Abstract
 [*] New understanding of Large Magellanic Cloud structure, dynamics and
orbit from carbon star kinematics
 van der Marel R.P., Alves D.R., Hardy E., Suntzeff N.B.
 AJ, 124, 26392663, 2002

 [*]
Citations to
this paper in the ADS
We formulate a new, revised and coherent understanding of the
structure and dynamics of the Large Magellanic Cloud (LMC), and its
orbit around and interaction with the Milky Way. Much of our
understanding of these issues hinges on studies of the LMC
lineofsight kinematics. The observed velocity field includes
contributions from the LMC rotation curve V(R'), the LMC transverse
velocity vector v_t, and the rate of inclination change di/dt. All
previous studies have assumed di/dt = 0. We show that this is
incorrect, and that combined with uncertainties in v_t this has led to
incorrect estimates of many important structural parameters of the
LMC. We derive general expressions for the velocity field which we fit
to kinematical data for 1041 carbon stars. We calculate v_t by
compiling and improving LMC proper motion measurements from the
literature, and we show that for known v_t all other model parameters
are uniquely determined by the data. The position angle of the line
of nodes is Theta = 129.9 +/ 6.0 degrees, consistent with the value
determined geometrically by van der Marel and Cioni (2001). The rate of
inclination change is di/dt = 0.37 +/ 0.22 mas/yr = 103 +/ 61
degrees/Gyr. This is similar in magnitude to predictions from Nbody
simulations by Weinberg (2000), which predict LMC disk precession and
nutation due to Milky Way tidal torques. The LMC rotation curve V(R')
has amplitude 49.8 +/ 15.9 km/s. This is 40% lower than what has
previously (and incorrectly) been inferred from studies of HI, carbon
stars, and other tracers. The lineofsight velocity dispersion has an
average value sigma = 20.2 +/ 0.5 km/s, with little variation as
function of radius. The dynamical center of the carbon stars is
consistent with the center of the bar and the center of the outer
isophotes, but it is offset by 1.2 +/ 0.6 degrees from the
kinematical center of the HI. The enclosed mass inside the last data
point is M(8.9 kpc) = (8.7 +/ 4.3) x 10^9 solar masses more than half
of which is due to a dark halo. The LMC has a considerable vertical
thickness; its V/sigma = 2.9 +/ 0.9 is less than the value for the
Milky Way's thick disk (V/sigma = 3.9). Simple arguments for models
stratified on spheroids indicate that the (outofplane) axial ratio
could be 0.3 or larger. Isothermal disk models for the observed
velocity dispersion profile confirm the finding of Alves and Nelson
(2000) that the scale height must increase with radius. A substantial
thickness for the LMC disk is consistent with the simulations of
Weinberg, which predict LMC disk thickening due to Milky Way tidal
forces. These affect LMC structure even inside the LMC tidal radius,
which we calculate to be r_t = 15.0 +/ 4.5 kpc (i.e., 17.1 +/ 5.1
degrees). The new insights into LMC structure need not significantly
alter existing predictions for the LMC selflensing optical depth,
which to lowest order depends only on sigma. The compiled proper
motion data imply an LMC transverse velocity v_t = 406 km/s in the
direction of position angle 78.7 degrees (with errors of 40 km/s in
each coordinate). This can be combined with the observed systemic
velocity, v_sys = 262.2 +/ 3.4 km/s, to calculate the LMC velocity in
the Galactocentric rest frame. This yields v_LMC = 293 +/ 39 km/s,
with radial and tangential components v_{LMC,rad} = 84 +/ 7 km/s and
v_{LMC,tan} = 281 +/ 41 km/s, respectively. This is consistent with
the range of velocities that has been predicted by models for the
Magellanic Stream. The implied orbit of the LMC has an apocenter to
pericenter distance ratio 2.5:1, a perigalactic distance 45 kpc, and
a present orbital period around the Milky Way 1.5 Gyr. The constraint
that the LMC is bound to the Milky Way provides a robust limit on the
minimum mass and extent of the Milky Way dark halo: M_MW > 4.3 x
10^{11} solar masses and r_h > 39 kpc (68.3% confidence). Finally, we
present predictions for the LMC proper motion velocity field, and we
discuss how measurements of this may lead to kinematical distance
estimates of the LMC.
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Last modified November 29, 2002.
Roeland van der Marel,
marel@stsci.edu.
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