Astrophysical validation of Gaia parallaxes

Sammanfattning: The Gaia satellite, to be launched in August 2012, will measure highly accurate absolute parallaxes of hundreds of millions of stars. This is done by comparing parallactic displacement of stars in different parts of the sky. The accuracy of this method highly depends on the stability of the so-called basic angle between the two fields of view of the Gaia instrument, and periodic variations could lead to a global zero-point error in the measured parallaxes. Small variations of the basic angle are closely monitored by on-board instruments, but independent verification methods are also needed. In this project, we use Galactic Cepheid variables as standard candles to compare with the observed parallaxes at a wide range of distances. If there is a parallax zero-point error, the observed parallaxes will not be consistent with a single Period-Luminosity relation. A model is formulated where the complete Galactic Cepheid population is generated and observed in a simulated Gaia mission. Using the observed Cepheids, we then make simultaneous fits to the P-L relation and the parallax zero-point in order to determine whether using Cepheids is a viable zero-point verification method. Our simulations show that Gaia will observe about 9000 Galactic Cepheids, fifteen times the currently known number. Gaia will alone result in large improvements in the accuracy with which the Galactic P-L relation can be determined. Both constants in the relation can be determined with an accuracy of omega a,b < 0.05. We show that using Galactic Cepheids, the parallax zero-point can be determined with an accuracy of omega c = 0.3 myas, with the largest error contribution coming from the uncertainty with which we can determine the extinction. This is very good, but not enough for the most demanding tasks of Gaia. We conclude that the global verification of the parallax zero-point ultimately will depend on a combination of many different methods.