  We have measured astrometry for members of the Orion Nebula Cluster with
images obtained in 2015 with the Wide Field Camera 3 on board the Hubble Space
Telescope. By comparing those data to previous measurements with NICMOS on
Hubble in 1998, we have discovered that a star in the Kleinmann-Low Nebula,
source x from Lonsdale et al. (1982), is moving with an unusually high proper
motion of 29 mas/yr, which corresponds to 55 km/s at the distance of Orion.
Previous radio observations have found that three other stars in the
Kleinmann-Low Nebula (BN and sources I and n) have high proper motions (5-14
mas/yr) and were near a single location ~540 years ago, and thus may have been
members of a multiple system that dynamically decayed. The proper motion of
source x is consistent with ejection from that same location 540 years ago,
which provides strong evidence that the dynamical decay did occur and that the
runaway star BN originated in the Kleinmann-Low Nebula rather than the nearby
Trapezium cluster. However, our constraint on the motion of source n is
significantly smaller than the most recent radio measurement, which indicates
that it did not participate in the event that ejected the other three stars.
