The Proper Motion of the Magellanic Clouds: The UCAC2-Hipparcos Inconsistency

Using the USNO CCD Astrograph all-sky Catalog (UCAC2), we measure the mean proper motion of the two Magellanic Clouds. Appropriately selected LMC populations show a proper motion (<mu RA>, <mu DEC>) ~ (+0.84,+4.32) that is significantly higher, in <mu DEC>, than currently accepted Hipparcos-like values, <mu RA>, <mu DEC> ~ (+1.94,-0.14). A higher <mu RA> value is also found for the SMC. Interestingly, the mean UCAC2 LMC proper motion agrees very well with the only work in the literature (Anguita et al. 2000) pointing to an unbound Magellanic Clouds-Milky Way interaction. Nonetheless, the implications of the UCAC2 proper motion are hard to reconcile with our present day understanding of the Clouds-Galaxy interaction unless one assumes a more massive Milky Way. Consequently, although no sources of systematic error have been identified, it is perhaps most likely that the UCAC2 catalog has an as yet unidentified systematic error resulting in an inconsistency between UCAC2 and Hipparcos based results for the Magellanic Clouds.