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Session 93 - Binary and Variable Stars.
Oral session, Friday, January 09

[93.03] VLBI Mapping of Candidate Guide Stars and Their Astrometric Reference Sources for the NASA/Stanford Gravity Probe B Mission

R. R. Ransom, N. Bartel, M. F. Bietenholz (York U.), M. I. Ratner, D. E. Lebach, I. I. Shapiro (CfA), J. -F. Lestrade (Obs. de Meudon)

The Gravity Probe B (GP-B) mission is being developed by NASA and Stanford University to test two unverified predictions of general relativity through measurements of the precessions of gyroscopes placed in low-Earth orbit. For the measurement-accuracy goals of the mission to be met, the proper motion of a ``guide-star,'' whose position provides a reference against which the precessions are measured, must be determined in an extragalactic reference frame with a standard error much less than 0.5 mas/yr. Very-long-baseline interferometry (VLBI) is the only technique currently available that is capable of measuring, in a few years time, the proper motions of stars at this level of accuracy. During the past several years we have made VLBI observations of three guide-star candidates, together with extragalactic reference sources nearby on the sky to each. To reduce the uncertainty in the VLBI-determined positions of the stars, we mapped the brightness distributions of each star and reference source. We present maps for GP-B guide-star candidate HR 5110 and its reference sources J1328+363, J1334+371 and J1340+379, candidate HR 1099 and its reference sources J0331+022 and J0339-017 (CTA 26), and candidate HR 8703 and its reference sources J2250+194 and J2253+161 (3C454.3). We found significant source structure in HR 5110 and HR 1099, but only very marginal structure in HR 8703. Since the VLA-monitored flux-density revealed intra-day variability in the candidate guide stars, we also computed ``snap-shot'' maps and models from temporal subsets of the stellar VLBI observations to both check the reliability of the inferred source structure and also look for intra-day variations in that source structure. We discuss the effects that the complex emission structure found in some of the stars and reference sources could have on the accuracy of proper-motion determinations for the stars and indicate how we can partially correct for such effects.

Program listing for Friday