1. Observing binaries with GAIA: large gains after much effort... Staffan Söderhjelm Lund Observatory

2. Pros and cons... Millions of binaries of all kinds Complete statistics near sun Partial statistics over large parts of the Galaxy Orbits and masses Increased size of sampling windows Greatly complicated reductions Orbital p.m. bias in the galactic dynamics Photometric bias from unresolved binaries + —

3. Most stars are binary/multiple Most stars look single a-distribution is very wide (10 -2 – 10 5 a.u.) q-distribution peaks at low q (Msec/Mpr << 1) evolution makes a lot of wd:s because Observationally, a ‘typical’ star has a companion, but with dm > 5-10. It will be easily discovered by GAIA if the period is less than 30 years, but not if it is above 100 years.

4. Census of binaries Huge numbers (10 7-8 resolved, 10 7 astrometric, 10 6-7 eclipsing, 10 6 spectroscopic), but still most undetected... Non-uniform sampling at different distances (resolved/unresolved, magnitude-effects) Possibilities to check statistics in different parts of the Galaxy, not only solar neighborhood

5. Orbit determinations (1-10 yr) resolved bin => indiv. masses astrometric bin => m 2 3 /(m 1 +m 2 ) 2 (1-β/μ) 3 (incl. extrasolar planets and BD, β=0) eclipsing bin => low-prec masses and radii SB1+astrometric => m 2 3 /(m 1 +m 2 ) 2, (β/μ) SB1+ecl => m 2 3 /(m 1 +m 2 ) 2, β SB2+ecl => masses and radii

6. Common problem: determination of periods few epochs, aliasing astrometric signal very non-explicit (RV and/or photometry simpler) too many objects for ‘manual’ intervention

7. ‘Design’ problems for resolved binaries Binaries need larger sampling windows => Conflict between ‘more faint singles’ or ‘better data for secondaries’ ‘Optimal’ strategy needs to be simple but not simplistic... Realistic Galaxy models needed

8. ‘Design’ problems for unresolved binaries Systematic astrometry errors due to mean color PSF instead of correct individual ones? MBP filter optimization needs to consider binaries RV/MBP pixels much larger => worse overlap Influence of variability (‘VIM’-doubles) Realistic Galaxy models needed

9. Some critical issues 2002-2004 Effects of the GAIA-2 design! Detection algortithm Sampling windows/obs strategy Spatial resolution for MBP/Spectro Binarity effects in the MBP filter selections SB-observing with different Spectro designs Galaxy model with realistic doubles Binaries/multiples in reduction prototype Database principles (what is a multiple system?)

10. Development of reduction methods Resolved doubles (how to get a priori positions?) Period-finding (for resolved, astrometric, eclipsing binaries) Resolved binary orbits (mass-determination) Astrometric binary orbits (with or without RV) Extrasolar planet orbits Eclipsing binaries/planetary transits Plus a never-ending list of ’special cases’...

11. Scientific issues (guide to priorities) Binary statistics (age/place resolved) Binary origin and evolution Individual stellar parameters Extrasolar planets (masses and orbits) +....