The Rotation Curve of the Large Magellanic Cloud and the Implications for Microlensing

The rotation of the disk of the Large Magellanic Cloud (LMC) is derived from the published radial velocities of 422 carbon stars. New aspects of this analysis include the propagation of uncertainties in the LMC proper motion with a Monte Carlo and a self-consistent modeling of the rotation curve and disk kinematics. The LMC rotation curve reaches a maximum circular velocity of 72 ± 7 km s -1 at R = 4.0 kpc and then declines. The rotation curve is well fitted by a truncated, finite-thickness exponential disk model with no dark halo, implying a total mass of 5.3 ± 1.0 × 10 9 M ☉ . The velocity dispersion in concentric radial bins from R = 0.5 to 5.6 kpc decreases from 22 to 15 km s -1 and then increases to ~20 km s -1 at larger radii. Constant-thickness disk models in virial equilibrium cannot be reconciled with the data even if the effects of LMC or Galactic dark halos are included. If the disk is virialized, the scale height rises from h = 0.3 to 1.6 kpc over the range of R = 0.5-5.6 kpc. Thus, the LMC disk is flared. We model the velocity dispersion at large radii ( R > 6 kpc) as a maximal flared disk under the influence of the Galactic dark halo, which favors a mean density for the latter of ~ 2.5 × 10 -4 M ☉ pc -3 at the LMC distance. LMC stellar kinematics plays an important role in elucidating the nature of MACHOs, a dark population inferred from LMC microlensing. We have constructed a truncated and flared maximal disk model for the LMC that is kinematically based. Our model does not include a nonvirialized component such as tidal debris. The instantaneous probability of microlensing from LMC stars in our model is τ < 1.0 × 10 -8 sec 2 i , where i is the disk inclination. Our upper limit on the self-lensing optical depth is in good agreement with that obtained from less sophisticated models and is an order of magnitude too small to account for the MACHO microlensing signal.