Orbits of Globular Clusters in the Outer Galaxy: NGC 7006

We present a proper motion study of the distant globular cluster NGC 7006based on the measurement of 25 photographic plates spanning a 40-year interval.The absolute proper motion determined with respect to extragalactic objects is(-0.96, -1.14) +- (0.35, 0.40) mas/yr. The total space velocity of NGC 7006 ina Galactocentric rest frame is 279 km/s, placing the cluster on one of the mostenergetic orbits (Ra =102 kpc) known to date for clusters within 40-kpc fromthe Galactic center. We compare the orbits of four clusters that haveapocentric radii larger than 80 kpc (NGC 5466, NGC 6934, NGC 7006 and Pal 13)with those of Galactic satellites with well-measured proper motions. Theseclusters have orbits that are highly eccentric and of various inclinations withrespect to the Galactic plane. In contrast, the orbits of the Galacticsatellites are of low to moderate eccentricity and highly inclined. Based onorbit types, chemical abundances and cluster parameters, we discuss theproperties of the hypothetical host systems of the remote globular clusters inthe Searle-Zinn paradigm. It is apparent that clusters such as NGC 5466, NGC6934 and NGC 7006 formed in systems that more likely resemble the Fornax dSph,rather than the Sagittarius dSph. We also discuss plausible causes for thedifference found so far between the orbit type of outer halo clusters and thatof Galactic satellites and for the tentative, yet suggestive phase-spacescatter found among outer halo clusters.Comment: 27 pages, 5 figures, to be published in the Astronomical Journa