1 Introduction

Isolated extragalactic H  II regions, which we will simply refer to as H  II galaxies, are powered by clusters of hot, massive stars that ionize their environment. Recent analyses of such objects, comparing the observed nebular emission lines, colors, and stellar features with evolutionary models (Mas-Hesse & Kunth 1991, 1999; Schaerer et al. 1999; Stasinska & Schaerer 1999) found that most have experienced a recent, quasi instantaneous burst of star formation. In addition, there is growing evidence for an older stellar population (Telles & Terlevich 1997; Papaderos et al. 1998; Raimann et al. 2000a). Some clues regarding the evolution of giant H  II regions, their properties and stellar content may be obtained by considering H  II galaxies in different evolutionary stages with respect to the most recent star formation event.

The present paper follows this approach: we compare the emission line properties of a large sample of H  II galaxies with those of photoionization models in which the ionizing continuum is provided by synthetic models of evolving starbursts. In a previous paper (Stasinska & Leitherer 1996, hereinafter SL96), we considered a sample of metal poor galaxies in which the oxygen abundance could be derived directly from observations using the electron temperature sensitive [O  III] $\lambda$4363 line. Thus the derived abundances were unambiguous and model independent. Unfortunately, we were automatically restricted to young starbursts by this requirement. As the most massive stars disappear, the radiation field gradually softens inducing a decrease of the [O  III] emission, and the weak [O  III] $\lambda$4363 becomes undetectable after about 5 Myr. Alternatively, the oxygen abundances can be estimated via strong line methods (Pagel et al. 1979; McGaugh 1991, 1994). The relevance and accuracy of these methods is however still under debate. We address this question in Sect. 4.2.

Lifting the restriction to the youngest starbursts immediately leads to inclusion of objects with a priori unknown metallicities. However, the distribution in metallicities should be the same for old and young objects, permitting an interpretation of diagnostic diagrams based on emission lines. This advantage of a large sample of bona fide young and old H  II galaxies is explored in the present work. With respect to SL96, we also use updated evolutionary synthesis models with more recent stellar tracks and stellar atmospheres as input for our photoionization models.

The study in the present paper is complementary to work on giant H  II regions in spiral galaxies, such as published by García-Vargas & Díaz (1994), García-Vargas et al. (1995), and more recently Bresolin et al. (1999) and Dopita et al. (2000). First, our observational samples are based on extragalactic H  II regions, for which the equivalent width of H$\beta$ can be used as a direct first-order indicator of the age of the latest starburst (Dottori 1981). Second, our samples are biased towards dwarf galaxies with correspondingly low average metallicities.

In Sect. 2 we present the observational samples, discuss possible selection effects, and display a series of observational diagrams. In particular, we show that emission line trends are seen not only in classical emission line ratio diagrams, but also in diagrams relating emission line ratios with the H$\beta$ equivalent width. In Sect. 3 we describe the models, present our model grid, and show theoretical diagrams for evolving starbursts. In Sect. 4 observations and models are compared. The main conclusions are summarized in Sect. 5.