An Attempt to Measure the Annual Parallax or Proper Motion of Four Radio Stars

Abstract

PREVIOUS attempts to measure the annual parallax of intense radio stars^1,2 have indicated that their distances exceed 1/50 parsec. These results were obtained by using an interferometer arrangement to determine the positions of individual stars over a period of many months. Observations of this type are limited by inconstancy of the phase retardations in the transmission lines connected to the aerials of the interferometer. This source of error, which is analogous to the collimation error of a visual transit telescope, may, however, be largely eliminated if observations are made of the relative positions of a number of radio stars. The difference of the right ascensions of two stars of considerable angular separation should vary periodically as the earth goes round its orbit. It can be shown that a determination of the amplitude and phase of this periodic variation leads to a knowledge of the true parallax of each star separately. In addition, if no relative angular motion between a number of stars can be detected, it is unlikely that any of them has a proper motion greater than the accuracy of the observations.