Proper-Motion Membership Tests for Four Planetary Nebulae in Galactic Globular Clusters

Howard E. Bond, Andrea Bellini, Kailash C. Sahu

Published 2020-02-26Version 1

Four planetary nebulae (PNe) are considered to be probable 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 in the sky, all of these PNe have radial velocities that are consistent, within the errors, 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). Their PMs are statistically consistent with cluster membership, although Ps 1 is a mild outlier; if confirmed by subsequent Gaia data, this could suggest that the central star received a "kick" during PN formation. For the other two PNe, we used archival Hubble Space Telescope (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, but the measured PM for the central star is again statistically consistent with cluster membership.