Hubble Space Telescope Snapshot Search for Planetary Nebulae in Globular Clusters of the Local Group

TL;DR

This study used Hubble Space Telescope imaging to search for planetary nebulae in globular clusters of the Local Group, finding very few candidates and suggesting a low but non-zero occurrence rate.

Contribution

First systematic HST snapshot survey of PNe in extragalactic globular clusters, providing new constraints on their occurrence and evolutionary channels.

Findings

No convincing PNe found in 75 GCs imaged.

Some candidate PNe likely originate from ambient interstellar medium or bright point sources.

The incidence rate of PNe in GCs is small but non-zero across the Local Group.

Abstract

Single stars in ancient globular clusters (GCs) are believed incapable of producing planetary nebulae (PNe), because their post-asymptotic-giant-branch evolutionary timescales are slower than the dissipation timescales for PNe. Nevertheless, four PNe are known in Galactic GCs. Their existence likely requires more exotic evolutionary channels, including stellar mergers and common-envelope binary interactions. I carried out a snapshot imaging search with the Hubble Space Telescope (HST) for PNe in bright Local Group GCs outside the Milky Way. I used a filter covering the 5007 A nebular emission line of [O III], and another one in the nearby continuum, to image 66 GCs. Inclusion of archival HST frames brought the total number of extragalactic GCs imaged at 5007 A to 75, whose total luminosity slightly exceeds that of the entire Galactic GC system. I found no convincing PNe in these clusters, aside from one PN in a young M31 cluster misclassified as a GC, and two PNe at such large angular separations from an M31 GC that membership is doubtful. In a ground-based spectroscopic survey of 274 old GCs in M31, Jacoby et al. (2013) found three candidate PNe. My HST images of one of them suggest that the [O III] emission actually arises from ambient interstellar medium rather than a PN; for the other two candidates, there are broad-band archival UV HST images that show bright, blue point sources that are probably the PNe. In a literature search, I also identified five further PN candidates lying near old GCs in M31, for which follow-up observations are necessary to confirm their membership. The rates of incidence of PNe are similar, and small but non-zero, throughout the GCs of the Local Group.