Radial Velocity Measurements with Objective Prisms for the Hipparcos Program

Abstract

In 1986, the European Space Agency (ESA) will launch an astrometrie satellite, Hipparcos (see ESA SP-177) with the objective of measuring positions for 200,000 stars. Proper motions will be determined with an accuracy of 0.002 "/yr. It is also very important to have radial velocities for the program stars.We will show here that an accuracy of 4 to 5 km/s for the radial velocity is good enough for a number of studies. For stellar studies, we need highly accurate radial velocities, and correlation methods (Griffin, Mayor et al.) allow that but are restricted to cool stars. Radial velocities are also needed for studies of stellar dynamics. In the case of star clusters (open and globular) we need a high accuracy, however the study of motions of stars in our galaxy do not require such a high accuracy. Only by increasing the sample can we improve our knowledge. In Table I we give data for the velocity ellipsoid. Kinematical data are from Allen and we have added absolute magnitudes and distances for stars having mv = 7.5. From an examination of this table we see that an accuracy of 4-5 km/s is good enough to study B star motions. The last column gives the dispersion in proper motion, and some of the values are astonishing, but it is well known that the proper motions for the nearby stars show a very high dispersion.