Runaway OB Stars in the Small Magellanic Cloud: Dynamical Versus
  Supernova Ejections
  Runaway OB stars are ejected from their parent clusters via two mechanisms,
both involving multiple stars: the dynamical ejection scenario (DES) and the
binary supernova scenario (BSS). We constrain the relative contributions from
these two ejection mechanisms in the Small Magellanic Cloud (SMC) using data
for 304 field OB stars from the spatially complete, Runaways and Isolated
O-Type Star Spectroscopic Survey of the SMC (RIOTS4). We obtain stellar masses
and projected rotational velocities $v_r\sin i $ for the sample using RIOTS4
spectra, and use transverse velocities $v_{\rm loc}$ from $\it{Gaia}$ DR2
proper motions. Kinematic analyses of the masses, $v_r\sin i $, non-compact
binaries, high-mass X-ray binaries, and Oe/Be stars largely support predictions
for the statistical properties of the DES and BSS populations. We find that
dynamical ejections dominate over supernova ejections by a factor of $\sim 2-3$
in the SMC, and our results suggest a high frequency of DES runaways and binary
ejections. Objects seen as BSS runaways also include two-step ejections of
binaries that are reaccelerated by SN kicks. We find that two-step runaways
likely dominate the BSS runaway population. Our results further imply that any
contribution from $\it{in-situ}$ field OB star formation is small. Finally, our
data strongly support the post-mass-transfer model for the origin of classical
Oe/Be stars, providing a simple explanation for the bimodality in the $v_r\sin
i $ distribution and high, near-critical, Oe/Be rotation velocities. The close
correspondence of Oe/Be stars with BSS predictions implies that the
emission-line disks are long-lived.
