What will Gaia tell us about the Galactic disk ?

Gaia will provide parallaxes and proper motions with accuracy ranging from 10 to 1000 microarcsecond on up to one billion stars. Most of these will be disk stars: for an unreddened K giant at 6 kpc, it will measure the distance accurate to 2% and the transverse velocity to an accuracy of about 1 km/s. Gaia will observe tracers of Galactic structure, kinematics, star formation and chemical evolution across the whole HR diagram, including Cepheids, RR Lyrae, white dwarfs, F dwarfs and HB stars. Onboard low resolution spectrophotometry will permit – in addition to an effective temperature estimate – dwarf/giant discrimination, metallicity measurement and extinction determination. For the first time, then, Gaia will provide us with a three-dimensional spatial/properties map and at least a two-dimensional velocity map of these tracers. (3D velocities will be obtained for the brighter sources from the onboard RV spectrograph). This will be a goldmine of information from which to learn about the origin and evolution of the Galactic disk. I briefly review the Gaia mission, and then show how the expected astrometric accuracies translate into distance and velocity accuracies and statistics. I then briefly examine the impact Gaia should have on a few scientific areas relevant to the Galactic disk, specifically disk structure and formation, the age–metallicity–velocity relation, the mass– luminosity relation, stellar clusters and spiral structure. Concerning spiral arms, I note how a better determination of their locations and pattern speed from their OB star population, plus a better reconstruction of the Sun's orbit over the past billion years (from integration through the Gaia-measured gravitational potential) will allow us to assess the possible role of spiral arm crossings in ice ages and mass extinctions on the Earth. 1. Gaia overview Gaia is a high-accuracy astrometric satellite to be launched by ESA at the end of 2011. By measuring the positions of stars tens of times over a five year baseline, it can derive the mean position, the parallax and two-dimensional proper motion of each star. These are five components of the six dimensional phase space, the sixth component – the radial velocity – being provided for the brighter stars by an onboard spectrograph. Gaia delivers absolute parallaxes tied to an inertial (quasar–based) reference frame. Gaia, which is currently under construction, represents a major step beyond the enormously successful Hipparcos mission, and is currently the only large-scale astrometric mission beyond the planning stage. Gaia will …