Formation of wide binary stars from adjacent cores

Wide gravitationally bound pairs of stars can be formed from adjacent prestellar cores that happen to move slowly enough relative to each other. These binaries are remnants of the primordial clustering. It is shown that the expected fraction of wide bound pairs in low-density star formation regions can be larger than the fraction of wide pairs in the field. On the other hand, wide binaries do not form or survive in dense clusters. Recent works on the separation distribution of young binaries, summarized here, confirm these expectations. Alternative formation mechanisms of wide binaries such as cluster dissolution or unfolding of triple stars cannot explain the large observed fraction of young wide pairs and therefore are not dominant. The fact that more than a half of wide pairs contain subsystems matches the general multiplicity statistics and does not imply that hierarchical multiplicity and wide binaries are genetically related.Comment: Accepted by MNRAS. 7 pages, 4 figures, 1 tabl

Spectroscopic subsystems in nearby wide binaries

Radial velocity (RV) monitoring of solar-type visual binaries has been conducted at the CTIO/SMARTS 1.5-m telescope to study short-period systems. Data reduction is described, mean and individual RVs of 163 observed objects are given. New spectroscopic binaries are discovered or suspected in 17 objects, for some of them orbital periods could be determined. Subsystems are efficiently detected even in a single observation by double lines and/or by the RV difference between the components of visual binaries. The potential of this detection technique is quantified by simulation and used for statistical assessment of 96 wide binaries within 67pc. It is found that 43 binaries contain at least one subsystem and the occurrence of subsystems is equally probable in either primary or secondary components. The frequency of subsystems and their periods match the simple prescription proposed by the author (2014, AJ, 147, 87). The remaining 53 simple wide binaries with a median projected separation of 1300AU have the distribution of the RV difference between their components that is not compatible with the thermal eccentricity distribution f(e)=2e but rather matches the uniform eccentricity distribution.Comment: Accepted by Astronomical Journal. 10 pages, 7 figures, 1 electronic tabl

Imaging survey of subsystems in secondary components to nearby southern dwarfs

To improve the statistics of hierarchical multiplicity, secondary components of wide nearby binaries with solar-type primaries were surveyed at the SOAR telescope for evaluating the frequency of subsystems. Images of 17 faint secondaries were obtained with the SOAR Adaptive Module that improved the seeing; one new 0.2" binary was detected. For all targets, photometry in the g', i', z' bands is given. Another 46 secondaries were observed by speckle interferometry, resolving 7 close subsystems. Adding literature data, the binarity of 95 secondary components is evaluated. We found that the detection-corrected frequency of secondary subsystems with periods in the well-surveyed range from 10^3 to 10^5 days is 0.21+-0.06, same as the normal frequency of such binaries among solar-type stars, 0.18. This indicates that wide binaries are unlikely to be produced by dynamical evolution of N-body systems, but are rather formed by fragmentation.Comment: Accepted by the Astronomical Journal; 7 pages, 7 figure

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 67pc of the Sun are presented. This distance-limited sample based on the Hipparcos catalog contains 4847 primary stars (targets) with 0.590% 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 ~100yr 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.Comment: Accepted by AJ. 14 pages, 12 figures. Full tables are available at http://www.ctio.noao.edu/~atokovin/papers/multiples-tables.tar.g

Spectroscopic orbits of subsystems in multiple stars. III

Spectroscopic orbits are computed for inner pairs in six nearby hierarchical multiple systems (HIP 35733, 95106/95110, 105441, 105585/105569, 105947, and 109951). Radial velocities and resolved measurements, when available, are used to derive combined sets of outer orbital elements for three systems. Each multiple system is discussed individually. Additionally, HIP 115087 is a simple 7.9 day single-lined binary. Although the minimum companion mass is sub-stellar (in the brown dwarf desert regime), it appears to be a 0.2 solar-mass star in a low-inclination orbit.Comment: Accepted by AJ. 7 pages, 5 figures. Electronic tables are available from the author upon reques

