2002A&A...386..558M

We present a detailed photoionization model of G29.96-0.02 (hereafter G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the Galaxy. This source has been observed extensively at radio and infrared wavelengths. The most recent data include a complete ISO (SWS and LWS) spectrum, which displays a remarkable richness in atomic fine-structure lines. The number of observables is twice as great as the number available in previous studies. In addition, most atomic species are now observed in two ionization stages. The radio and infrared data on G29.96 are best reproduced using a nebular model with two density components: a diffuse (n_e∼680cm^–3) extended (∼1pc) component surrounding a compact (∼0.1pc) dense (n_e∼57000cm^–3) core. The properties of the ionizing star were derived using state-of-the-art stellar atmosphere models. CoStar models yield an effective temperature of ∼30⁺²_–1kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values, T_eff∼32-38kK. This range in T_eff is compatible with all observational constraints, including near-infrared photometry and bolometric luminosity. The range 33-36kK is also compatible with the spectral type O5-O8 determined by WH97 when recent downward revisions of the effective temperature scale of O stars are taken into account. The age of the ionizing star of G29.96 is found to be a few 10⁶yr, much older than the expected lifetime of UCHII regions. Accurate gas phase abundances are derived with the most robust results being Ne/S=7.5 and N/O=0.43 (1.3 and 3.5 times the solar values, respectively).