Molecular abundance variations in interstellar clouds

Abstract

The abundances of several molecular species have been obtained for 13 regions in a variety of molecular clouds. The abundances were obtained by establishing a temperature and density model of the regions through the use of CO and H2CO observations. Then the abundances of HCO(+), H2CO, HNC, HCN, and (C-13)O were determined by comparing the model with the observed lines. The abundances of some species vary by up to three orders of magnitude from cloud to cloud. The (C-13)O abundance is less variable, but its abundance in dense regions is about 10 times less than values which are in current use. The regions in this sample appear to divide into two groups with regard to temperature, density, and abundance. The regions of high density and temperature have lower abundances. The low abundances found in this study require that hot dense regions have less CO than predicted by steady-state ion-molecule chemistry and that the cosmic-ray ionization rate be less than or approximately 5 x 10 to the-18th power per sec.