Orbits of four young triple-lined multiple systems

Each of the nearby triple systems HIP 7601, 13498, 23824, and 113597 (HD 10800, 18198, 35877, 217389) consists of solar-type dwarfs with comparable masses, where all three components are resolved spectrally, while the outer pairs are resolved both visually and spectrally. These stars are relatively young (between 100 and 600 Myr) and chromospherically active (X-ray sources), although they rotate slowly. Spectroscopic orbits of the inner subsystems (periods 19.4, 14.1, 5.6, 20.3 days) and orbits of the outer systems (periods 1.75, 51, 27, 500 yrs, respectively) are determined. For HIP 7601 and 13498, the combined spectro-interferometric outer orbits produce direct measurement of masses of all components, allowing comparison with stellar models. The 6708A lithium line is present and its strength is measured in each component individually by subtracting the contributions of other components. The inner and outer orbits of HIP 7601 are nearly circular, likely co-planar, and have a modest period ratio of 1:33. This study contributes to the characterization of hierarchical multiplicity in the solar neighborhood and provides data for testing stellar evolutionary models and chronology.Comment: Accepted by the Astronomical Journal. 16 pages, 14 figures, 10 tabl

Kappa Fornaci, a triple radio-star

Bright and nearby (22pc) solar-type dwarf Kappa Fornaci (HIP 11072) is a triple system. The close pair of M-type dwarfs Ba,Bb with a tentative period of 3.7 days moves around the main component A on a 26-year orbit. The mass of the "dark companion" Ba+Bb is comparable to the mass of A, causing large motion of the photo-center. The combined spectro-interferometric orbit of AB is derived and the relative photometry of the components A and B is given. A weak signature of Ba and Bb is detected in the high-resolution spectra by cross-correlation and by variable emission in the Bahlmer hydrogen lines. The activity of the M-dwarfs, manifested by a previously detected radio-flare, is likely maintained by synchronization with their tight orbit. We discuss the frequency of similar hidden triple systems, methods of their detection, and the implications for multiple-star statistics.Comment: Accepted by AJ. 7 pages, 5 figures, 5 table

Dancing twins: stellar hierarchies that formed sequentially?

This paper attracts attention to the class of resolved triple stars with moderate ratios of inner and outer periods (possibly in a mean motion resonance) and nearly circular, mutually aligned orbits. Moreover, stars in the inner pair are twins with almost identical masses, while the mass sum of the inner pair is comparable to the mass of the outer component. Such systems could be formed either sequentially (inside-out) by disk fragmentation with subsequent accretion and migration or by a cascade hierarchical fragmentation of a rotating cloud. Orbits of the outer and inner subsystems are computed or updated in four such hierarchies: LHS 1070 (GJ 2005, periods 77.6 and 17.25 years), HIP 9497 (80 and 14.4 years), HIP 25240 (1200 and 47.0 years), and HIP 78842 (131 and 10.5 years).Comment: 8 pages, 7 figures, 4 tables. Accepted by Astronomical Journa

Ten years of speckle interferometry at SOAR

Since 2007, close binary and multiple stars are observed by speckle interferometry at the 4.1 m Southern Astrophysical Research (SOAR) telescope. The HRCam instrument, observing strategy and planning, data processing and calibration methods, developed and improved during ten years, are presented here in a concise way. Thousands of binary stars were measured with diffraction-limited resolution (29mas at 540nm wavelength) and a high accuracy reaching 1mas; two hundred new pairs or subsystems were discovered. To date, HRCam has performed over 11000 observations with a high efficiency (up to 300 stars per night). An overview of the main results delivered by this instrument is given.Comment: Accepted by PASP. 12 pages, 11 figure

The Updated Multiple Star Catalog

The catalog of hierarchical stellar systems with three or more components is an update of the original 1997 version of the MSC. For two thousand hierarchies, the new MSC provides distances, component's masses and periods, as well as supplementary information (astrometry, photometry, identifiers, orbits, notes). The MSC content and format are explained, its incompleteness and strong observational selection are stressed. Nevertheless, the MSC can be used for statistical studies and it is a valuable source for planning observations of multiple stars. Rare classes of stellar hierarchies found in the MSC (with 6 or 7 components, extremely eccentric orbits, planar and possibly resonant orbits, hosting planets) are briefly presented. High-order hierarchies have smaller velocity dispersion compared to triples and are often associated with moving groups. The paper concludes by the analysis of the ratio of periods and separations between inner and outer subsystems. In wide hierarchies, the ratio of semimajor axes, estimated statistically, is distributed between 3 and 300, with no evidence of dynamically unstable systems.Comment: Accepted by ApJS. 11 pages, 9 figures, 6 tables. The catalog is available at http://www.ctio.noao.edu/~atokovin/stars