AAS 203rd Meeting, January 2004
Session 3 Space Missions: Planet Finding, Astrobiology and Others
Poster, Monday, January 5, 2004, 9:20am-6:30pm, Grand Hall

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[3.15] SIM preparatory science: stability of optical centroids of extragalactic reference frame link sources

N. Zacharias, C. Dahn, D. Boboltz, A.L. Fey, R. A. Gaume, K.J. Johnston (USNO)

In December 2001 a program started at the 1.55m Strand Telescope at the US Naval Observatory Flagstaff Station to monitor a dozen optical counterparts of extragalactic reference frame sources. First results for 0241+622 and 0552+398 are presented based on 13 and 18 CCD frames, respectively. The sources are handled as "parallax stars" and positions on the few mas level are obtained for a variety of candidates (low and high redshift, 14 to 18 mag bright, QSO, BL-Lac, ICRF defining, ICRF candidate and sources with and without radio structure) as representative sample.

The goal is to verify the optical position stability over time. The currently best optical positions are on the 30 to 50 mas level for about 400 sources. Several "outliers" were detected and a larger radio-optical position offset (by about 10 mas on average) is claimed for sources with radio structure (sub-mas level) as compared to no radio structure. Our investigation will assist in selecting the best ICRF candidates for use by the Space Interferometry Mission (SIM) in defining the radio/optical link. SIM will provide optical positions on the micro-arcsec level but will be affected by structure on the 10 to 100 mas level (i.e. on angular scales larger that the fringe spacing of the SIM interferometer).

The author(s) of this abstract have provided an email address for comments about the abstract: nz@usno.navy.mil

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.