Scientific impact of interferometric and space astrometry

Abstract

Four types of astrometry are distinguished in this paper, depending upon the field ϕ where the measurements are made. For each of them, the expectations for astronomic results due to the advent of interferometry and space techniques are discussed. In the case of very narrow field astrometry (ϕ < 5”)speckle and Michelson interferometry in visual and infrared light will permit an increase by two or three orders of magnitude on the precision of double star and diameter measurements. In narrow field astrometry ( ϕ < 0°,5) space telescope, mask detection systems and Michelson interferometry, the main results will be accurate’ relative parallaxes of selected objects and a search for invisible companions of stars.For large field astrometry (ø < 5°), photographic techniques will be still used. Finally, while Earth based semi-global astrometry will increase the number of observed objects, the breakthrough will come from space astrometry which will permit to get 0"001 precision for parallaxes and annual proper notions.The scientific expectations for stellar and galactic astronomy, astrophysics, stellar and galactic evolution, etc… are sketched, in particular for more than 100 000 stars parallaxes with an overall precision of one to two milliseconds of arc. This should provide:- a remarkable basis for the determination of the masses about 300 double stars,- the calibration of luminosities for most main sequence stars, type III giants and population II horizontal branch,- a new cosmic distance scale, with a direct trigonometric calibration of new categories of stars that are used as distance indicators,- Using the extended distance scale and the proper motion that will be obtained for all the stars in the program, it will be possible, for the first time, to analyse the variations of the kinematic properties within a sphere of about 1000 parsecs, providing data for the analysis of the galactic gravitational potential and the kinematics of various types of stars.- In connection with VLBI and narrow field astrometry using interferometers, the catalogue will provide an accessible frame of reference for all studies where an absolute system of reference is needed.