The proper motion, parallax, and origin of the isolated neutron star rx J185635-3754

The isolated neutron star RX J185635-3754 is the closest known neutron star to the Sun. Based on Hubble Space Telescope Wide Field Planetary Camera 2 observations over a 3 yr baseline, I report its proper motion (332 +/- 1 mas yr(-1) at a position angle of 100.3 degrees +/- 0.1 degrees) and parallax (16.5 +/- 2.3 mas, 61 pc). This proper motion brings the neutron star from the general vicinity of the Sco-Cen OB association. For an assumed neutron star radial velocity between -55 and -60 km s(-1), the runaway O star zeta Oph, the Upper Sco OB association, and the neutron star come into spatial coincidence between 0.9 x 10(6) and 1.0 x 10(6) yr ago. RX J185635-3754 may be the remnant of the original primary of the zeta Oph system. If so, the space velocity suggests that the neutron star received a kick of about 200 km s(-1) at birth.