Observational Constraints on Solar Nebula Nitrogen Chemistry: N 2/NH 3

Observations of N2(+) and NH2 in Comet Halley and N2H(+) and NH3 in nine Galactic star-forming regions are used to determine the average N2/NH3 abundance ratio in these objects. For Comet Halley, N2/NH3 of about 0.1 is found, and for the quiescent gas in a sample of star-forming regions, N2/NH3 of about 170 +/- 100. The cometary N2/NH3 ratio corrected for gas-phase elemental nitrogen depletion in Comet Halley indicates that the gas component in the comet-forming region of the solar nebula was N2/NH3 of about 4. It is concluded that more realistic models which include condensation fractionation effects are required before the N2 and NH3 abundances in comet volatiles can be related to star-forming regions.