The Magellanic Stream: break up and accretion onto the hot Galactic
  corona
  The Magellanic HI Stream (~2x10^9 Msun [d/55 kpc]^2) encircling the Galaxy at
a distance 'd' is arguably the most important tracer of what happens to gas
accreting onto a disk galaxy. Recent observations reveal that the Stream's mass
is in fact dominated (3:1) by its ionised component. Here we revisit the origin
of the mysterious H-alpha recombination emission observed along much of its
length that is overly bright (~150-200 milli-Rayleigh) for the known Galactic
ultraviolet background (~20-40 mR / [d/55 kpc]^2). In an earlier model, we
proposed that a slow shock cascade was operating along the Stream due to its
interaction with the extended Galactic hot corona. We find that, for a smooth
coronal density profile, this model can explain the bright H-alpha emission if
the coronal density satisfies 2 < (n / 10^{-4} cm^{-3}) < 4 at d = 55 kpc. But
in view of updated parameters for the Galactic halo and mounting evidence that
most of the Stream must lie far beyond the Magellanic Clouds (d>55 kpc), we
revisit the shock cascade model in detail. At lower densities, the HI gas is
broken down by the shock cascade but mostly mixes with the hot corona without
significant recombination. At higher densities, the hot coronal mass (including
the other baryonic components) exceeds the baryon budget of the Galaxy. If the
H-alpha emission arises from the shock cascade, the upper limit on the smooth
coronal density constrains the Stream's mean distance to < 75 kpc. If, as some
models indicate, the Stream is even further out, either the shock cascade is
operating in a regime where the corona is substantially mass-loaded with recent
gas debris, or an entirely different ionization mechanism is responsible.
