VLA Observations of Hydrogen and Helium Recombination Lines from Partially and Fully Ionized Gas in S88B

Abstract

We present VLA observations of radio recombination lines of hydrogen (H92α, H110α, and H166α) and helium (He92α) made toward the S88B massive star forming region, which contains compact (S88B2) and cometary (S88B1) regions of ionized gas. We find that the profiles of the hydrogen line emission from the compact H II region exhibit remarkable differences with principal quantum number. The H110α profile is composed of two distinct components, a broad (Δv ~ 31 km s-1) and a narrow (Δv ~ 7 km s-1) component, having similar intensities. The H92α 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 H166α line was not detected, to a limit of 4 mJy. From a model of the observed line intensities and profiles we conclude that the emission from the narrow line corresponds to stimulated emission arising from a partially ionized medium contiguous with, and along the line of sight toward, the compact H II region. We find that the partially ionized gas has an electron temperature of ~800 K, an electron density of ~250 cm-3, and an emission measure of ~2 × 103 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 ~104 K, an electron density of ~7.5 × 103 cm-3, and an emission measure of ~8 × 106 pc cm-6. The profiles of the H92α and H110α line emission from the cometary-like H II region show the presence of a single broad (~25 km s-1) component. The intensities and profiles of these lines are well reproduced by a model in which the emission arises from a homogeneous, isothermal region of ionized gas with an electron density of ~4.8 × 103 cm-3, an electron temperature of ~13,000 K, and an emission measure of ~1 × 107 pc cm-6. Emission in the helium line was detected 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 ≤0.02. The low value of the observed He+/H+ abundance ratio of the compact H II region B2 can be simply explained as due to the low effective temperature of its ionizing source.