AAS 196th Meeting, June 2000
Session 53. Surveys, Catalogs, Database
Display, Thursday, June 8, 2000, 9:20am-4:00pm, Empire Hall South

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[53.08] Interferometric Astrometry with HST Fine Guidance Sensor 3: Parallaxes of the Distance Scale Calibrators \delta Cep and RR Lyr

G.F. Benedict, B.E. McArthur (McDonald Obs., U. Texas), L.W. Fredrick (Astronomy, U. Virginia), J. Lee, Wm. van Altena (Astronomy, Yale U.), W.H. Jefferys (Astronomy, U. Texas), E. Nelan (STScI), D. Story (Jackson & Tull), R.L. Duncombe (Aerospace Eng., U. Texas), P.J. Shelus (McDonald Obs., U. Texas), P.D. Hemenway (U. Rhode Island), O.G. Franz, L.H. Wasserman (Lowell Obs.), A.L. Whipple, A. Bradley (Allied-Signal Aerospace)

We report absolute parallaxes of the fundamental distance scale calibrators \delta Cephei and RR Lyrae. These were derived using relative astrometry from HST Fine Guidance Sensor 3, and included seven and nine sets ( HST orbits) of data, respectively. Observation epochs spanned two and a half and two years for these objects. Challenges included independent calibration of the FGS 3 neutral density filter, FND5, required to reduce the target flux to a safe level. Spectral classification (including luminosity class) of the stars used in the astrometric reference frames comprised our second challenge, met only recently with spectra from the multi-object spectrograph, Hydra, on the WIYN telescope. These spectra provide the means to transform parallaxes, relative to our reference stars, into absolute parallaxes. This transformation represents the dominant source of error for these very distant objects.

We compare our absolute parallaxes with those from HIPPARCOS (1997) and Gatewood (1993).

This work was supported by NASA HST GTO Grant NAG5-1603, patiently administered through the Goddard Spaceflight Center.

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