the pulsation frequency as quintuplet, septuplet, nonuplet, undecaplet, tredecaplet, quindecaplet and sepdecaplet structures. Additional frequencies, not belonging to the classical multiplet structures, are detected, as well as their linear combinations with the fundamental radial mode. We interpret these additional terms as non-radial modes. During the five consecutive Blazhko cycles, there is a shift of the maximum phase around 0.011 pulsation phase which is likely the consequence of a long term modulation.
This paper analyses 3 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 2 n 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.
Context. The CoRoT (Convection, Rotation and planetary Transits) space mission provides a valuable opportunity to monitor stars with uninterrupted time sampling for up to 150 days at a time. The study of RR Lyrae stars, performed in the framework of the Additional Programmes belonging to the exoplanetary field, will particularly benefit from such dense, long-duration monitoring. Aims. The Blazhko effect in RR Lyrae stars is a long-standing, unsolved problem of stellar astrophysics. We used the CoRoT data of the new RR Lyrae variable CoRoT 101128793 ( f 0 = 2.119 d −1 , P = 0.4719296 d) to provide us with more detailed observational facts to understand the physical process behind the phenomenon. Methods. The CoRoT data were corrected for one jump and the long-term drift. We applied different period-finding techniques to the corrected timeseries to investigate amplitude and phase modulation. We detected 79 frequencies in the light curve of CoRoT 101128793. They have been identified as the main frequency f 0 and its harmonics, two independent terms, the terms related to the Blazhko frequency f m , and to several combination terms. Results. A Blazhko frequency f m = 0.056 d −1 and a triplet structure around the fundamental radial mode and harmonics were detected, as were a long-term variability on the Blazhko modulation. Indeed, the amplitude of the main oscillation is decreasing along the CoRoT survey. The Blazhko modulation is one of the smallest observed in RR Lyrae stars. Moreover, the additional modes f 1 = 3.630 and f 2 = 3.159 d −1 are detected. Taking its ratio with the fundamental radial mode into account, the term f 1 could be the identified as the second radial overtone. Detecting of these modes in horizontal branch stars is a new result obtained by CoRoT.
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