Ionization State Within interstellar clouds

Abstract

We have studied the chemical evolution in some interstellar clouds, focussing on their charge density. The objects that have been treated are of different physical and chemical structures. IC 63 is studied as one of the PDRs regions. TMC-1 and NGC 2264 are studied as examples of dense clouds. c; Ophiuchi is considered as a famous example of a diffuse cloud.

For each object the study includes the following points;

1- We discussed the main physical and chemical features of the object.

2-The calculated chemical abundances of atomic and molecular species are investigated and compared with observations. 3- The electron density and the dominant ionic species are also studied. The main conclusions are summarized in the following: In the photo-dominated region (IC 63 cloud), the chemical structure and ionizatiori state depend directly on the intensity of the incident UV radiation. The electron density is also affected by the incident UV radiation. It decreases gradually with the increase of the cloud depth. It ranges between 5.9 x 10"5,atthecloudsurface,and9.6x I0-9,atthecloud interiors. The ionic carbon (C+) in the outer region dominates the electron density, while in the deepest region, the ionic metals and some other ions (H+, CH2D+ and HCO+) are the most dominant. In dense clouds, we have studied the chemical evolution in contracting interstellar clouds. Two cloud objects are studied, TMC-1 and NGC 2264. Most of the resulting fractional abundances are in good agreement with the observations. As expected, most of the observed •species increase with c;, in particularly the ionic species, hence the electron density.

Most of the molecular species increase also with the increase of the initial abundance ofPAH, while the electron density decreases. The effect ofchanging the abundance of deuterium on