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The Stellar Halo in the Large Magellanic Cloud: Mass, Luminosity, and Microlensing Predictions
Recently obtained kinematic data has shown that the Large Magellanic Cloud
(LMC) possesses an old stellar halo. In order to further characterize the
properties of this halo, parametric King models are fit to the surface density
of RR Lyrae stars. Using data from both the MACHO and OGLE II microlensing
surveys, the model fits yield the center of their distribution at RA =
05:21.1+-0.8, Dec = -69:45+-6 (J2000) and a core radius of 1.42+-0.12 kpc. As a
check the halo model is compared with RR Lyrae star counts in fields near the
LMC's periphery previously surveyed with photographic plates. These data,
however, require a cautious interpretation. Several topics regarding the LMC
stellar halo are discussed. First, the properties of the halo imply a global
mass-to-light ratio of M/L_V = 5.3+-2.1 and a total mass of 1.6+-0.6 10^10
M_sun for the LMC in good agreement with estimates based on the rotation curve.
Second, although the LMC's disk and halo are kinematically distinct, the shape
of the surface density profile of the halo is remarkably similar to that of the
young disk. For example, the best-fit exponential scale length for the RR Lyrae
stars is 1.47+-0.08 kpc, which compares to 1.46 kpc for the LMC's blue light.
In the Galaxy, the halo and disk do not resemble each other like this. Finally,
a local maximum in the LMC's microlensing optical depth due to halo-on-disk
stellar self-lensing is predicted. For the parameters of the stellar halo
obtained, this maximum is located near MACHO events LMC-4 and LMC-23, and is
large enough to possibly account for these two events, but not for all of the
observed microlensing.Comment: 11 pages, 1 figure, accepted to ApJ Letter
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