Binary Stars
In the area of binary and multiple stars Gaia's important contribution will be to make controlled statistical studies of multiple star distribution functions.
Resolved binaries: Gaia will be unable to resolve binaries with separations below about 20 milliarcsec. At separations of a few arcsec, even very faint secondaries can be detected. Resolved binaries will be studied in different populations of stars: nearby star-forming regions, open clusters and associations as well as field binaries of different ages. Gaia's high precision parallaxes and proper motions will also allow identifying the very widest binaries (amongst the individually observed stars) and studying their relation to common proper motion pairs and associations.
Gaia's precision astrometry will also be exploited for visual binary orbit and mass determinations of unusual accuracy. Although the number of resolved orbital systems will be modest and only a fraction of these will have short enough periods for the orbits to be well-defined, the absolute numbers of new high-precision masses will be very impressive. Simulations of Gaia observations of thousands of close (20-100 milliarcsec) but resolved binaries have been performed. Analysis of the simulated data shows that the full set of orbital elements can be recovered for periods in the 4-40 year range. From the Gaia observations, more than 10000 masses accurate to 1 per cent can be expected. With these mass determinations detailed checks of the stellar evolution models can be made.
Astrometric binaries: Gaia is extremely sensitive to non-linear proper motions. Large fractions of astrometric binaries with periods in the range 0.03 to 30 years will be immediately recognized by their poor fit to the standard 5 parameter single star astrometric model. While most of these will be unresolved, for many a photocentre orbit will be determined allowing the absolute and relative binary frequency to be established, and investigation into variations with age and place of formation in the Galaxy.
Gaia will detect a majority (approximately 60 per cent) of the estimated 10 million binaries (down to 20 mag) closer than 250 pc to the Sun. This drops to 35 per cent out to 1000 pc. In total about 60 million binaries will be detected. This huge sample can be used to investigate age-groups, variability and possible evolutionary trends of the binary frequency at different orbital periods. For many of the astrometric binary systems low-precision mass-functions will be obtained.
Spectroscopic binary: The spectra collected by Gaia will allow orbit determination for many spectroscopic binaries. A velocity resolution of about 1.5 km per second and about 100 observations per object over the five years of the mission, will allow derivation of the masses of the components to better than 1 per cent.
Eclipsing binaries: The multi-epoch photometry of Gaia will provide a sample of several million eclipsing binary light curves. Absolute stellar radii will be inferred from the colour and absolute magnitudes. The semi-major axis and the mass of the system then follow from the relative radii.
Very few eclipsing binaries are known in critical stellar populations such as globular clusters and old open clusters, or star forming regions. Gaia will discover large numbers of them. The surprising number of new eclipsing and contact binaries discovered by Hipparcos at magnitudes as bright as 7 mag demonstrates that even bright binary systems are not well surveyed. Gaia will make a dramatic impact in this area, with stellar masses and radii measured to better than 1 per cent for thousands of eclipsing binaries.
Contact binaries: Other types of variables while quite common, are poorly understood.
W UMa-type contact binaries make up about 1 per cent of the total number of solar type stars. They must therefore represent a relatively normal and long-lived stage of the evolutions of solar-type stars, yet their origin, evolution and structure is poorly understood.
The known contact binaries show severe incompleteness at V~9 mag - hence Gaia will discover large numbers of them, some perhaps in the critical, short-lived stages leading to their formation. The Gaia sample will be unbiased and will thus allow the spatial distribution and evolutionary state as a population to be studied. This is not possible with current (or foreseen) ground-based samples.
Algol-type binaries are also relatively common but the existing sample is already severely incomplete at V~9 mag. Gaia will identify large numbers of them down to faint magnitudes allowing a study of their evolutionary status.
RS CV type binaries are also quite common, and are prominent in the X-ray sky. Establishing their space density and distribution is essential for understanding their contribution to the Galactic component of the X-ray background.
Interacting binary systems: A rich variety of astrophysical problems related to interacting binary systems will become accessible with parallaxes in the 10 microarcsec range. The evolutionary history of interacting binary systems and the origin of Type I supernovae, millisecond pulsars, low mass X-ray binaries and globular cluster X-ray sources is intimately bound up with the behaviour of compact binaries with mass transfer and mass loss. Accurate knowledge of the stellar masses and orbital separation can be derived from astrometric measurements combined with estimates of the mass function determined by radial velocity measurements. Many specific questions about accretion rates, precursors, mass distributions and kinematic behaviour can be addressed with these data, including studies of black hole candidates.