Planetary Nebula - Membership in Clusters

Membership in Clusters

Planetary nebulae have been detected as members in four globular clusters: Messier 15, Messier 22, NGC 6441 and Palomar 6. However, there has yet to be an established case of a planetary nebula discovered in an open cluster as based on a consistent set of distances, reddenings, and radial velocities. The cases of NGC 2348 in Messier 46, and NGC 2818 in the respective open cluster that is designated by the same name, are often cited as bona fide instances, however, they are instead line-of-sight coincidences granted the radial velocities between the clusters and planetary nebulae are discrepant.

Partly because of their small total mass, open clusters have relatively poor gravitational cohesion. Consequently, open clusters tend to disperse after a relatively short time, typically from 100 to 600 million years, because of external gravitational influences amid other factors. Under exceptional conditions, open clusters can remain intact for up to one billion years or more.

Theoretical models predict that planetary nebulae can form from main-sequence stars of between eight and one solar masses, which puts their age at 40 million years and older. Although there are a few hundred known open clusters within that age range, a variety of reasons limit the chances of finding a member of an open cluster in a planetary nebula phase. One such reason is that the planetary nebula phase for more massive stars belonging to younger clusters is on the order of thousands of years—a blink of the eye in cosmic terms.