2010AJ....140.1934V

Absolute proper motions are determined for stars and galaxies to V = 17.5 over a 450 deg² area that encloses both Magellanic Clouds. The proper motions are based on photographic and CCD observations of the Yale/San Juan Southern Proper Motion program, which span a baseline of 40 years. Multiple, local relative proper-motion measures are combined in an overlap solution using photometrically selected Galactic disk stars to define a global relative system that is then transformed to absolute using external galaxies and Hipparcos stars to tie into the ICRS. The resulting catalog of 1.4 million objects is used to derive the mean absolute proper motions of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC); (µ_α cos δ, µ_δ)_LMC= (1.89, + 0.39)±(0.27, 0.27) mas/yr and (µ_αcos δ, µ_δ)_SMC= (0.98, - 1.01)±(0.30, 0.29) mas/yr. These mean motions are based on best-measured samples of 3822 LMC stars and 964 SMC stars. A dominant portion (0.25 mas/yr) of the formal errors is due to the estimated uncertainty in the inertial system of the Hipparcos Catalog stars used to anchor the bright end of our proper motion measures. A more precise determination can be made for the proper motion of the SMC relative to the LMC; (µ_αcos δ_, µ_δ)_SMC–LMC= (-0.91, - 1.49)±(0.16, 0.15) mas/yr. This differential value is combined with measurements of the proper motion of the LMC taken from the literature to produce new absolute proper-motion determinations for the SMC, as well as an estimate of the total velocity difference of the two clouds to within ±54 km/s. The absolute proper-motion results are consistent with the Clouds' orbits being marginally bound to the Milky Way, albeit on an elongated orbit. The inferred relative velocity between the Clouds places them near their binding energy limit and, thus, no definitive conclusion can be made as to whether or not the Clouds are bound to one another.