The optical turbulence above Dome C in winter is mainly concentrated in the first tens of meters above the ground. The properties of this so-called surface layer were investigated during the last two ...winterover by a set of sonics anemometers placed on a 45 m high tower. These anemometers provide measurements of the temperature and the wind speed vector. The sampling rate of 10 Hz allows to derivate the refractive index structure constant Cn2. We report here the first analysis of these data.
Context. The winter seeing at Concordia is bimodal, i.e. either excellent or quite poor, depending on the altitude above the snow surface. We study the temporal behavior of the good seeing sequences. ...Efficient exploitation of extremely good seeing conditions with an adaptive optics system requires long integrations. Aims. We examine the temporal distribution of time intervals providing excellent seeing at Concordia. Methods. We create temporal windows of good seeing by applying a simple binary process: good or bad. We correct the autocorrelations of these windows for those of the existing data sets, since these are not continuous, often being interrupted by technical problems in addition to the adverse weather gaps. We infer the typical duration of good seeing sequences from these corrected autocorrelations. This study has to be a little detailed as its results depend on the season, summer or winter. Results. When we adopt a threshold of 0.5 arcsec to define “good seeing”, we find that three characteristic numbers describe the temporal evolution of the good seeing windows. The first number is the mean duration of an uninterrupted good seeing sequence, which is τ0 = 7.5 h at 8 m above the ground and 15 h at 20 m. These sequences are randomly distributed in time, following a negative exponential law of damping time τ1 = 29 h (at elevation 8 m and 20 m), which represents our second number. The third number is the mean time between two 29 h episodes, which 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, at which we can achieve these exceptionally high quality seeing conditions.
GOLF in-flight commissioning and calibration was carried out during the first four months, most of which represented the cruise phase of SOHO towards its final L1 orbit. The initial performance of ...GOLF is shown to be within the design specification, for the entire instrument as well as for the separate sub-systems. Malfunctioning of the polarising mechanisms after 3 to 4 months operation has led to the adoption of an unplanned operating sequence in which these mechanisms are no longer used. This mode, which measures only the blue wing of the solar sodium lines, detracts little from the detection and frequency measurements of global oscillations, but does make more difficult the absolute velocity calibration, which is currently of the order of 20%. Data continuity in the new mode is extremely high and the instrument is producing exceptionally noise-free p-mode spectra. The data set is particularly well suited to the study of effects due to the excitation mechanism of the modes, leading to temporal variations in their amplitudes. The g modes have not yet been detected in this limited data set. In the present mode of operation, there are no indications of any degradation which would limit the use of GOLF for up to 6 years or more.
Solar diameter measurements performed with the Calern Observatory astrolabe (O.C.A - France) during more than two solar cycles show temporal variations. Due to the weather, seasonal effects and ...instrumental characteristics, recorded solar data are non uniformly sampled and present temporal gaps. Thus, to analyze these data, diameter measurements averaged over one or more months have generally been considered. This limits the accessible harmonic terms to a low frequency range. To determine short-term periodicities from the observed variations, all daily solar data need to be considered and also corrected from the zenithal distance. In the present work, we use two methods to analyze solar diameter measurements recorded at Calern Observatory astrolabe during the observation period 1975-1996. They are based on least square fits and deconvolution of the observation window function. Results deduced from the analysis confirm harmonic terms already found by other authors but also reveal new higher frequencies.
Two sets of radiosounding measurements were taken at Dome C (Antarctica) in December 2003 and January 2003 and 2004, using RS80‐A, RS80‐H, and RS90 Vaisala radiosondes, and from March to May 2005, ...employing the RS92 model. They were examined following accurate correction procedures to remove the main relative humidity dry bias and the temperature and humidity lag errors. The results showed that a strong cooling usually characterizes the thermal conditions of the whole troposphere from December/January to April/May, with an average temperature decrease from 245 to 220 K at the ground, of around 10 K at upper tropospheric levels, and of more than 15 K in the lower stratosphere. The relative humidity data were found to be affected by dry bias of 5–10%, on average, for the RS80‐A and RS80‐H Humicap sensors and by smaller percentages for the other sensors. The mean monthly vertical profiles of absolute humidity were found to decrease sharply throughout the troposphere, especially within the first 3 km, and to diminish considerably passing from December/January to March/April/May, with average values of precipitable water decreasing from 0.75 to 0.28 mm, median values from 0.69 to 0.25 mm, and first and third quartiles from 0.60 to 0.22 mm and from 0.87 to 0.34 mm, respectively. The results demonstrate that Dome C (where a permanent scientific station has been open for winter operations since austral winter 2005) is a site of comparable quality to the South Pole for both validation of satellite radiance measurements and astronomic observations in the infrared, submillimetric, and millimetric wavelength range, performed with large telescopes that cannot be carried on satellites.
