Vla observations of carbon radio recombination lines toward the H II region complex S88B

Abstract

We present high angular resolution VLA observations of the C92a, C110a, and C166a radio recombination lines of carbon from the region of massive star formation known as S88B. The observations reveal that the carbon emission arises from two distinct components that are intimately associated with the compact (S88B2) and cometary (S88B1) regions of ionized gas within the complex. The brighter carbon component has an angular size of D6A.6, an average line-center velocity of 21.0^0.5 km s~1, and an average line width of 5.1^1.0 km s~1; it is associated with the compact H II region. The second component has an angular size of D16A and is found projected toward the head of the cometary-like H II region. The average center velocity and width of the carbon line emission are 21.1^0.7 km s~1 and 5.1^1.7 km s~1, respectively. The spatial location and velocity of both carbon regions suggest that the emission arises in layers of photodissociated gas at the interface between the molecular cloud and the regions of ionized gas that are undergoing a champagne phase. From a model analysis of the dependence of the recombination line intensity with principal quantum number, we conclude that the carbon emission originates in warm photodissociated regions. The electron temperatures and electron densities of the photodissociated gas range between 400 and 600 K and between 40 and 80 cm~3, respectively. Further, we Ðnd that stimulated ampliÐcation of the background H II region continuum radiation contributes signiÐcantly to the carbon emission in both components. We also detected emission in sulfur radio recombination lines from both components. We Ðnd that the ratios of sulfur to carbon line intensities are considerable larger than the [S/C] cosmic abundance ratio and that they vary with principal quantum number, with values in the range between 0.3 and 0.6. We attribute the large values of the intensity ratios to depletion of carbon in the gas phase by a factor of D5 and the variations with principal quantum number to stimulated emission e†ects in a region of low electron density (neD3 cm~3) and low temperature (TeD50 K) that surrounds the C` region.