A White Dwarf Discovery Machine Stefan Jordan

Gaia is a satellite mission of the ESA, aiming at absolute astro-metric measurements of about one billion stars (V < 20) with unprecedented accuracy. Additionally, magnitudes and colors will be obtained for all these stars, while radial-velocities will be determined only for bright objects (V < 17.5). However, the wavelength range for the radial-velocity instrument is rather unsuitable for most white dwarfs. Gaia will probably discover about 400,000 white dwarfs; up to 100 pc the detection probability for white dwarfs is almost 100 %. This survey of white dwarfs will have very clear, easy to understand selection criteria , and will therefore be very suitable for statistical investigations. The Gaia data will help to improve the construction of a luminosity function for the disk and the halo and will provide a more accurate determination of the age of our solar neighborhood. Moreover, reliable stellar dynamical investigations of the disk and halo components will be possible. For the first time it will be possible to test the mass-radius relation of white dwarfs in great detail. Moreover, more accurate masses of magnetic and cool white dwarfs can be expected. Gaia is also expected to discover many new pulsating white dwarfs. The Gaia measurements can also complement the measurements of gravitational waves from close white dwarf binaries with Lisa. The orbit of the Gaia mission has been chosen to be a controlled Lissajous orbit around the Lagrangian point L2 of the Sun-Earth system in order to have a quiet environment for the payload in terms of mechanical and thermo-mechanical stability. Another advantage of this position is the possibility of uninterrupted observations, since the Earth, Moon and Sun all lay within Gaia's orbit. The aim of Gaia is to perform absolute astrometry rather than differential measurements in a small field of view. For this reason, Gaia – like Hipparcos – (i) simultaneously observes in two fields of view (FoVs) separated by a large basic angle of 106.5 • , (ii) roughly scans along a great circle leading to strong mathematical closure conditions, (iii) performing mainly one-dimensional measurements, and (iv) scanning the same area of sky many times during the mission under varying orientations. These conditions are fulfilled by Gaia's nominal scanning law (see Fig. 1): The satellite will spin around its axis with a constant rotational period of 6 hours. The spin axis will precess around the solar direction with a fixed aspect angle …