FAR-INFRARED LINES FROM G45.13+0.14-A AND K3-50-A - DENSITY-FLUCTUATIONS IN COMPACT H-II REGIONS

The far-infrared lines of [O III] 51.8 and 88.4 μm, [N III] 57.3 μm, [S III] 33.5 μm, and [Ne III] 36.0 μm have been measured in the compact H II regions G45.13 + 0.14 A and K3-50 A. These measurements were made with the facility cooled grating spectrometer on flights of NASA's Kuiper Airborne Observatory. For both sources, the ratio of the two O++ lines indicates an electron density Ne ∼ 103 cm-3. For K3-50 A, this is a factor of 10 to a hundred times lower than the density determined from near-infrared and optical line observations of lower excitation species and from radio measurements of the peak continuum emission. A comparison with other far-infrared measurements for both sources shows that the lower excitation, higher critical density S++ lines originate from higher density material than do the O++ lines. Detailed, spherically symmetric models for both sources are presented. These models require clumping, different abundances than the Orion Nebula, and an enhancement in the standard Kurucz stellar atmospheres at energies E > 41 eV to obtain reasonable agreement with the measurements. The average nitrogen-to-oxygen abundance ratio for these two H II regions is N/O ∼ 0.2, in agreement with other far-infrared studies at Galactic radii ≳ 6 kpc.