  The redshifted 21-cm line of hydrogen holds great potential for the study of
cosmology, as it can probe otherwise unobservable cosmic epochs. In particular,
measurements of the 21-cm power spectrum during cosmic dawn---the era when the
first stars were formed---will provide us with a wealth of information about
the astrophysics of stellar formation, as well as the origin of fluctuations in
our Universe. In addition to their usually considered density fluctuations,
dark matter and baryons possess large relative velocities with respect to each
other, due to the baryon acoustic oscillations (BAOs) suffered by the latter,
which suppress the formation of stars during cosmic dawn, leaving an imprint on
21-cm observables during this era. Here we present 21cmvFAST, a version of the
publicly available code 21cmFAST modified to account for this effect. Previous
work has shown that the inclusion of relative velocities produces an acoustic
modulation on the large-scale 21-cm power spectrum during cosmic dawn. By
comparing analytic calculations with simulations from 21cmvFAST, here we
demonstrate that this modulation takes the form of robust velocity-induced
acoustic oscillations (VAOs), during both the Lyman-$\alpha$ coupling era and
the subsequent epoch of heating. The unique shape of these VAOs, which is
determined by the acoustic physics that generated the relative velocities, can
be analytically computed and is easily distinguishable from the usual
density-sourced fluctuations. We find that, under a broad range of
astrophysical assumptions, the VAOs are detectable at high significance by the
upcoming HERA interferometer, which could therefore confirm the presence of
acoustic oscillations at cosmic dawn.
