SIMBAD references

The effect of frequency-dependent AGN core positions (``core-shifts'') on radio Very Long Baseline Interferometry (VLBI) global astrometry measurements is investigated. The basic equations relating to VLBI astrometry are reviewed, including the effects of source structure. A power-law representation of core-shifts, based on both observations and theoretical considerations of jet conditions, is incorporated. It is shown that, in the presence of core-shifts, phase and group-delay astrometry measurements yield different positions. For a core displacement from the jet base parametrized by Δx(λ)=kλ<sup>β</sup>, group delays measure a ``reduced'' core-shift of (1-β)Δx(λ). For the astrophysically-significant case of β=1, group delays measure no shift at all, giving the position of the jet base. At 8.4GHz an estimated typical offset between phase and group-delay positions of ∼170µas is smaller than the current ∼250µas precision of group-delay positions of the sources used to define the ICRF; however, this effect must be taken into account for future measurements planned with improved accuracy when comparing with optical positions of AGN to be obtained with the GAIA mission.