The HIPPARCOS proper motion of the Magellanic Clouds

The proper motion of the Large (LMC) and Small (SMC) Magellanic Cloud using data acquired with the Hipparcos satellite is presented. Hipparcos measured 36 stars in the LMC and 11 stars in the SMC. A correctly weighted mean of the data yields the presently available most accurate values, μ_αcos( δ) = 1.94±0.29 mas/yr, μ_δ = -0.14±0.36 mas/yr for the LMC. For the SMC, μ_αcos( δ) = 1.23±0.84 mas/yr, μ_δ = -1.21±0.75 mas/yr is obtained, whereby care is taken to exclude likely tidal motions induced by the LMC. Both galaxies are moving approximately parallel to each other on the sky, with the Magellanic Stream trailing behind. The Hipparcos proper motions are in agreement with previous measurements using PPM catalogue data by Kroupa et al. (1994) [MNRAS, 266, 412] and by Jones et al. (1994) [AJ, 107, 1333] using background galaxies in a far-outlying field of the LMC. For the LMC the Hipparcos data suggest a weak rotation signal in a clockwise direction on the sky. Comparison of the Hipparcos proper motion with the proper motion of the field used by Jones et al. (1994) [AJ, 107, 1333], which is about 7.3 kpc distant from the center of the LMC, also suggeests clockwise rotation. Combining the three independent measurments of the proper motion of the LMC and the two independent measurements of the proper motion of the SMC improves the estimate of the proper motion of the LMC and SMC. The corresponding galactocentric space motion vectors are computed. Within the uncertainties, the LMC and SMC are found to be on parallel trajectories. Recent theoretical work concerning the origin of the Magellanic System is briefly reviewed, but a unique model of the Magellanic Stream, for the origin of the Magellanic Clouds, and for the mass distribution in the Galaxy cannot yet be decided upon. Future astrometric space missions are necessary to significantly improve our present knowledge of the space motion of the two most conspicous galactic neighbours of the Milky Way.