Andrei Tokovinin

Exceptional astronomical seeing conditions above Dome C in Antarctica.

One of the most important considerations when planning the next generation of ground-based optical astronomical telescopes is to choose a site that has excellent ‘seeing’—the jitter in the apparent position of a star that is caused by light bending as it passes through regions of differing refractive index in the Earths atmosphere. The best mid-latitude sites have a median seeing ranging from 0.5 to 1.0 arcsec (refs 1–5). Sites on the Antarctic plateau have unique atmospheric properties that make them worth investigating as potential observatory locations. Previous testing at the US Amundsen-Scott South Pole Station has, however, demonstrated poor seeing, averaging 1.8 arcsec (refs 6, 7). Here we report observations of the wintertime seeing from Dome C (ref. 8), a high point on the Antarctic plateau at a latitude of 75° S. The results are remarkable: the median seeing is 0.27 arcsec, and below 0.15 arcsec 25 per cent of the time. A telescope placed at Dome C would compete with one that is 2 to 3 times larger at the best mid-latitude observatories, and an interferometer based at this site could work on projects that would otherwise require a space mission.

From Differential Image Motion to Seeing

The theory of the differential image motion monitor (DIMM), a standard and widely spread method of measuring astronomical seeing, is reviewed and extended. More accurate coefficients for computing the Fried parameter from the measured variance of image motion are given. They are tested by numerical simulations that show that any DIMM measures Zernike tilts, not image centroids as generally assumed. The contribution of CCD readout noise to image motion variance is modeled. It can substantially bias DIMM results if left unsubtracted. The second most important DIMM bias comes from the used exposure time, which is typically not short enough to freeze image motion completely. This effect is studied quantitatively for real turbulence and wind profiles, and its correction by interlaced short and long exposures is validated. Finally, the influence of turbulence outer scale reduces image size in large telescopes by 10% or more compared to the standard theory; new formulae to compute FWHM and half-energy diameter of the atmospheric point-spread function that take into account outer scale are provided.

MASS: a monitor of the vertical turbulence distribution

The MASS (Multi-Aperture Scintillation Sensor) instrument consists of a 14-cm off-axis reflecting telescope and a detector unit which measures the scintillations of single stars in four concentric zones of the telescope pupil using photo-multipliers. Statistical analysis of these signals yields information of the vertical turbulence profile with a resolution of dh/h=0.5. We describe the instrument and present the results of its first field tests, including comparisons with DIMM seeing monitor and generalized SCIDAR. MASS will be used to obtain the extensive statistics of turbulence profiles at potential sites of future giant telescopes, as needed to predict the quality of adaptive seeing compensation.

Statistics of spectroscopic sub-systems in visual multiple stars

A large sample of visual multiples of spectral types F5-M has been surveyed for the presence of spectroscopic sub-systems. Some 4200 radial velocities of 574 components were measured in 1994-2000 with the correlation radial velocity meter. A total of 46 new spectroscopic orbits were computed for this sample. Physical relations are established for most of the visual systems and several optical components are identified as well. The period distribution of sub-systems has a maximum at periods from 2 to 7 days, likely explained by a combination of tidal dissipation with triple-star dynamics. The fraction of spectroscopic sub-systems among the dwarf components of close visual binaries with known orbits is similar to that of field dwarfs, from 11% to 18% per component. Sub-systems are more frequent among the components of wide visual binaries and among wide tertiary components to the known visual or spectroscopic binaries - 20% and 30%, respectively. In triple systems with both outer (visual) and inner (spectroscopic) orbits known, we find an anti-correlation between the periods of inner sub-systems and the eccentricities of outer orbits which must be related to dynamical stability constraints.

FROM BINARIES TO MULTIPLES. II. HIERARCHICAL MULTIPLICITY OF F AND G DWARFS

Statistics of hierarchical multiplicity among solar-type dwarfs are studied using the distance-limited sample of 4847 targets presented in the accompanying Paper I. Known facts about binaries (multiplicity fraction 0.46, log-normal period distribution with median period 100 yr and logarithmic dispersion 2.4, and nearly uniform mass-ratio distribution independent of the period) are confirmed with a high statistical significance. The fraction of hierarchies with three or more components is 0.13 ± 0.01, the fractions of targets with n = 1, 2, 3, . . . components are 54:33:8:4:1. Sub-systems in the secondary components are almost as frequent as in the primary components, but in half of such cases both inner pairs are present. The high frequency of those 2+2 hierarchies (4%) suggests that both inner pairs were formed by a common process. The statistics of hierarchies can be reproduced by simulations, assuming that the field is a mixture coming from binary-rich and binary-poor environments. Periods of the outer and inner binaries are selected recursively from the same log-normal distribution, subject to the stability constraint and accounting for the correlation between inner sub-systems. The simulator can be used to evaluate the frequency of multiple systems with specified parameters. However, it does not reproduce the observed excess of inner periods shorter than 10 d, caused by tidal evolution. Subject headings: stars: binaries; stars: solar-type; stars: statistics

