We present a detailed photoionization model of G29.96-0.02 ( hereafter G29.96 ) , one of the brightest Galactic Ultra Compact H ii ( 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 much 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 } \sim 680 cm ^ { -3 } ) extended ( \sim 1 pc ) component surrounding a compact ( \sim 0.1 pc ) dense ( n _ { e } \sim 57000 cm ^ { -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 \sim 30 ^ { +2 } _ { -1 } kK whereas more recent non-LTE line blanketed atmospheres with stellar winds indicate somewhat higher values , T _ { \mathrm { eff } } Â \sim 32–38 kK .
This range in T _ { \mathrm { eff } } Â is compatible with all observational constraints , including near-infrared photometry and bolometric luminosity .
The range 33-36Â kK is also compatible with the spectral type O5-O8 determined by Watson & Hanson ( 1997 ) 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 10 ^ { 6 } Â yr , much older than the expected lifetime of UCHII regions .
Accurate gas phase abundances are derived with the most robust results being Ne/SÂ =Â 7.5 and N/OÂ =Â 0.43 ( 1.3 and 3.5 times the solar values , respectively ) .