I will discuss several tests to gauge the accuracy of pre–main-sequence (PMS) models. Methods to determine the mass of young stars are overviewed, with emphasis on the information provided by double-lined, spectroscopic binary systems. A comparison of the dynamically determined masses with those estimated using the PMS models of Palla & Stahler (1999) is presented. Good agreement between empirical and theoretical masses is found. The analysis of the inferred ages from the isochrones shows a remarkable coevality within each binary system. A complete assessment of the accuracy of PMS tracks needs the identification of eclipsing systems of low-mass.

We present evolutionary models for young low mass stars (m ≤ 1M⊙) based on recent improvement of the theory: equation of state, atmosphere models, etc. We concentrate on early evolutionary phases from the initial deuterium burning phase to the zero-age Main Sequence. Evolutionary models for young brown dwarfs are also presented. We discuss the uncertainties of the present models. We analyse the difficulties arising when comparing models with observations for very young objects, in particular concerning the problem of reddening.

Based on the theory of stellar structure and evolution combined with the theory of stellar atmospheres theoretical properties of young stars can be calculated. These calculations of pre-main sequence evolution have been refined over the last decades and do now provide theoretical spectra and colours even for very cool objects like young stars brown dwarfs and planets. Two of their key assumptions must become invalid towards the formation phases: (1) the hydrostatic equilibrium of pressure forces and gravity that assumes stellar matter to be at rest and (2) the non-dependence on the initial thermal structure. The former (1) is violated by accretion- and collapse flows, the latter (2) because a new born young star is observed with the specific thermal structure produced by the cloud collapse. I discuss changes in the theoretical properties of young stars that follow from calculating the pre-main sequence evolution as the consequence of the collapse of Bonnor-Ebert spheres.

We consider how positions of unresolved pre-main-sequence binaries in the HR diagram compare with the positions of their corresponding primaries and secondaries. As one might expect from simple considerations, the composites appear younger than their components, and have masses intermediate between those of the two stars. However, the amount of the age difference between the composite and its components ranges from nearly zero to as much as an order of magnitude depending upon the specifics of the system such as the reddening and relative spectral types of the objects. We present four examples that illustrate these issues.

We have carried out a thought experiment in which we generate a random pairing of objects drawn from a pre-assumed single star power-law IMF, which we call the fundamental IMF. We show how the mass function of primary stars, secondary stars (if we could resolve them) and the mass function of the total mass of systems differ from the underlying fundamental IMF. We also compare our results with observations and conclude that the fundamental IMF of subsolar mass stars could be steeper than currently believed. In other words: the low-mass turn-over of the observed (“apparent”) IMF could be spurious, if the binary fraction of field stars is close to 100% (perhaps due to invisible companions).

We performed populations synthesis calculations of single stars and binaries and show that binary evolution is extremely important for Galactic astronomy. We review several binary evolution models and conclude that they give quite different results. These differences can be understood from the assumptions related to how mass is transfered in the binary systems. Most important are 1) the fraction of mass that is accreted by the companion star during mass transfer, 2) the amount of specific angular momentum which is carried away with the mass that leaves the binary system.

Among the 50 discovered extrasolar planets orbiting main-sequence solar type stars only three are confirmed to be in a binary star system. However, the majority of stars seem to form in binary or even multiple stellar systems. Standard planet formation scenarios consider the creation of planets or planetary systems only for isolated solitary stars. The presence of a companion creates tidal torques on the protoplanetary disk, which may influence the formation process of planets in disks. In this contribution the consequences of the companion's perturbation on the formation scenario of planets is briefly discussed.

This paper compares the statistical features of the sample of discovered extrasolar planets with those of the secondaries in nearby spectroscopic binaries, in order to enable us to distinguish between the two populations. Based on 32 planet candidates discovered until March 2000, we find that their eccentricity and period distribution are surprisingly similar to those of the binary population, while their mass distribution is remarkably different. The mass distributions definitely support the idea of two distinct populations, suggesting the planet candidates are indeed extrasolar planets. The transition between the two populations probably occurs at 10–30 Jupiter masses. We point out a possible negative correlation between the orbital period of the planets and the metallicity of their parent stars, which holds only for periods less than about 100 days. These short-period systems are characterized by circular or almost circular orbits.

Programs designed to observe gravitational microlensing are poised to provide a great deal of information about binary populations far from Earth, including those in the Galactic Bulge, in the Magellanic Clouds, in M31, and perhaps in other external galaxies. Because many millions of stars are monitored, microlensing observations allow us to study binaries in three ways: (1) when they are “involved” in a microlensing event (as either a lens or lensed source), (2) when variability due to binarity produces significant variations in the light curve, and (3) when light from a more distant star is attenuated or refracted by matter associated with the binary system (e.g., in a disk).

