Testing Cluster Membership of Planetary Nebulae with High-precision Proper Motions. I. HST Observations of JaFu 1 Near the Globular Cluster Palomar 6

If a planetary nebula (PN) is shown to be a member of a star cluster, we obtain important new constraints on the mass and chemical composition of the PN’s progenitor star, which cannot be determined for PNe in the field. Cluster membership can be tested by requiring the projected separation between the PN and cluster to be within the tidal radius of the cluster, and the objects to have nearly identical radial velocities (RVs) and interstellar extinctions, and nearly identical proper motions (PMs). In an earlier study, we used PMs to confirm that three PNe, which had already passed the other tests, are highly likely to be members of Galactic globular clusters (GCs). For a fourth object, the PN JaFu 1, which lies in the Galactic bulge near the GC Palomar 6 on the sky and has a similar RV, the available PM measurement gave equivocal results. We have now obtained new high-resolution images of the central star of JaFu 1 with the Hubble Space Telescope (HST), which, combined with archival HST frames taken 14 and 16 yr earlier, provide a high-precision PM. Unfortunately, we find that the PM of the central star differs from that of the cluster with high statistical significance. Nevertheless, JaFu 1 is of astrophysical interest because its nucleus appears to be a member of the rare class of “EGB 6-type” central stars, which are associated with compact emission-line knots.