VLA OBSERVATIONS OF HYDROGEN AND HELIUM RECOMBINATION LINES FROM PARTIALLY AND FULLY IONIZED-GAS IN S88B

We present VLA observations of radio recombination lines of hydrogen ( H92 alpha, H110 alpha, and H166 alpha) and helium (He92 alpha) made to ward the S88B massive star forming region, which contains compact (S88 B2) and cometary (S88B1) regions of ionized gas. We find that the prof iles of the hydrogen line emission from the compact H II region exhibi t remarkable differences with principal quantum number. The H100 alpha profile is composed of two distinct components, a broad (Delta v simi lar to 31 km s(-1)) and a narrow (Delta v similar to 7 km s(-1)) compo nent, having similar intensities. The H92 alpha profile also shows the presence of the broad and narrow components, but the bulk of the line emission is dominated by the broad component. Emission in the H166 al pha line was not detected, to a limit of 4 mJy. From a model of the ob served line intensities and profiles we conclude that the emission fro m the narrow line corresponds to stimulated emission arising from a pa rtially ionized medium contiguous with, and along the line of sight to ward, the compact H II region. We find that the partially ionized gas has an electron temperature of similar to 800 K, an electron density o f similar to 250 cm(-3), and an emission measure of similar to 2 x 10( 3) pc cm(-6), and suggest that the most likely source of ionization is X-rays from stellar winds. The broad-line emission originates from an H II region with an electron temperature of similar to 10(4) K, an el ectron density of similar to 7.5 x 10(3) cm(-3), and an emission measu re of similar to 8 x 10(6) pc cm(-6). The profiles of the H92 alpha an d H110 alpha line emission from the cometary-like H II region show the presence of a single broad (similar to 25 km s(-1)) component. The in tensities and profiles of these lines are well reproduced by a model i n which the emission arises from a homogeneous, isothermal region of i onized gas with an electron density of similar to 4.8 x 10(3) cm(-3), an electron temperature of similar to 13,000 K, and an emission measur e of similar to 1 x 10(7) pc cm(-6). Emission in the helium line was d etected only toward the cometary B1 region. We find that the helium-to -hydrogen integrated intensity ratio is 0.08 for the B1 region, while for the B2 region we derive an upper limit of less than or equal to 0. 02. The low value of the observed He+/H+ abundance ratio of the compac t H II region B2 can be simply explained as due to the low effective t emperature of its ionizing source.