2008AJ....135.2013C

We present a 3D kinematical analysis of stars located in Kapteyn Selected Area 71 (l = 167.1°, b = -34.7°), where previously a stellar excess was found by Dinescu et al. in (2002). Previous findings indicated that the stellar excess has a cold kinematical signature as inferred from proper motions, and was initially associated with debris from the Sagittarius dwarf galaxy (Sgr)–namely the southern trailing tail. We have obtained radial velocities using the Hydra multi-object spectrograph on the WIYN 3.5 m telescope.
Result. for 183 proper-motion-selected stars indicate that the dominant population in this stellar excess is not debris from Sgr, but rather a population that kinematically belongs to the ring-like stream that is now known as the Monoceros (Mon) stream. The orbit determined for this population agrees very well with the predictions for the Mon stream from Peñarrubia et al. in (2005). The radial-velocity dispersion of this population is between 20 and 30 km/s, lower than that of the Galactic field. Also, the shape of the radial-velocity distribution shows a sharp cut-off on one side, which is more in line with model predictions of the disruption of a satellite rather than with the distribution of the Galactic field. Despite the fact that we now believe most of the stars in the stellar excess to be a part of Mon, about ten stars in this stellar excess have highly negative radial velocities, which is a clear indication of their membership to the Sgr trailing tidal tail.