Proper-Motion Membership Tests for Four Planetary Nebulae in Galactic
  Globular Clusters
  Four planetary nebulae (PNe) are considered to be probable or possible
members of Galactic globular clusters (GCs). These are Ps 1 = K648 in M15, GJJC
1 = IRAS 18333-2357 in M22, JaFu 1 in Palomar 6, and JaFu 2 in NGC 6441. In
addition to lying close to the host GCs on the sky, these PNe have radial
velocities that are consistent, within the errors and stellar velocity
dispersions, with cluster membership. The remaining membership criterion is
whether the proper motions (PMs) of the central stars are in agreement with
those of the host clusters. We have carried out the PM test for all four PNe.
Two of the central stars--those of Ps 1 and GJJC 1--have PMs listed in the
recent Gaia Data Release 2 (DR2). We updated the PM of the Ps 1 central star to
a more precise value using archival Hubble Space Telescope (HST) frames. Both
PMs are statistically consistent with cluster membership. For the other two
PNe, we used archival HST images to derive the PMs of their nuclei. For JaFu 2,
there are HST images at several epochs, and the measured PM of the nucleus is
in excellent agreement with that of the host cluster. For JaFu 1 the available
archival HST images are less optimal and the results are less conclusive; the
measured PM for the central star is marginally consistent with cluster
membership, but additional astrometric observations are desirable for a more
robust membership test.
