Planetary nebula candidates in extragalactic young star clusters

Abstract

Context. During an analysis of optical spectra of 80 young star clusters in several nearby spiral galaxies, [Oiii] and [Nii] emission lines were noted in some cases. Three of these emission line sources are associated with clusters older than 30 Myr, and are identified as likely planetary nebula (PN) candidates. Aims. These objects may represent a rare opportunity to study PNe whose progenitor stars are known to be of intermediate masses, although detailed analysis is challenging because of the underlying strong continuum from the cluster stars. This paper presents and discusses basic properties of the PN candidates and their host clusters. Methods. 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. Cluster ages and masses are estimated from broad-band colours and by fitting model SSP spectra to the observed spectra. Results. The two PN candidates where central star luminosities and temperatures can be estimated are found to be consistent with post-AGB model tracks for a central star mass of about 0.60 M . One of the host clusters has an age of 32–65 Myr, corresponding to a main sequence turn-off mass of MTO = 6.6–9.0 M . For the other cluster the age is 282–407 Myr, corresponding to MTO = 3.2–3.6 M . By estimating the number of stars evolving off the main sequence per year, a total of 6 PNe are expected in the full sample of 80 clusters for a PN lifetime of 104 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. Conclusions. While PNe have previously been found in some old globular clusters, the candidates identified here are among the first identified in young star clusters.