Microlensing observations will discover large numbers of binaries and planetary systems in a variety of galactic environments. Thus, comparative statistical studies of binary properties (distributions of mass ratios and orbital separations) are possible.

An intriguing sign that we have already begun to learn about binaries through microlensing observations comes from work indicating that all of the lenses detected to date may in fact be binaries. For observations along the direction of the Magellanic Clouds this would imply that, if the lenses are primarily located in the Halo, then MACHOs tend to be binaries. If, on the other hand, most of the lenses are located in the Magellanic Clouds, microlensing observations are giving us a unique way to explore a distant stellar population of binaries.

Interferometric observations of binary stars have a profound impact on many areas of stellar astrophysics. This article gives a brief review of interferometric techniques applied to binaries, and of orbit determination and binary surveys with optical and infrared interferometers.

ALMA, the Atacama Large Millimeter / Sub-millimeter Array will be the first instrument allowing very high angular resolution (down to 0.01″) with sufficient sensitivity to image thermal emission from dust and molecules in proto-planetary disks at wavelengths where these disks are optically thin. Its unsurpassed characteristics will make it a premier instrument to study the formation of binary and multiple systems. I present here the projected characteristics of ALMA, in particular the expected sensitivities and frequency coverage, and illustrates some possible applications relevant to the study of binary star formation.

I briefly describe how diffraction-limited imaging with adaptive optics (AO) can detect low mass companions (young massive brown dwarfs for example). I review how current curvature AO systems can already detect point sources 1 million times fainter at separations of 3 arcsec in median seeing (0.65 arcsec). I show real examples of very faint companion detections made with the University of Hawaii AO system located at CFHT on Mauna Kea around the young (2 Myr) nearby (132 pc) Herbig Ae/Be star MWC480. Moreover, I show that the four faint (H=18–19 mag) companions within 6 arcsec of MWC480 (H=7.0 mag) are unlikely to be physical since they are non-common proper motion objects. I point out that the current 8–10m class AO systems will detect even fainter companions at closer separations with 0.03–0.06 arcsec NIR imaging.

Ambitious infrared and astrometric space observatories are planned by both NASA and ESA for the first decade of the new millenium. I present a brief overview of their capabilities and suggest some of their likely contributions to research on young binary stars.

The advantages and disadvantages of two approaches to astrophysical hydrodynamics, Smoothed Particle Hydrodynamics (SPH) and Adaptive Mesh Refinement (AMR), are briefly discussed together with some current problems of computational hydrodynamics.

Detailed radiative transfer calculations are an essential tool for the analysis of observations related to fragmented molecular cloud cores, protobinaries, and circumbinary and circumstellar disks. In this contribution, recent progress in line and continuum radiative transfer models will be reviewed.

Hans Zinnecker has asked me to express my impressions of this conference. My first impression here was this very modern lecture room. It contains no blackboard any more, and instead very modern tools for projecting slides and films. The room also has a supply of fresh air at EVERY seat of this room, so nobody has an excuse any more to fall asleep. When I heard the lectures on Monday, I had the feeling the whole universe was made out of binaries only. This impression reminds me of a story I have heard about Bohr and Pauli: Pauli visited Bohr at his summer cottage one day for a discussion on Quantum Theory. When Pauli approached the cottage, he saw a horse-shoe over the entrance door - a sign of good luck as you may know. Pauli, a very critical person, asked Bohr: “Do you believe in it, and that it will bring luck to you?” Bohr answered: “No, I don't believe in it. But I was told that it helps even if one does not believe in it.” This story shows you the deeply positive way of how physicists think, and may therefore also encourage us to solve our problem.

Let me begin by taking a very broad historical perspective on the study of pre-main-sequence binaries. You have seen reference several times at this meeting to that first paper on T Tauri binaries by Joy & Van Biesbroeck (1944). Following that paper there followed a long period that might be called the Decades of Ignorance. I refer here to a period of time in the study of star formation where we largely ignored the fact that we knew that most stars were binaries. The high frequency of binaries among stars is something that we've known for a very long time, well over a century. Indeed, looking back, it is remarkable that the paper in 1982 by Mel Dyck and his collaborators identifying T Tauri as a binary caused such a stir. This paper is sometimes cited as the birth of modern research into young binaries. But how many of our colleagues in the realm of main-sequence binaries had already told us over and over again that most of these young stars had to be binaries? Of course, the reason this important paper caused much ado was in part because T Tauri is the very prototype star of star-formation study.