IMPROVING GALACTIC CENTER ASTROMETRY BY REDUCING THE EFFECTS OF GEOMETRIC DISTORTION

Abstract

We present significantly improved proper motion measurements of the Milky\nWay's central stellar cluster. These improvements are made possible by refining\nour astrometric reference frame with a new geometric optical distortion model\nfor the W. M. Keck II 10 m telescope's Adaptive Optics camera (NIRC2) in its\nnarrow field mode. For the first time, this distortion model is constructed\nfrom on-sky measurements, and is made available to the public. When applied to\nwidely dithered images, it produces residuals in the separations of stars that\nare a factor of ~3 smaller compared to the outcome using previous models. By\napplying this new model, along with corrections for differential atmospheric\nrefraction, to widely dithered images of SiO masers at the Galactic center, we\nimprove our ability to tie into the precisely measured radio Sgr A*-rest frame.\nThe resulting infrared reference frame is ~2-3 times more accurate and stable\nthan earlier published efforts. In this reference frame, Sgr A* is localized to\nwithin a position of 0.6 mas and a velocity of 0.09 mas/yr, or ~3.4 km/s at 8\nkpc (1 sigma). While earlier proper motion studies defined a reference frame by\nassuming no net motion of the stellar cluster, this approach is fundamentally\nlimited by the cluster's intrinsic dispersion and therefore will not improve\nwith time. We define a reference frame with SiO masers and this reference\nframe's stability should improve steadily with future measurements of the SiO\nmasers in this region. This is essential for achieving the necessary reference\nframe stability required to detect the effects of general relativity and\nextended mass on short-period stars at the Galactic center.\n