On the Absolute Proper Motion of NGC 6397

Astronomy and Astrophysics – Astronomy

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Scientific paper

The absolute proper motion of the globular cluster NGC 6397 has previously been determined relative to field stars (Cudworth & Hanson 1993, AJ, 105, 68) and by HST measurements relative to galaxies (Milone et al. 2006, A&A 456, 517; Kalirai et al. 2007, ApJ, 657, L93). However, none of these determinations are in agreement at the 1-σ level. We have applied the method of Cudworth & Hanson to a new reduction of the same plate material with no significant change in the derived motion. We have also determined the cluster motion from a weighted average of 20 cluster members in the Tycho-2 Catalog; this determination is in agreement with the Cudworth & Hanson method. A similar determination using cluster members in the PPMX catalog is in progress and the result will be presented at the meeting. We are also exploring the usefulness of a similar approach using UCAC3 proper motions. We have found evidence for systematic errors in the proper motions from the HST studies. We have also identified a plausible source for systematic errors in the Cudworth & Hanson method when applied to NGC 6397, but we note that the only evidence that this plausible error is present is external, namely that the result is in conflict with the HST results. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF. This research has made use of the VizieR catalogue access tool, CDS, Strasbourg, France. This research has been partially supported by the NSF.

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