Modeling the ionizing spectra of H II regions: individual stars versus stellar ensembles

¹ Instituto de Astrofísica de Andalucía (CSIC). Camino bajo de Huétor 50, Granada 18080, Spain
e-mail: mcs@iaa.es
² Instituto de Astrofísica de Canarias, C/vía Láctea s/n, 38205 La Laguna, Spain
³ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain

Received: 27 July 2009
Accepted: 4 June 2010

Abstract

Aims. We study how IMF sampling affects the ionizing flux and emission line spectra of low mass stellar clusters.

Methods. We performed 2 × 10⁶ Monte Carlo simulations of zero-age solar-metallicity stellar clusters covering the 20−10⁶ M_⊙ mass range. We study the distribution of cluster stellar masses, M_clus, ionizing fluxes, Q(H⁰), and effective temperatures, . We compute photoionization models that broadly describe the results of the simulations and compare them with photoionization grids.

Results. Our main results are: (a) a large number of low mass clusters (80% for M_clus = 100 M_⊙) are unable to form an H ii region. (b) There are a few overluminous stellar clusters that form H ii regions. These overluminous clusters preserve statistically the mean value of  ⟨Q(H⁰)⟩  obtained by synthesis models, but the mean value cannot be used as a description of particular clusters. (c) The ionizing continuum of clusters with M_clus ≲ 10⁴ M_⊙ is more accurately described by an individual star with self-consistent effective temperature () and Q(H⁰) than by the ensemble of stars (or a cluster ) produced by synthesis models. (d) Photoionization grids of stellar clusters cannot be used to derive the global properties of low mass clusters.

Conclusions. Although variations in the upper mass limit, m_up, of the IMF would reproduce the effects of IMF sampling, we find that an ad hoc law that relates m_up to M_clus in the modeling of stellar clusters is useless, since: (a) it does not cover the whole range of possible cases; and (b) the modeling of stellar clusters with an IMF is motivated by the need to derive the global properties of the cluster: however, in clusters affected by sampling effects we have no access to global information of the cluster but only particular information about a few individual stars.