During an analysis of optical spectra of 80 young star clusters in several
nearby spiral galaxies, [O III] and [N II] emission lines were noted in some
cases. Three of these emission line sources are identified as likely planetary
nebula (PN) candidates and may represent a rare opportunity to study PNe whose
progenitor stars are known to be of intermediate masses. This paper presents
and discusses basic properties of the PN candidates and their host clusters.
Based on the observed emission line fluxes, the excitation parameters and
luminosities of the nebulae are derived. This allows a crude placement of the
central stars in two of the objects on the H-R diagram, and their temperatures
and luminosities are found to be consistent with post-AGB model tracks for a
central star mass of about 0.60 Msun. Host cluster ages and masses are
estimated from broad-band colours and by comparison with model SSP spectra. One
of the host clusters has an age of 32-65 Myrs, corresponding to a main sequence
turn-off mass of M(TO) = 6.6-9.0 Msun. For the other cluster the age is 282-407
Myrs, corresponding to M(TO) = 3.2-3.6 Msun. By estimating the number of stars
evolving off the main sequence per year, a total of 6 PNe are expected in our
full sample of 80 clusters for aPN lifetime of 10000 years. The factor of two
disagreement with the actual observed number may be due, among other things, to
uncertainties in PN lifetimes. It is interesting to note that all three PN
candidates are associated with clusters which are more diffuse than average.
While PNe have previously been found in some old globular clusters, the
candidates identified here are among the first identified in young star
clusters.Comment: 10 pages, 9 figures. Accepted for publication in A&
