VLA OBSERVATIONS OF CARBON RADIO RECOMBINATION LINES TOWARD THE H-II REGION COMPLEX S88B

We present high angular resolution VLA observations of the C92 alpha, C110 alpha, and C166 alpha radio recombination lines of carbon from th e region of massive star formation known as S88B. The observations rev eal 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 compo nent has an angular size of similar to 6''.6, an average line-center v elocity 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 seco nd component has an angular size of similar to 16 degrees and is found projected toward the head of the cometary-like H II region. The avera ge 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 locat ion and velocity of both carbon regions suggest that the emission aris es in layers of photodissociated gas at the interface between the mole cular cloud and the regions of ionized gas that are undergoing a champ agne phase. From a model analysis of the dependence of the recombinati on line intensity with principal quantum number, we conclude that the carbon emission originates in warm photodissociated regions. The elect ron temperatures and electron densities of the photodissociated gas ra nge between 400 and 600 K and between 40 and 80 cm(-3), respectively. Further, we find that stimulated amplification of the background H II region continuum radiation contributes significantly to the carbon emi ssion in both components. We also detected emission in sulfur radio re combination lines from both components. We find that the ratios of sul fur to carbon line intensities are considerable larger than the [S/C] cosmic abundance ratio and that they vary with principal quantum numbe r, with values in the range between 0.3 and 0.6. We attribute the larg e values of the intensity ratios to depletion of carbon in the gas pha se by a factor of similar to 5 and the variations with principal quant um number to stimulated emission effects in a region of low electron d ensity (n(e) similar to 3 cm(-3)) and low temperature (T-e similar to 50 K) that surrounds the C+ region.