Abstract. The Ninth Catalogue of Spectroscopic Binary Orbits (http://sb9.astro.ulb.ac.be) continues the series of compilations of spectroscopic orbits carried out over the past 35 years by Batten and collaborators. As of 2004 May 1st, the new Catalogue holds orbits for 2386 systems. Some essential differences between this catalogue and its predecessors are outlined and three straightforward applications are presented: (1) completeness assessment: period distribution of SB1s and SB2s; (2) shortest periods across the H-R diagram; (3) period-eccentricity relation.

We have surveyed a sample of 165 solar-type spectroscopic binaries (SB) with periods from 1 to 30 days for higher-order multiplicity. A subsample of 62 targets were observed with the NACO adaptive optics system and 13 new physical tertiary companions were detected. An additional 12 new wide companions (5 still tentative) were found using the 2MASS all-sky survey. The binaries belong to 161 stellar systems; of these 64 are triple, 11 quadruple and 7 quintuple. After correction for incompleteness, the fraction of SBs with additional companions is found to be 63% ± 5%. We find that this fraction is a strong function of the SB period P, reaching 96% for P < 3 d and dropping to 34% for P > 12 d . Period distributions of SBs with and without tertiaries are significantly different, but their mass ratio distributions are identical. The statistical data on the multiplicity of close SBs presented in this paper indicates that the periods and mass ratios of SBs were established very early, but the periods of SB systems with triples were further shortened by angular momentum exchange with companions.

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.

ABSTRACT. The CHIRON optical high-resolution echelle spectrometer was commissioned at the 1.5 m telescope at CTIO in 2011. The instrument was designed for high throughput and stability, with the goal of monitoring radial velocities of bright stars with high precision and high cadence for the discovery of low-mass exoplanets. Spectral resolution of R ¼ 79 000 is attained when using a slicer with a total (including telescope and detector) efficiency of 6% or higher, while a resolution of R ¼ 136 000 is available for bright stars. A fixed spectral range of 415-880 nm is covered. The echelle grating is housed in a vacuum enclosure and the instrument temperature is stabilized to AE0:2°. Stable illumination is provided by an octagonal multimode fiber with excellent light-scrambling properties. An iodine cell is used for wavelength calibration. We describe the main optics, fiber feed, detector, exposure-meter, and other aspects of the instrument, as well as the observing procedure and data reduction.

Abstract. The MSC catalogue contains data on 612 physical multiple stars of multiplicity 3 to 7 which are hierarchical with few exceptions. Orbital periods, angular separations and mass ratios are estimated for each subsystem. Orbital elements are given when available. The catalogue can be accessed through CDS (Strasbourg). Half of the systems are within 100 pc from the Sun. The comparison of the periods of close and wide sub-systems reveals that there is no preferred period ratio and all possible combinations of periods are found. The distribution of the logarithms of short periods is bimodal, probably due to observational selection. In 82% of triple stars the close subsystem is related to the primary of a wide pair. However, the analysis of mass ratio distribution gives some support to the idea that component masses are independently selected from the Salpeter mass function. Orbits of wide and close sub-systems are not always coplanar, although the corresponding orbital angular momentum vectors do show a weak tendency of alignment. Some observational programs based on the MSC are suggested.

Abstract.A sample of 115 B-type stars in the Sco OB2 association is examined for existence of visual companions in the J and Ks bands, using the ADONIS near-infrared adaptive optics system and coronograph. Practically all the components in the separation range 0. 3-6. 4 (45-900 AU) and magnitudes down to K = 16 were detected. The K and J −K photometry of the primaries and differential photometry and astrometry of the 96 secondaries are presented. Ten secondaries are new physical components, as inferred from the photometric and statistical criteria, while the rest of the newly detected objects are faint background stars. After a small correction for detection incompleteness and a conversion of the fluxes into masses, an unbiased distribution of the components mass ratio q was derived. The power law f (q) ∝ q −0.5 fits the observations well, whereas a q −1.8 distribution, which corresponds to a random pairing of stars, is rejected. The companion star fraction is 0.20 ± 0.04 per decade of separation which is comparable to the highest measured binary fraction among low-mass PMS stars and ∼1.6 times higher than the binary fraction of low-mass dwarfs in the solar neighborhood and in open clusters in the same separation range.

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 Earth's 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 degrees 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.

ABSTRACT. 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.