A QUANTITATIVE STUDY OF THE 7.7-MU-M AND 11.3-MU-M EMISSION BANDS BASED ON IRAS/LRS SPECTRA

This paper presents a quantitative analysis of the emission bands at 7.7 and 11.3-mu-m in the mid-infrared spectra of HII regions, reflection nebulae, planetary nebulae and galaxies. The study is based on a sample of 113 sources whose spectra have been extracted from the IRAS/LRS data base. Relations of the intensity of the emission bands with physical conditions such as the excitation, and the relative contributions of emission bands and continuum around 10-mu-m to the mid-infrared and far-infrared fluxes are presented and analyzed. A strong linear correlation is found between the 7.7 and 11.3-mu-m band luminosities. The ratio of the 7.7 and 11.3-mu-m band intensities is equal to 8.3(-2.6)+3.8. The good correlation and the small dispersion (30%, compatible with the measurement errors) confirm, on a large sample of sources, that the carriers of the 7.7 and 11.3-mu-m emission features belong to the same family, most likely aromatic molecular species such as PAH molecules. The average contribution of these features to the mid-infrared luminosity is 50%. The values range from 5 to 90% where the smallest values are derived for sources with high excitation conditions. The contribution of the near-infrared emission bands to the total infrared luminosity is a few percent implying that one to five percent of the cosmic carbon is locked in the aromatic molecules. From simple energetical considerations, it is shown that the excitation of the 7.7 and 11.3-mu-m emission bands is dominated by ultraviolet photons in the range 365 to 91 nm. Finally, the importance of the underlying continuum is stressed since it accounts, in a number of the sources, for more than half of the mid-infrared luminosity. However, the precise nature of its carriers is still unclear.