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 ± 0.°1) 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 ζ Oph, the Upper Sco OB association, and the neutron star come into spatial coincidence between 0.9 × 106 and 1.0 × 106 yr ago. RX J185635-3754 may be the remnant of the original primary of the ζ Oph system. If so, the space velocity suggests that the neutron star received a kick of about 200 km s-1 at birth.