Appendix B: The neutron star RX J185635-3754

After this paper was submitted, Walter (2000) suggested that perhaps the past trajectory of the isolated neutron star RX J185635-3754 intersected that of $\zeta$ Oph. He used HST WFPC2 observations over a three-year baseline to obtain a very accurate proper motion and parallax, and approximated the past orbit by a straight line. He concluded that the neutron star came very near $\zeta$ Oph 1.15 Myr ago in Upper Scorpius for an assumed radial velocity of -45 km s^-1.

We have used the simulation machinery described in Sect. 3 to analyse this case in the same way as done there for $\zeta$ Oph and PSR J1932+1059. We have again run three million simulations, covering the range $-50\pm50$ km s^-1 for the radial velocity of the neutron star. Figure B1 presents the resulting distribution of minimum separations $D_{\rm min}$ and associated kinematic ages $\tau_0$. This figure can be compared directly with Fig. 3. We have seen in Sect. 3 that $30\,822$simulations put PSR J1932+1059 within 10 pc of $\zeta$ Oph, and in 4214 of these the encounter took place in Upper Scorpius. By contrast, only 748 simulations put RX J185635-3754 within 10 pc of $\zeta$ Oph, about 1.5 Myr ago. None of these encounters occur within 15 pc of Upper Scorpius. We conclude that it is unlikely that RX J185635-3754 is the remnant of the supernova that gave $\zeta$ Oph its large space velocity. We suspect that this neutron star formed long ago somewhere else in the Galactic plane.

Figure B1: Left: Distribution of minimum separations, $D_{\rm min}(\tau_0)$, between $\zeta$ Oph and RX J185635-3754. The solid line denotes the expected distribution of $D_{\rm min}$, see Sect. 3.3. Right: Distribution of the times $\tau_0$ at which the minimum separation was reached