Astrometric Effects of Secular Aberration

One of the main endeavors of fundamental astrometry is to establish a practical realization of a non-rotating, inertial reference frame anchored to celestial objects whose positions are defined in the barycentric coordinates of the solar system matching the current level of astrometric observational accuracy. The development of astrometric facilities operating from space at a microarcsecond level of precision makes the non-uniformity of the galactic motion of the barycenter an observable and non-negligible effect that violates the desired inertiality of the barycentric frame of the solar system. Most of the observable effect is caused by the nearly-constant (secular) acceleration of the barycenter with respect to the center of the Galaxy. The acceleration results in a pattern of secular aberration which is observable astrometrically as a systematic vector field of the apparent proper motions of distant quasars. We employ the classic approximations of planar epicycle and vertical harmonic oscillation for the Sun’s galactic motion to estimate the magnitude of secular acceleration components in the galactic coordinates and show that these approximations are adequate for the SIM space mission. We employ the vector spherical harmonic formalism to describe the predicted field of proper motions and evaluate the amplitude of this field at each point on the celestial sphere. It is shown that the pattern of secular aberration is fully represented by three low-order electric-type vector harmonics, and hence, it is easily distinguishable from the residual rotations of the reference frame and other possible effects, such as the hypothetical long-period gravitational waves, which are described by other types of vector or tensor harmonics. Comprehensive numerical simulations of the grid astrometry with SIM PlanetQuest are conducted assuming that 110 optically bright quasars are included as grid objects and observed on the same schedule