On the motion of the Magellanic Clouds

We have measured the proper motion of the Large and Small Magellanic Clouds using Magellanic Cloud stars in the PPM Catalogue, and obtain $$\mu=1.7\pm 0.9 \ {\rm mas \ yr}^{-1}$$ for the LMC. Systematic uncertainties are discussed. Bound and unbound orbits of the Magellanic Clouds around the Galaxy are consistent with our result. The various models of the Magellanic Stream and their predictions for the motion of the Magellanic Clouds are discussed. The predictions by several authors for the same type of model differ by up to $$0.3 \ {\rm mas \ yr}^{-1}$$. All models proposed to date that assume the Magellanic Clouds to lead the Magellanic Stream predict a proper motion for the LMC of between 1.5 and $$2.0 \ {\rm mas \ yr}^{-1}$$, the smallest value being a prediction for a Galaxy with no halo. Other independent measurements of the proper motion of the LMC are discussed. These lie between 1 and $$1.5 \ {\rm mas \ yr}^{-1}$$. Future astrometry will have to allow measurement of the proper motion of the LMC with an uncertainty no larger than one-tenth of a milliarcsecond per year in order to help to distinguish significantly between models of the halo of the Galaxy. The proper motion of the LMC cannot by itself distinguish between models of the Magellanic Stream.