Trigonometric parallaxes of Planetary Nebulae

Abstract

The accuracy of parallaxes obtained with ground-based CCD cameras now often reaches or exceeds 0.5 milliarcseconds. This improved accuracy permits us to measure significant parallaxes and determine distances for the nearest planetary nebulae. At present, the central stars of 11 planetary nebulae have been observed as part of the USNO parallax program. We now have determined distances with accuracies better than 20 percent for four central stars and better than 50 percent for five more. This paper gives the present status of the program, a brief interpretation of the results, and future prospects.