The Space Motion of the Globular Cluster NGC 6397
Anderson, Jay; Lépine, Sébastien; Rich, R. Michael; Stetson, Peter B.; Hansen, Brad M. S.; Richer, Harvey B.; Shara, Michael M.; Fahlman, Gregory G.; King, Ivan R.; Carraro, Giovanni; Kalirai, Jasonjot S.; Reitzel, David B.; Brewer, James P.; Davis, Saul D.; Hurley, Jarrod R.
United States, Canada, Chile, Australia
Abstract
As a by-product of high-precision, ultradeep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly pointlike. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find μαcosδ=3.56+/-0.04 mas yr-1 and μδ=-17.34+/-0.04 mas yr-1. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way and find that the cluster has made frequent passages through the Galactic disk.
Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with proposal GO-10424.