Isoplanatism in a multiconjugate adaptive optics system

Turbulence correction in a large field of view by use of an adaptive optics imaging system with several deformable mirrors (DMs) conjugated to various heights is considered. The residual phase variance is computed for an optimized linear algorithm in which a correction of each turbulent layer is achieved by applying a combination of suitably smoothed and scaled input phase screens to all DMs. Finite turbulence outer scale and finite spatial resolution of the DMs are taken into account. A general expression for the isoplanatic angle thetaM of a system with M mirrors is derived in the limiting case of infinitely large apertures and Kolmogorov turbulence. Like Frieds isoplanatic angle theta0,thetaM is a function only of the turbulence vertical profile, is scalable with wavelength, and is independent of the telescope diameter. Use of angle thetaM permits the gain in the field of view due to the increased number of DMs to be quantified and their optimal conjugate heights to be found. Calculations with real turbulence profiles show that with three DMs a gain of 7-10x is possible, giving the typical and best isoplanatic field-of-view radii of 16 and 30 arcseconds, respectively, at lambda = 0.5 microm. It is shown that in the actual systems the isoplanatic field will be somewhat larger than thetaM owing to the combined effects of finite aperture diameter, finite outer scale, and optimized wave-front spatial filtering. However, this additional gain is not dramatic; it is less than 1.5x for large-aperture telescopes.

SPECKLE INTERFEROMETRY AT THE BLANCO AND SOAR TELESCOPES IN 2008 AND 2009

The results of speckle interferometric measurements of binary and multiple stars conducted in 2008 and 2009 at the Blanco and SOAR 4 m telescopes in Chile are presented. A total of 1898 measurements of 1189 resolved pairs or sub-systems and 394 observations of 285 un-resolved targets are listed. We resolved for the first time 48 new pairs, 21 of which are new sub-systems in close visual multiple stars. Typical internal measurement precision is 0.3 mas in both coordinates, typical companion detection capability is Δm ~ 4.2 at 015 separation. These data were obtained with a new electron-multiplication CCD camera; data processing is described in detail, including estimation of magnitude difference, observational errors, detection limits, and analysis of artifacts. We comment on some newly discovered pairs and objects of special interest.

FROM BINARIES TO MULTIPLES I: DATA ON F AND G DWARFS WITHIN 67 PC OF THE SUN.

Data on the multiplicity of F- and G-type dwarf stars within 67 pc of the Sun are presented. This distance-limited sample based on the Hipparcos catalog contains 4847 primary stars (targets) with 0.5 90% complete. There are 2196 known stellar pairs, some of them belong to 361 hierarchical systems from triples to quintuples. Models of companion detection by radial velocity, astrometric acceleration, direct resolution, and common proper motion are developed. They serve to compute completeness for each target, using the information on its coverage collected here. About 80% of companions to the primary stars are detected, but the census of sub-systems in the secondary components is only about 30%. Masses of binary components are estimated from their absolute magnitudes or by other methods, the periods of wide pairs are evaluated from their projected separations. A third of binaries with periods shorter than ∼100 yr are spectroscopic and/or astrometric pairs with yet unknown periods and mass ratios. These data are used in the accompanying Paper II to derive unbiased statistics of hierarchical multiple systems. Subject headings: stars: binaries; stars: solar-type; stars: statistics 1. GOALS AND STRATEGY

From the grating scale monitor to the generalized seeing monitor

An instrument named the grating scale monitor for measuring the outer scale L0 from the angle-of-arrival (AA) fluctuations of a perturbed wave front was developed a few years ago at Nice University. The AA is detected with a 5-ms time resolution by modulation of the stellar image in a small telescope with a grating. One uses the normalized covariance of AA fluctuations to estimate L0. A new version of this instrument, the generalized seeing monitor (GSM) is described. It consists of four identical modules for measuring the AA at four locations on the wave front. A spatiotemporal analysis of these data leads to the determination of seeing epsilon0, outer scale L0, and the wave-front speed. In addition, isoplanatic angle theta0 is determined from scintillation, making the characterization of turbulence with the GSM almost complete. We describe the instrument and make a detailed analysis of its performance and accuracy. Several site-testing campaigns have been conducted with the GSM: at La Silla (Chile), Oukaïmeden (Morocco), Maidanak (Uzbekistan), and Cerro Pachon and Cerro Paranal (Chile). The main results of these campaigns are presented and discussed.

Accurate seeing measurements with MASS and DIMM

Astronomical seeing is quantified by a single parameter, the turbulence integral, in the framework of the Kolmogorov turbulence model. This parameter can be routinely measured by a Differential Image Motion Monitor (DIMM). A new instrument, the multi-aperture scintillation sensor (MASS), permits one to measure the seeing in the free atmosphere above ∼0.5 km and, together with a DIMM, to estimate the ground-layer seeing. The absolute accuracy of both methods is studied here using analytical theory, numerical simulation and experiments. A modification of the MASS data processing to compensate for partially saturated scintillation is developed. We find that the DIMM can be severely biased by optical aberrations (e.g. defocus) and propagation. Seeing measurements with DIMM and MASS can reach absolute accuracy of ∼ 10 per cent when their biases are carefully controlled. Pushing this limit to 1 per cent appears unrealistic because the seeing itself is just a model-dependent parameter of a non-stationary random process.