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# The fine-structure transition of CII at 157.7 m (between levels 2P3/2 and 2P1/2 ) represents the important way for cooling of gas in HI regions.

The fine-structure transition of CII at 157.7 µm (between levels 2P3/2 and 2P1/2 ) represents the important way for cooling of gas in HI regions. The CII (2s22p) 2P3/2 level can be excited by e − collisions with the CII ion in the (2s22p) 2P1/2 level. This is followed by radiative de-excitation producing the 157.7 µm photon which then escapes from the region. The net result is conversion of e− kinetic energy into radiative energy which is lost from the region. The process therefore cools the region.

(a) Typical warm neutral regions have a kinetic temperature of T = 6000 K. If the region is in TE what is the ratio of densities of CII ions in the (2s22p) 2P3/2 and (2s22p) 2P1/2 levels? Why is the actual number density ratio likely to be different from this estimate?

(b) Assuming that a HI region has a temperature of 1000 K, what would be the shape of the resultant emission from a collection of radiating CII ions? Give an analytic expression.

(c) Calculate the full width at half maximum of the line and express the result in both wavelength and velocity units