Observational Studies of Interstellar and Solar Nebula Nitrogen Chemistry.
Abstract
Despite the high cosmic abundance of nitrogen, chemical models of nitrogen-bearing species in the interstellar medium and the primordial solar nebula have not been constrained significantly by observations. Essentially nothing is known about the abundance of the simplest and most stable nitrogen compound N_2 in astronomical sources. Ion-molecule chemistry models of interstellar clouds predict molecular ions to be formed primarily in cold, quiescent gas. The only well-studied molecular ion in dense clouds, HCO^+, is very abundant in regions of energetic outflows from young stars, contrary to model predictions. Therefore, observations of another molecular ion are needed to better understand ion-molecule models. Here we present millimeter-wave observations of rm N_2H^+ toward galactic molecular clouds. These data can provide constraints on both the nitrogen and ion-molecule chemistry in the interstellar medium. Measured excitation temperatures of rm N_2H^+ were found to be low (T ex ~ 5-30K), and the line profiles narrow (Delta v_{1/2} ~ 3-5 km s^{-1}), which imply that rm N_2H^+ is produced primarily in quiescent gas, in agreement with ion-molecule models. In addition, a map of rm N_2H^+ emission toward Orion-KL revealed for the first time that two distinct clouds in collision may be triggering the star formation processes in this star-forming region. Abundances calculated for rm N_2H^+ imply that the quiescent gas in dense clouds has reached a steady-state chemistry. Abundances of interstellar N_2 were derived for the first time directly from the rm N _2H^+ data, and are ~ 100 times lower than those predicted by ion-molecule models. These observations provide the first critical diagnostic for the extent of chemical processing in the early solar nebula. The abundance ratio, N_2 /NH_3, in star-forming environments determines the initial conditions for chemical models of the early solar nebula. The N_2/NH _3 ratio observed in comets provides constraints on the extent of chemical processing prior to solid body accumulation and solar nebula dissipation. The N_2/NH_3 ratios calculated for six star-forming regions are ~100 times lower than the initial ratio generally assumed in protosolar nebula models. Furthermore, the average N_2/NH_3 ratio for the sample of star-forming regions is approximately equal to that observed for comet Halley, which suggests that little nitrogen processing occurred in the outer protosolar nebula.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 1991
- Bibcode:
- 1991PhDT.........4W
- Keywords:
-
- Physics: Astronomy and Astrophysics;
- Interstellar Chemistry;
- Interstellar Matter;
- Molecular Clouds;
- Molecular Ions;
- Nitrogen Compounds;
- Planetary Nebulae;
- Star Formation;
- Astronomical Models;
- Cold Gas;
- K Lines;
- Millimeter Waves;
- Steady State;
- Astronomy