Secular aberration drift in stellar proper motions

Context. The motion of the Solar System barycenter (SSB), the spatial origin of the International Celestial Reference System, causes a directional displacement known as secular aberration. The secular aberration drift caused by the galactocentric acceleration of the SSB has been modeled in the third generation of the International Celestial Reference Frame. Aims. We aim to address another secular aberration drift effect due to the change in the line-of-sight direction and study its implications for stellar proper motions. Methods. We derived a complete formula for the secular aberration drift and computed its influence on stellar proper motion based on the astrometric data in Gaia Data Release 3. Results. We find that the secular aberration drift due to the change in the line-of-sight direction tends to decrease the observed proper motions for stars with galactic longitudes between 0° and 180°, and increase the observed proper motion for stars in the remaining region. If this secular aberration drift effect is ignored, it will induce an additional proper motion of > 1 mas yr −1 for 84 stars and > 0.02 mas yr −1 for 5 944 879 stars, which is comparable to or several times greater than the typical formal uncertainty of the Gaia proper motion measurements at G < 13. Conclusions. The secular aberration drift due to the change in the line-of-sight direction and the acceleration of the SSB should be modeled to make the stellar reference frame consistent with the extragalactic reference frame.