The model

The hydrodynamical equations are solved together with the transfer of H-ionizing photons on a two-dimensional cylindrical grid. To provide a refined resolution mainly in the central part around the star, the grid is structured using a nested scheme. The time-dependent ionization and recombination of hydrogen are calculated in each time step and we carefully take stock of all the important energy-exchange processes in the system. A detailed description of the numerical method and further references are given in Freyer et al. (2003).

As initial condition an undisturbed homogeneous background gas with Solar abundances (Anders & Grevesse 1989), hydrogen number density n0 = 20 cm-3, and temperature T0 = 200 K was applied for the reasons described in Freyer et al. (2003). The models were then started with the sudden turn-on of the ZAMS stellar radiation field and stellar wind. Since the gas is assumed to be devoid of molecular material the radiation field commences immediately to ionize the environment outwards of the SWB without an enveloping photo-dissociation region. From the series of models of radiation- and wind-driven H ii regions around single massive stars mentioned above, for our purpose the 85 MQ star (Kroeger et al. 2006b) looks the most appropriate with respect to its self-enrichment. The time-dependent parameters of this star with "standard" mass-loss rate and Solar metallicity (Z = 0.02) during its H-burning main-sequence stage and its subsequent evolution are taken from Schaller et al. (1992). The model analysis has already been published (Kroeger et al. 2006a).

The investigation starts not before the onset of the WR stage, but with the onset of the WN stage at an age of t = 2.83 Myr. The WR star enriches the combined SWB-H ii region with 12C, 14N, and 16O. During its WN phase the star releases 0.143Mq of 14N, which is more than half of its total release, but hardly any extra 12C or 16O is supplied. As the condition for observability within the H ii region only the "warm" gas (6.0 x 103 K < T < 5.0 x 104 K) is accounted for. The mass fractions of 12C, 14N, and 16O with respect to Solar are set according to Anders &

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Figure 34.2. Time-dependent abundances of 12C, 14N, and 16O in the hot (left panel) and in the warm gas phase (right panel). The plot does not start before the onset of the WN phase at 2.83 Myr.

Figure 34.2. Time-dependent abundances of 12C, 14N, and 16O in the hot (left panel) and in the warm gas phase (right panel). The plot does not start before the onset of the WN phase at 2.83 Myr.

Grevesse (1989) to 4.466 x 10-3, 1.397 x 10-3, and 1.061 x 10-2, respectively, normalized with respect to H.

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