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IAU Symposium No. 240

Binary Stars as Critical Tools & Tests
in Contemporary Astrophysics

22-25 August 2006
Prague, Czech Republic

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International Astronomical Union

26th IAU General Assembly

Czech Republic

The narrative from the original symposium proposal follows, outlining the reasoning behind the proposal and the research topics which will be covered during the meeting.

IAU Colloquium 135, "Complementary Approaches to Double and Multiple Star Research", was held at Callaway Gardens (near Atlanta, Georgia) in 1992, and was meant to emphasize the expanding overlap of observational opportunities offered to binary star researchers by advances in precise radial velocity techniques, interferometry, etc. Additional topics included recent advances in our knowledge of duplicity for young stars and pre-main sequence stars, the latest theories of binary and multiple star formation, and the tantalizing first results from HST and Hipparcos. The meeting successfully brought together nearly one hundred astronomers with a diversity of expertise until then rarely found at a single meeting on binary stars.

Much has changed in the ensuing 13 years! HST is now approaching the end of its life, and its successor is under construction. Hipparcos results have been published and well-studied, and the next generation of astrometry satellites now under development promise results orders of magnitude greater in both precision and quantity. Speckle interferometry and adaptive optics have replaced visual micrometry as the routine methods of measuring visual binaries, while long-baseline interferometry has now produced a significant body of results for closer binaries with the promise of an outburst of activity in the resolution of spectroscopic binary systems during the next few years. Interferometric results from the HST FGS have also become plentiful. All of these represent dramatic changes in the way we conduct astrometry of binaries.

Similar tremendous advances continue in other areas as well. Variability-induced motion and other color-based detection methods are being used to mine the SDSS database for new binaries, and may be used with GAIA and SIM data as well. Superior infrared detectors and techniques, as well as the recent availability of infrared spectroscopy and imaging from space with the Spitzer Space Telescope (SST), are revolutionizing the study of pre-main sequence binaries and binaries with cool star and brown dwarf components. Ever more accurate radial velocity techniques have yielded long-period orbits that further blur the distinction between the traditional spectroscopic and visual separation/period regimes and yield accurate stellar masses across the spectral range - not to mention the new short-period systems and 120+ exoplanets discovered over the last decade! Within the next several years hundreds of additional exoplanets (with diameters as small as the Earth) are expected to be discovered by the COROT and Kepler missions.

Binary systems are now observable in increasing numbers in other galaxies within our Local Group. For example, several thousand new eclipsing systems have been already discovered in the Magellanic Clouds by the MACHO and OGLE programs alone, while additional systems are being discovered in M31 and M33. Within the next decade hundreds of thousands new eclipsing systems are expected to be discovered from automated wide field search programs. Orbital inclinations and therefore accurate masses will soon be within our grasp even for non- eclipsing spectroscopic and visual binaries beyond our Milky Way, providing an unprecedented opportunity to obtain by far the best determinations of the distances to nearby galaxies. Observations of these stars (in our Galaxy and in exterior galaxies with large ranges in metal abundances) will significantly improve our understanding of stellar atmospheres (effects such as gravity darkening, etc.) by allowing us to compare the spectral appearance of rapidly-rotating stars seen equator-on with those of equal-mass binaries seen more pole- on.

Studies of binary and multiple star systems are being strongly stimulated by high-precision (milli- and micro-magnitude) photometric observations from MOST and soon from COROT and KEPLER. There are corresponding exciting developments in theory and in modern methods such as Doppler tomography and synthetic spectrum libraries that, for example, are capable of disentangling individual component spectra even for binaries and multiple systems with heavily blended spectral lines. Another exciting area is the rapidly developing investigation of pulsating stars in binary systems. Ultimately, asteroseismology as well as apsidal motion studies could help to test and improve the theory of stellar structure and evolution, again contributing to the improvement of the methodology over a wide range of astrophysical applications.

Computer modelling is now possible with detail undreamed of a decade ago, giving us insights into the formation of binary and multiple star systems within condensing molecular clouds or the flow of material between components of an interacting binary. These mass-exchanging binaries are vital tools to study and improve the theory of accretion disks, again with far-reaching applications to disks on galactic scales (such as AGN) and on planetary scales (proto-planetary disks).

The study of wide binaries and multiple star systems have not received much attention since a colloquium held in Brussels in 1987. Yet, research in this area has seen a similar revolution. For example, high- precision positions and proper motions from astrometry projects such as Tycho-2, 2MASS, Super COSMOS, and UCAC are yielding many new faint common-proper motion pairs, as well as astrometric binaries. Binaries (both wide and close) are also being searched for in clusters and associations, even in other galaxies. In the past several years wide pairs have been used as probes of our Galaxy's dark matter content, in calibrating the chromospheric activity vs. age relation for late type main sequence stars, in determining the white-dwarf and main-sequence luminosity functions, in setting constraints on post-main sequence mass loss (via orbital expansion), and in assessing the intrinsic completeness of new deep surveys. Wide binaries, especially if they have different spectral types, can also be used to test the assumption of identical initial abundances for binaries and confront theoretical evolutionary tracks and models.

Not even envisioned a decade ago were the variety and ubiquity of substellar companions, including L- and T-dwarfs, brown dwarf pairs, and exoplanets. Following initial discoveries of these objects in the mid-1990's, an explosion of advancements has occurred in these fields. We now have two new classes of spectral types, along with evidence that the frequencies of substellar versus stellar companions are profoundly different. There are also strong suspicions that the typical exoplanet has an orbital and cosmogonical history different than that of our own solar system planets. Both the techniques for discovery and theoretical origins for these new classes of stellar systems overlap with the interests of the traditional binary and multiple star community.

We therefore proposed a follow-up to the "Complementary Approaches" colloquium, this time expanded into a symposium to highlight the many advances in our field mentioned above and broaden the cross fertilization theme of the first meeting by including members of both the "wide" and "close" binary communities. It is appropriate that such a meeting be held in the Czech Republic at the General Assembly, since much of the pioneering work on binary and variable stars has been carried out in Central and Eastern Europe for over a century.

As this proposal was a merger of two similar efforts, the Scientific Organizing Committee (SOC) is co-chaired by Prof. Edward F. Guinan (Villanova University, USA), Dr. Petr Harmanec (Charles University, Prague, Czech Republic), and Dr. William I Hartkopf (U.S. Naval Observatory, USA).

The format for the symposium is a mix of invited oral review presentations(~30 min) and more narrowly focused topical (~15- 20 min) presentations. There will also be contributed posters, with some dissertation-related talks by advanced graduate students and plenty of time for discussion. It is expected that this comprehensive symposium should attract a large number of astronomers attending the General Assembly. The topics covered cut across a wide range of interests and we expect about 250 - 300 to attend and participate.