Improving Galactic Center Astrometry by Reducing the Effects of Geometric Distortion

Abstract

We present significantly improved proper motion measurements of the Milky Way's central stellar cluster. These improvements are made possible by refining our astrometric reference frame with a new geometric optical distortion model for the W. M. Keck II 10 m telescope's Adaptive Optics camera (NIRC2) in its narrow field mode. For the first time, this distortion model is constructed from on-sky measurements, and is made available to the public in the form of FITS files. When applied to widely dithered images, it produces residuals in the separations of stars that are a factor of ∼3 smaller compared to the outcome using previous models. By applying this new model, along with corrections for differential atmospheric refraction, to widely dithered images of SiO masers at the Galactic center, we improve our ability to tie into the precisely measured radio Sgr A*-rest frame. The resulting infrared reference frame is ∼2-3 times more accurate and stable than earlier published efforts. In this reference frame, Sgr A* is localized to within a position of 0.6 mas and a velocity of 0.09 mas yr −1 , or ∼3.4 km s −1 at 8 kpc (1σ). Also, proper motions for members of the central stellar cluster are more accurate, although less precise, due to the limited number of these wide field measurements. These proper motion measurements show that, with respect to Sgr A*, the central stellar cluster has no rotation in the plane of the sky to within 0.3 mas yr −1 arcsec −1 , has no net translational motion with respect to Sgr A* to within 0.1 mas yr −1 , and has net rotation perpendicular to the plane of the sky along the Galactic plane, as has previously been observed. While earlier proper motion studies defined a reference frame by assuming no net motion of the stellar cluster, this approach is fundamentally limited by the cluster's intrinsic dispersion and therefore will not improve with time. We define a reference frame with SiO masers and this reference frame's stability should improve steadily with future measurements of the SiO masers in this region (∝ t 3/2 ). This is essential for achieving the necessary reference frame stability required to detect the effects of general relativity and extended mass on short-period stars at the Galactic center.

An Improved Distance and Mass Estimate for SGR A* From a Multistar Orbit Analysis

“…We tie the NIR measurements of these masers to astrometric and velocity measurements made in the radio to construct an absolute reference frame with Sgr A * at rest at the origin (see Reid et al 2007;Ghez et al 2008;Gillessen et al 2009b;Yelda et al 2010Yelda et al , 2014. Here we repeat the exact procedure detailed in Yelda et al (2014) to construct the absolute reference frame, adding three new epochs of NIR maser observations from 2011 to 2013.…”

High-Precision Radio and Infrared Astrometry of LSPM J1314+1320ab. Ii. Testing Pre-Main-Sequence Models at the Lithium Depletion Boundary With Dynamical Masses

“…StarFinder iteratively solves for both the binary parameters and an image of the point-spread function. We then corrected our derived (x, y) positions using the NIRC2 distortion solution of Yelda et al (2010), which has a pixel scale of 9.952±0.002 mas pixel −1 and a +0°.252±0°. 0.009 offset added to the orientation given in the NIRC2 image headers.…”

Circumstellar discs in Galactic centre clusters: Disc-bearing B-type stars in the Quintuplet and Arches clusters

“…2.3 in Habibi 2014 for details) 2 . During the NACO observations, the influence of optical distortions on the relative astrometric uncertainty is minimised by conducting a small-scale dither pattern to avoid large image shifts, and by using the same observational setup and positioning in each field during each epoch (see also Ghez et al 2008;Yelda et al 2010). …”