1 Introduction

The properties of young massive stars are still poorly known. This is due to the fact that massive stars are relatively rare and that their lifetime is short. In their early stages, massive stars are still embedded in their parental molecular clouds and suffer heavy extinction. Our knowledge of the stellar energy distribution in the visible and the ultraviolet of young massive stars therefore relies on indirect measurements, such as atomic fine-structure lines observed at infrared wavelengths. A comparison of the line fluxes with predictions of detailed photo-ionizing codes coupled with stellar atmosphere models allows one to constrain the properties of the ionizing stars.

Recently, Peeters et al. (2002) and Martín-Hernández et al. (2002), hereafter Papers I and II, presented the results of an infrared spectral survey of 34 galactic compact H II regions based on complete ISO grating spectra. For most of the sources, the ISO data display a remarkable richness in spectral lines including all the fine-structure lines of N, O, Ne, S, and Ar. Using additional data from the literature, the elemental abundances and their variation across the Galactic disc were derived (Paper II).

Some of the H II regions in the sample are well known and were studied in detail at other wavelengths. Those additional data provide useful contraints to derive the properties of the ionizing star and to fine tune the elemental abundance estimates. One such source is the compact H II region IRAS 18434-0242 (G29.96-0.02, herafter G29.96). This compact source has one of the richest ISO spectrum of the entire sample (Paper I) and is one of the few compact H II regions for which the ionizing star has been identified and characterized through direct infrared spectroscopy (Watson & Hanson 1997; Hanson et al. 2002).

The availability of high-quality data on G29.96 together with the recent developments of the models of stellar atmosphere of massive stars prompted us to make a detailed photionization model of this source with the aim to further constrain the nature of the ionizing stars and to derive accurate elemental abundances. The paper is organized as follows: the observational properties are summarized in Sect. 2; Sect. 3 describes the photoionization code, the input stellar parameters and the methodology; Sect. 4 presents the results of the best model which are discussed in Sect. 5; the conclusions are given in Sect. 6.