THE PARALLAX AND PROPER MOTION OF PSR1451-68

MEASUREMENTS of annual parallax give distances for stellar objects which are largely model-independent. Such measurements provide a basis for the calibration of other methods for estimating distances which are applicable to more distant objects. In the case of pulsars, parallax measurements allow estimation of the local interstellar electron density and hence calibration of the distance scale based on pulse dispersion. Here we use data from the Parkes-Tidbinbilla interferometer to determine the parallax and proper motion of PSR1451-68, obtaining a value of 2.2±0.3 milli-arcseconds for the parallax and 41.4±0.9 milliarcseconds per year for the proper motion. When combined with the dispersion measure, the parallax yields a value of 0.019±0.003 cm-3 for the mean density of free electrons along the line of sight to this pulsar. Together with other measurements, this result indicates that the interstellar medium within 500 pc of the Sun is typical of that at greater distances. © 1990 Nature Publishing Group.