Chemical Transitions for Interstellar C 2 and CN in Cloud Envelopes
Abstract
Observations were made of absorption from CH, C2, and CN toward moderately reddened stars in Sco, OB2, Ceo OB3, and Taurus/Auriga. For these directions, most of the reddening is associated with a single cloud complex, for example, the rho Ophiuchus molecular cloud, and as a result, the observations probe moderately dense material. When combined with avaliable data for nearby directions, the survey provides the basis for a comprehensive analysis of the chemistry for these species. The chemical transitions affecting C2 and CN in cloud envelopes were analyzed. The depth into a cloud at which a transition takes place was characterized by tauuv, the grain optical depth at 1000 A. One transition at tauuv approx. = 2, which arises from, the conversion of C(+) into CO, affects the chemistries for both molecules because of the key role this ion plays. A second one involving production terms in the CN chemistry occurs at tauuv of approx. = 3; neutral reactions which C2 and CH is more important at larger values for tauuv. The transition from photodissociation to chemical destruction takes place at tauuv approx. = 4.5 for C2 and CN. The observational data for stars in Sco OB2, Cep OB3, and Taurus/Auriga were studied with chemical rate equations containing the most important production and destruction mechanisms. Because the sample of stars in Sco OB2 includes sight lines with Av ranging from 1-4 mag, sight lines dominated by photochemistry could be analyzed separately from those controlled by gas-phase destruction. The analysis yielded values for two poorly known rate constants for reactions involved in the production of CN; the reactions are C2 + N yields CN + C and C(+) + NH yields all products. The other directions were analyzed with the inferred values. The predicted column densities for C2 and CN agree with the observed values to better than 50%, and in most instances 20%. When combining the estimates for density and temperature derived from chemical modeling and molecular excitation for a specific cloud, such as the rho Ophiuchus molecular cloud, the portion of the cloud envelope probed by C2 and CN absorption was found to be in pressure equilibrium.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 1994
- DOI:
- 10.1086/173930
- Bibcode:
- 1994ApJ...424..772F
- Keywords:
-
- Carbon;
- Cyanogen;
- Infrared Astronomy;
- Infrared Spectra;
- Interstellar Extinction;
- Interstellar Gas;
- Light (Visible Radiation);
- Methylidyne;
- Molecular Clouds;
- Molecular Physics;
- Astronomical Models;
- Carbon Monoxide;
- Density (Mass/Volume);
- Photochemical Reactions;
- Photodissociation;
- Spectrometers;
- Temperature;
- Astrophysics;
- ISM: CLOUDS;
- ISM: MOLECULES;
- MOLECULAR PROCESSES