DETECTION OF A STELLAR STREAM BEHIND OPEN CLUSTER NGC 188: ANOTHER PART OF THE MONOCEROS STREAM

摘要

We present results from a WIYN/Orthogonal Parallel Transfer Imaging Camera photometric and astrometric surveyof the field of the open cluster NGC 188 ((l, b) = (122.8, 22.5)). We combine these results with the proper-motionand photometry catalog of Platais et al. and demonstrate the existence of a stellar overdensity in the background ofNGC 188. The theoretical isochrone fits to the color–magnitude diagram of the overdensity are consistent with anage between 6 and 10 Gyr and an intermediately metal poor population (Fe/H=-0.5 to -1.0). The distance tothe overdensity is estimated to be between 10.0 and 12.6 kpc. The proper motions indicate that the stellar populationof the overdensity is kinematically cold. The distance estimate and the absolute proper motion of the overdensityagree reasonably well with the predictions of the Penarrubia et al. model of the formation of the Monoceros stream.Orbits for this material constructed with plausible radial-velocity values, indicate that dynamically, this material isunlikely to belong to the thick disk. Taken together, this evidence suggests that the newly found overdensity is partof the Monoceros stream.