  We report the discovery of HATS-70b, a transiting brown dwarf at the
deuterium burning limit. HATS-70b has a mass of Mp=12.9 +1.8/-1.6 Mjup and a
radius of Rp=1.384 +0.079/-0.074 Rjup, residing in a close-in orbit with a
period of 1.89 days. The host star is a M*=1.78 +/- 0.12 Msun A star rotating
at vsini=40.61 +0.32/-0.35 km/s, enabling us to characterize the spectroscopic
transit of the brown dwarf via Doppler tomography. We find that HATS-70b, like
other massive planets and brown dwarfs previously sampled, orbits in a low
projected-obliquity orbit with lambda=8.9 +5.6/-4.5 deg. The low obliquities of
these systems is surprising given all brown dwarf and massive planets with
obliquities measured orbit stars hotter than the Kraft break. This trend is
tentatively inconsistent with dynamically chaotic migration for systems with
massive companions, though the stronger tidal influence of these companions
makes it difficult to draw conclusions on the primordial obliquity distribution
of this population. We also introduce a modeling scheme for planets around
rapidly rotating stars, accounting for the influence of gravity darkening on
the derived stellar and planetary parameters.
