We present the fourth installment of the Yale/San Juan Southern Proper Motion Catalog, SPM4. The SPM4 contains absolute proper motions, celestial coordinates, and (B,V) photometry for over 103 million stars and galaxies between the south celestial po
le and -20 deg declination. The catalog is roughly complete to V=17.5 and is based on photographic and CCD observations taken with the Yale Southern Observatorys double-astrograph at Cesco Observatory in El Leoncito, Argentina. The proper-motion precision, for well-measured stars, is estimated to be 2 to 3 mas/yr, depending on the type of second-epoch material. At the bright end, proper motions are on the International Celestial Reference System by way of Hipparcos Catalog stars, while the faint end is anchored to the inertial system using external galaxies. Systematic uncertainties in the absolute proper motions are on the order of 1 mas/yr.
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 (Dinescu et al. 2002, Newberg et al. 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 multiobject spectrograph on the WIYN 3.5 m telescope. Results 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 stream. The orbit determined for this population agrees very well with the predictions for the Monoceros stream from Penarrubia et al. (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 part of Monoceros, 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.
