Astrometric Effects of Secular Aberration

One of the main endeavors of fundamental astrometry is to establish apractical realization of an inertial reference frame anchored to celestialobjects whose positions are defined in the barycentric coordinates of the solarsystem. The development of astrometric facilities operating from space at amicroarcsecond level of precision makes the non-uniformity of the galacticmotion of the barycenter an observable effect that violates the inertiality ofthe barycentric frame. Most of the observable effect is caused by secularacceleration of the barycenter with respect to the center of the Galaxy. Theacceleration results in a pattern of secular aberration which is observableastrometrically as a systematic vector field of the proper motions of distantquasars. We employ the vector spherical harmonics to describe the predictedfield of the proper motions and evaluate its amplitude at each point on thecelestial sphere. It is shown that the pattern of secular aberration is fullyrepresented by three low-order electric-type vector harmonics, and hence, it iseasily distinguishable from the residual rotations of the reference frame andother possible effects, such as the hypothetical long-period gravitationalwaves. Comprehensive numerical simulations of the grid astrometry with SIMPlanetQuest are conducted. The full covariance matrix of the simulated gridsolution is used to evaluate the covariances of the three electric harmoniccoefficients, representing the secular aberration pattern of proper motions. Weconclude that the grid astrometry with SIM PlanetQuest will be sensitive to themain galactocentric component of secular acceleration, while the peculiaracceleration of the Sun with respect to LSR is expected to be too small to bedetected with this astrometric space interferometer.Comment: 21 pages, 4 figure