Yelda, S. and Lu, J. R. and Ghez, A. M. and Clarkson, W. and Anderson, J. and Do, T. and Matthews, K. (2010) Improving galactic center astrometry by reducing the effects of geometric distortion. Astrophysical Journal, 725 (1). pp. 331-352. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20110110-091805738
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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 with 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 (GC), 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 GC.
|Additional Information:||© 2010 American Astronomical Society. Received 2010 September 9; accepted 2010 September 24; published 2010 November 18. We thank the staff of the Keck Observatory, especially Randy Campbell, Al Conrad, Jim Lyke, and Hien Tran for their help in obtaining the observations. We are grateful to Mark Reid for providing the latest VLA measurements of the Galactic center. We also thank Mark Morris, Quinn Konopacky, Marshall Perrin, and Leo Meyer for their helpful suggestions on this work and the manuscript. Support for this work was provided by the NSF grants AST-0406816 and AST-0909218 and the NSF Science & Technology Center for Adaptive Optics, managed by UCSC (AST-9876783). The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors also recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facilities: Keck: II (NIRC2)|
|Subject Keywords:||astrometry – Galaxy: kinematics and dynamics – instrumentation: adaptive optics Online-only material: color figures, machine-readable table|
|Classification Code:||PACS: 98.35.Jk; 98.35.Df; 98.20.Di; 95.85.Kr; 98.20.Gm; 98.70.Dk|
|Official Citation:||S. Yelda et al. 2010 ApJ 725 331 doi: 10.1088/0004-637X/725/1/331|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Benjamin Perez|
|Deposited On:||10 Jan 2011 17:55|
|Last Modified:||26 Dec 2012 12:49|
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