Clusters in extragalactic HII regions and their modelling.pdf
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Extragalactic Star Clusters
IAU Symposium Series, Vol. Vol. 207, 2001
Eva K. Grebel, Doug Geisler, and Dante Minniti, eds.
Clusters in extragalactic HII regions and their modelling
Rosa Mar??a Gonza?lez Delgado
Instituto de Astrof??sica de Andaluc??a (CSIC). Apdo. 3004, 18080
Granada, Spain
Abstract.
This review deals with properties of very young (≤ 10-20 Myr) stellar
clusters which are in the nebular phase and are embedded in photoionized
regions (classical extragalactic HII regions (RHIIs) or starburst galaxies).
Based on the analysis of the integrated light of these clusters at the UV
and optical wavelengths, different techniques are discussed that allow to
estimate in consistent the stellar content and the evolutionary state of
the ionizing clusters. Figures illustrating this contribution can be found
in: http://www.iaa.csic.es/～rosa/
1. Introduction
RHIIs are amongst the brightest objects in galaxies. They have sizes up to
a few 100 pc and their Hα luminosity is larger than 1039 erg s?1 (Kennicutt
1984). The most luminous and brightest RHII in the Local Group is 30 Doradus
in the LMC. It has a nebular size of 400 pc and an Hα luminosity 1040.2 erg
s?1, produced by more than one thousand ionizing stars. However, most of the
radiative and mechanical energy of the region is provided by the central compact
cluster R136. It contains more than one hundred stars within a few parcsecs,
having a stellar density of ～ 2 stars/pc2 (Campbell et al 1992). This cluster is
very young, ～ 3 Myr (Vacca et al 1995). Its compactness contrasts with that
of the second brightest RHII in the Local group, NGC 604 in M 33. As 30 Dor,
NGC 604 is photoionized by a very young cluster (3 Myr old, Gonza?lez Delgado
Pe?rez 2000) that contains a large population of O and B stars. However, it
extends in a larger area with an effective radius of 60 pc (Hunter et al 1996;
Drissen et al 1993). Several subclumps are detected, b
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