Mass, Structure and Orbit of LMC

There have been a wide variety of LMC censuses, most of which treat the LMC as a galaxy and use the standard mass and mass density estimates: rotation curves, star counts, surface brightness profiles. Two relatively recent rotation curve studies, Meatheringham et al. (1988) and Schommer et al. (1992), estimate LMC masses of $6\times10^9{\rm\,M_\odot}$ and $1.5\times10^{10}{\rm\,M_\odot}$. Similar limits follow from carbon star studies by Kunkel et al. (1997b). The main difference between these determinations is not the value of V_c for the Cloud but the radial extent of the rotation curve. Alternatively, from the Milky Way's point of view, the LMC is similar to an oversized globular cluster. Its tidal radius is measurable and depends on both the Milky Way rotation curve and the LMC mass (and, weakly, its profile). A preliminary estimate of the tidally inferred LMC mass (Nikolaev & Weinberg 1998) yields $2\times10^{10}{\rm\,M_\odot}$ but is consistent with the Schommer et al. estimate. A brief description of this result is provided in the Appendix.

Recent estimates of the LMC space velocity from archival plate (Jones et al. 1994) and Hipparcos (Kroupa & Bastian 1997) proper motions both lead to consistent estimates of the LMC orbital plane. The procedure used here to estimate the orbit is described in Weinberg (1995). For the Milky Way halo, I choose a W₀=3 King model (1966) with $r_t=200{\rm\,kpc}$ and mass scaled to $4\times10^{11}{\rm\,M_\odot}$ (Kochanek 1996). The rotation curve due to the Galaxy, then, is approximately flat in the region of the LMC orbit. Together with a disk, the overall rotation curve is a plausible representation of the observed Milky Way. With this halo model, both the space velocities estimated by Jones et al. and Kroupa & Bastian yield a similar perigalacticon of 46 kpc with apogalacticons of 72 kpc and 120 kpc. For lack of motivation to favor one of these over the other, I adopt the mean apogalacticon.