  [abridged] A "missing data" algorithm is developed to model (ie, reconstruct)
the three-dimensional velocity distribution function of a sample of stars using
data (velocity measurements) every one of which has one dimension unmeasured
(the radial direction). It also accounts for covariant measurement
uncertainties on the tangential velocity components. The algorithm is applied
to tangential velocities measured in a kinematically unbiased sample of 11,865
stars taken from the Hipparcos catalog. The local stellar velocity distribution
function of each of a set of 20 color-selected subsamples is modeled as a
mixture of two three-dimensional Gaussian ellipsoids of arbitrary relative
responsibility. In the fitting, one Gaussian (the "halo") is fixed at the known
mean velocity and velocity variance tensor of the Galaxy halo, and the other
(the "disk") is allowed to take arbitrary mean and arbitrary variance tensor.
The mean and variance tensor (commonly the "velocity ellipsoid") of the disk
velocity distribution are both found to be strong functions of stellar color,
with long-lived populations showing larger velocity dispersion, slower mean
rotation velocity, and smaller vertex deviation than short-lived populations.
The local standard of rest (LSR) is inferred in the usual way and the Sun's
motion relative to the LSR is found to be
(U,V,W)_{\odot}=(10.1,4.0,6.7)+/-(0.5,0.8,0.2) km/s. Artificial data sets are
made and analyzed, with the same error properties as the Hipparcos data, to
demonstrate that the analysis is unbiased. The results are shown to be
insensitive to the assumption that the velocity distributions are Gaussian.
