High-Precision VLBI Astrometric Observations of Radio-Emitting Stars for Detection of Extra-Solar Planets

Abstract

The displacement of a radio-emitting star around the barycenter of a possible planetary system can be measured by astrometric very long baseline interferometry (VLBI) observations. We have observed the radio-emitting starσ² CrB at 8 epochs over 5 years by VLBI and fitted its 5 astrometric parameters to the observed coordinates. The post-fit coordinate residuals have an rms scatter of 0.22 milliarcseconds and show no systematic behavior. We use this result to set a limit on the presence of planets aroundσ² CrB and conclude that our present VLBI astrometric precision corresponds to the threshold to detect a Jupiter-like planet around this star. We also discuss the astrometric monitoring program of 11 radio-emitting stars that we are conducting for the Hipparcos space mission and its possible contribution to a long-term planet search program.