Laboratory and theoretical simulation of 3.4 μm spectra of hydrocarbons in interstellar sources

The presence of hydrocarbons in interstellar clouds and in some emission objects can be inferred from the appearance of spectral features near 3.4 mu m that are characteristic of CH2 and CH3 groups. While the 3.4 mu m band attributable to these hydrocarbons has been found to be similar in sources such as GC IRS 6E and CRL 618 (Chiar et al. 1998), there are significant variations in the relative amplitude of individual components in other sources such as NGC 7538 IRS9 (Allamandola et al. 1992). This indicates that the composition of these hydrocarbons may depend on ambient conditions in interstellar clouds. To investigate this possibility, we have analyzed observational IR spectra of GC IRS 6E, CRL 618, IRAS 05341+0852, and NGC 7538 IRS9 to extract spectral bands associated with CH2 and CH3 groups in each of these sources. These components are compared with the features that appear in laboratory absorption and emission spectra of hydrogenated amorphous carbon. It is found that significant differences exist in the CH2/CH3 ratio in individual sources. In particular, we find that CH3 groups are suppressed in dense cloud dust but that CH2 groups are still abundant. The properties of individual spectral components within the 3.4 mu m interstellar band are discussed and compared to laboratory and theoretical data on IR spectra of certain hydrocarbon molecules, as well as that of hydrogenated amorphous carbon. Simulation of 3.4 mu m spectra using a random covalent network model is shown to provide a useful way to extract structural and bonding information for specific chemical groups in interstellar material.