2005A&A...431..183A

The luminosity of the central star of the compact HII regions of NGC 7538 was estimated from the solid angle of the IR sources subtended relative to the central star, and was found to be 5∼10 times as intense as that of IR sources. Under the single central star approximation, the luminosity gives a stellar UV photon rate N_U(*) (s^–1) of ∼3.0x10⁴⁸, ∼1.5x10⁴⁹, ∼5.1x10⁴⁹, and ∼1.7x10⁴⁷ for the compact HII regions of NGC 7538-IRS1(A/2), B, IRS2, and IRS3, respectively. N_U (*) and the observed electron density, n_e, provide the dust opacity of the ionizing photon, τ_Sd, for the optical path out to the Stroemgren sphere radius r_S, assuming a gas with standard dust content. Ionizing photon opacity over the same optical path but with the actual dust content τ_Sda is also derived from r_i/r_S, where r_i is the radius of the ionized sphere, which is estimated from N_U(*) and the observed volume emission measure n_e² (4πr_i³/3) (Spitzer, Physical Processes in the Interstellar Medium (New York: John Wiley and Sons, Inc.), 103]). An observational trend of γ{N_U(*)/4πr_i²}^1/2∼constant, where γ=τ_Sda/τ_Sd, was obtained for the 4 compact HII regions of the NGC 7538(N). Fourteen selected compact HII regions from data catalogued by VLA observations were examined for this trend, and a similar result was obtained. A limit of γ as 15≥γ≥0.1 was given for the 14 selected sources. The size of the dust-depleted cavity of the NGC 7538(N) suggested by Chini et al. (1986A&A...167..315C) coincides with that of the ionized sphere of the IRS2 of the region.