Astrometry with the Hubble Space Telescope: A Parallax of the Fundamental Distance Calibrator RR

We present absolute parallaxes and relative proper motions for the central stars of the planetary nebulae NGC6853 (The Dumbbell), NGC 7293 (The Helix), Abell 31, and DeHt 5. This paper details our reduction and analysis using DeHt 5 as an example. We obtain these planetary nebula nuclei (PNNi) parallaxes with astrometric data from Fine Guidance Sensors FGS 1R and FGS 3, white-light interferometers on the Hubble Space Telescope (HST). Proper motions, spectral classifications and VJHKT2M and DDO51 photometry of the stars comprising the astrometric reference frames provide spectrophotometric estimates of reference star absolute parallaxes. Introducing these into our model as observations with error, we determine absolute parallaxes for each PNN. Weighted averaging with previous independent parallax measurements yields an average parallax precision, σπ/π = 5%. Derived distances are: dNGC 6853 = 405 +28 −25pc, dNGC 7293 = 216 +14 −12pc, dAbell 31 = 621 +91 −70pc, and dDeHt 5 = 345 +19 −17pc. These PNNi distances are all smaller than previously derived from spectroscopic analyses of the central stars. To obtain absolute magnitudes from these distances requires estimates of interstellar extinction. We average extinction measurements culled McDonald Observatory, University of Texas, Austin, TX 78712 Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK Department of Astronomy, New Mexico State University, Las Cruces, New Mexico 88003 5 United States Naval Observatory, Flagstaff Station, Flagstaff, AZ 86001 6 Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218 7 Department of Astronomy, University of Virginia, PO Box 3818, Charlottesville, VA 22903 8 Department of Astronomy and Astrophysics, The Pennsylvania State University, University Park, PA 16802