The Milky Way has no thick disk
  Different stellar sub-populations of the Milky Way's stellar disk are known
to have different vertical scale heights, their thickness increasing with age.
Using SEGUE spectroscopic survey data, we have recently shown that
mono-abundance sub-populations, defined in the [\alpha/Fe]-[Fe/H] space, are
well described by single exponential spatial-density profiles in both the
radial and the vertical direction; therefore any star of a given abundance is
clearly associated with a sub-population of scale height h_z. Here, we work out
how to determine the stellar surface-mass density contributions at the solar
radius R_0 of each such sub-population, accounting for the survey selection
function, and for the fraction of the stellar population mass that is reflected
in the spectroscopic target stars given populations of different abundances and
their presumed age distributions. Taken together, this enables us to derive
\Sigma_{R_0}(h_z), the surface-mass contributions of stellar populations with
scale height h_z. Surprisingly, we find no hint of a thin-thick disk
bi-modality in this mass-weighted scale-height distribution, but a smoothly
decreasing function, approximately \Sigma_{R_0}(h_z)\propto \exp(-h_z), from
h_z ~ 200 pc to h_z ~ 1 kpc. As h_z is ultimately the structurally defining
property of a thin or thick disk, this shows clearly that the Milky Way has a
continuous and monotonic distribution of disk thicknesses: there is no 'thick
disk' sensibly characterized as a distinct component. We discuss how our result
is consistent with evidence for seeming bi-modality in purely geometric disk
decompositions, or chemical abundances analyses. We constrain the total visible
stellar surface-mass density at the Solar radius to be \Sigma^*_{R_0} = 30 +/-
1 M_\odot pc^{-2}.
