Microwave spectroscopic mapping of gaseous nebulae. II. Observations of hydrogen in NGC 7538

The R 94a recombination line is mapped at 12 locations across the galactic R ii region NGC 7538. The observed a-line parameters are used to determine some of the physical and kinematic properties of the nebula. The radial-velocity gradient implies a nebular rotation period of 10 years. The LTE electron temperature varies between 7000 and 12,000 K, and in the outer portion of the nebula appears to increase with distance from the center. The derived electron temperature at this frequency (7.8 GRz) is close to the gas-kinetic temperature, at least in the center of the source. Observation of helium, previously undetected in NGC 7538, yields a relative abundance n(Re)+i n(R+) 0.06. Subject headings: abundances, nebular - nebulae - radio lines