Issue |
A&A
Volume 459, Number 1, November III 2006
|
|
---|---|---|
Page(s) | 103 - 111 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20065816 | |
Published online | 12 September 2006 |
Planetary nebula candidates in extragalactic young star clusters
1
European Southern Observatory, ST-ECF, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
2
Astronomical Institute, University of Utrecht, Princetonplein 5, 3584 CC Utrecht, The Netherlands e-mail: larsen@astro.uu.nl
3
Astronomy Group, Universidad de Concepción, Departamento de Física, Casilla 160-C, Concepción, Chile
Received:
13
June
2006
Accepted:
16
August
2006
Context.During an analysis of optical spectra of 80 young star clusters in several
nearby spiral galaxies, [] and [
] 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 .
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
. For the other cluster the age is
282–407 Myr, corresponding to MTO = 3.2–3.6
.
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.
Key words: galaxies: spiral / galaxies: star clusters / stars: AGB and post-AGB / planetary nebulae: general
© ESO, 2006
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