Aims. We present a unique multi-epoch infrared interferometric study of the oxygen-rich Mira variable RR Aql in comparison to radiative transfer models of the dust shell. We investigate flux and ...visibility spectra at 8–13 μm with the aim of better understanding the pulsation mechanism and its connection to the dust condensation sequence and mass-loss process. Methods. We obtained 13 epochs of mid-infrared interferometry with the MIDI instrument at the VLTI between April 2004 and July 2007, covering minimum to pre-maximum pulsation phases (0.45–0.85) within four cycles. The data are modeled with a radiative transfer model of the dust shell where the central stellar intensity profile is described by a series of dust-free dynamic model atmospheres based on self-excited pulsation models. We examined two dust species, silicate and Al2O3 grains. We performed model simulations using variations in model phase and dust shell parameters to investigate the expected variability of our mid-infrared photometric and interferometric data. Results. The observed visibility spectra do not show any indication of variations as a function of pulsation phase and cycle. The observed photometry spectra may indicate intracycle and cycle-to-cycle variations at the level of 1–2 standard deviations. The photometric and visibility spectra of RR Aql can be described well by the radiative transfer model of the dust shell that uses a dynamic model atmosphere describing the central source. The best-fitting model for our average pulsation phase of \hbox{$\overline{\Phi_V}=0.64\pm0.15$}ΦV=0.64±0.15 includes the dynamic model atmosphere M21n (Tmodel = 2550 K) with a photospheric angular diameter of θPhot = 7.6 ± 0.6 mas, and a silicate dust shell with an optical depth of τV = 2.8 ± 0.8, an inner radius of Rin = 4.1 ± 0.7 RPhot, and a power-law index of the density distribution of p = 2.6 ± 0.3. The addition of an Al2O3 dust shell did not improve the model fit. However, our model simulations indicate that the presence of an inner Al2O3 dust shell with lower optical depth than for the silicate dust shell can not be excluded. The photospheric angular diameter corresponds to a radius of \hbox{$R_\mathrm{phot}=520^{+230}_{-140}~R_\odot$}Rphot=520-140+230 R⊙ and an effective temperature of Teff ~ 2420 ± 200 K. Our modeling simulations confirm that significant intracycle and cycle-to-cycle visibility variations are not expected for RR Aql at mid-infrared wavelengths within our uncertainties. Conclusions. We conclude that our RR Aql data can be described by a pulsating atmosphere surrounded by a silicate dust shell. The effects of the pulsation on the mid-infrared flux and visibility values are expected to be less than about 25% and 20%, respectively, and are too low to be detected within our measurement uncertainties.
Aims. We present the first NIR spectro-interferometry of the LBV η Carinae. The observations were performed with the AMBER instrument of the ESO Very Large Telescope Interferometer (VLTI) using ...baselines from 42 to 89 m. The aim of this work is to study the wavelength dependence of η Car's optically thick wind region with a high spatial resolution of 5 mas (11 AU) and high spectral resolution. Methods. The observations were carried out with three 8.2 m Unit Telescopes in the K-band. The raw data are spectrally dispersed interferograms obtained with spectral resolutions of 1500 (MR-K mode) and 12 000 (HR-K mode). The MR-K observations were performed in the wavelength range around both the $\ion{He}{i}$ 2.059 μm and the Brγ 2.166 μm emission lines, the HR-K observations only in the Brγ line region. Results. The spectrally dispersed AMBER interferograms allow the investigation of the wavelength dependence of the visibility, differential phase, and closure phase of η Car. In the K-band continuum, a diameter of $4.0\pm0.2$ mas (Gaussian FWHM, fit range 28–89 m baseline length) was measured for η Car's optically thick wind region. If we fit Hillier et al. (2001, ApJ, 553, 837) model visibilities to the observed AMBER visibilities, we obtain 50% encircled-energy diameters of 4.2, 6.5 and 9.6 mas in the 2.17$\,\mu$m continuum, the $\ion{He}{i}$, and the Brγ emission lines, respectively. In the continuum near the Brγ line, an elongation along a position angle of $120\degr\pm15\degr$ was found, consistent with previous VINCI/VLTI measurements by van Boekel et al. (2003, A&A, 410, L37). We compare the measured visibilities with predictions of the radiative transfer model of Hillier et al. (2001), finding good agreement. Furthermore, we discuss the detectability of the hypothetical hot binary companion. For the interpretation of the non-zero differential and closure phases measured within the Brγ line, we present a simple geometric model of an inclined, latitude-dependent wind zone. Our observations support theoretical models of anisotropic winds from fast-rotating, luminous hot stars with enhanced high-velocity mass loss near the polar regions.
Aims. Over the past few years a major effort has been put into the exploration of potential sites for the deployment of submillimetre astronomical facilities. Amongst the most important sites are ...Dome C and Dome A on the Antarctic Plateau, and the Chajnantor area in Chile. In this context, we report on measurements of the sky opacity at 200 μm over a period of three years at the French-Italian station, Concordia, at Dome C, Antarctica. We also present some solutions to the challenges of operating in the harsh polar environment. Methods. The 200-μm atmospheric opacity was measured with a tipper. The forward atmospheric model MOLIERE (Microwave Observation LIne Estimation and REtrieval) was used to calculate the atmospheric transmission and to evaluate the precipitable water vapour content (PWV) from the observed sky opacity. These results have been compared with satellite measurements from the Infrared Atmospheric Sounding Interferometer (IASI) on Metop-A, with balloon humidity sondes and with results obtained by a ground-based microwave radiometer (HAMSTRAD). In addition, a series of experiments has been designed to study frost formation on surfaces, and the temporal and spatial evolution of thermal gradients in the low atmosphere. Results. Dome C offers exceptional conditions in terms of absolute atmospheric transmission and stability for submillimetre astronomy. Over the austral winter the PWV exhibits long periods during which it is stable and at a very low level (0.1 to 0.3 mm). Higher values (0.2 to 0.8 mm) of PWV are observed during the short summer period. Based on observations over three years, a transmission of around 50% at 350 μm is achieved for 75% of the time. The 200-μm window opens with a typical transmission of 10% to 15% for 25% of the time. Conclusions. Dome C is one of the best accessible sites on Earth for submillimetre astronomy. Observations at 350 or 450 μm are possible all year round, and the 200-μm window opens long enough and with a sufficient transparency to be useful. Although the polar environment severely constrains hardware design, a permanent observatory with appropriate technical capabilities is feasible. Because of the very good astronomical conditions, high angular resolution and time series (multi-year) observations at Dome C with a medium size single dish telescope would enable unique studies to be conducted, some of which are not otherwise feasible even from space.
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