Resolved Kinematics of Runaway and Field OB Stars in the Small Magellanic Cloud

M. S. Oey(1), J. Dorigo Jones(1), N. Castro(1), P. Zivick(2), G. Besla(3), H. C. Januszewski(1), M. Moe(3), N. Kallivayalil(2) and D. J. Lennon(4,5)

(1)University of Michigan, (2)University of Virginia, (3)University of Arizona, (4)ESA/ESAC, (5)IAC

We use GAIA DR2 proper motions of the RIOTS4 field OB stars
in the Small Magellanic Cloud (SMC) to study the kinematics of runaway stars. The data reveal
that the SMC Wing has a systemic peculiar motion relative to the SMC Bar of
(v_\alpha, v_\delta)=(62\pm 7, -18\pm5) km/s
and relative radial velocity +4.5\pm 5.0 km/s. This unambiguously
demonstrates that these two regions are kinematically distinct: the
Wing is moving away from the Bar, and towards the Large Magellanic
Cloud with a 3-D velocity of 64\pm10 km/s.
This is consistent with models for a recent, direct collision between the Clouds.
We present transverse velocity distributions for our field OB stars, confirming
that unbound runaways comprise on the order of half our sample, possibly more.
Using eclipsing binaries and double-lined
spectroscopic binaries as tracers of dynamically ejected runaways, and
high-mass X-ray binaries (HMXBs) as tracers of runaways accelerated by supernova
kicks, we find significant contributions from both populations.
The data suggest that HMXBs have lower velocity dispersion relative to
dynamically ejected binaries, consistent with the former corresponding
to less energetic supernova kicks that failed to unbind the components.
Evidence suggests that our fast runaways are dominated by dynamical,
rather than supernova, ejections.

Reference: ApJ Letters
Status: Manuscript has been accepted