Physics
Scientific paper
Nov 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000esasp.456...59c&link_type=abstract
ISO beyond the peaks: The 2nd ISO workshop on analytical spectroscopy. Eds. A. Salama, M.F.Kessler, K. Leech & B. Schulz. ESA-SP
Physics
2
Infrared: Ism: Lines And Bands
Scientific paper
Using the Mid-Infrared Spectrometer on board the Infrared Telescope in Space and the low-resolution grating spectrometer on board the Infrared Space Observatory, we obtained 850 mid-infrared (5 - 12 μm) spectra of the diffuse interstellar medium (DIM) in the Galactic center, W51, and Carina nebula regions. These spectra indicate that the emission is dominated by the unidentified infrared (UIR) emission bands at 6.2, 7.7, 8.6, and 11.2 μm. The relative band intensities (6.2/7.7, 8.6/7.7, and 11.2/7.7 μm) were derived from these spectra, and no systematic variation in these ratios was found in our observed regions in spite of the fact that the total far infrared intensities differ by a factor of 100 (or G0 ≈ 5 - 4000, with G0 being the integrated radiation intensity in units of the solar neighborhood value). Our results imply constant physical and chemical properties of the carriers of the UIR emission bands in the DIM. Comparing our results with the polycyclic aromatic hydrocarbons (PAHs) model, we found that the candidate PAHs are mostly or completely ionized in the DIM. The finding that diffuse interstellar PAHs are mostly ionized is incompatible with past theoretical studies, in which a large fraction of neutral PAHs is predicted in low UV radiation environments such as the DIM. One possible resolution of this discrepancy is that the recombination coefficients for electron-PAH ion interactions are at least an order of magnitude less than those adopted in the past theoretical studies. If the PAH model requires a mixture of PAHs with different carbon atom sizes and degree of thermodynamical stabilities to fit the observed UIR emission bands, it may have difficulty to keep the constancy of this complex mixture of PAHs in the DIM covering a wide range of UV radiation field strengths. Larger PAHs may solve this problem.
Chan Kai Wing
Mizutani Masashi
Nakagawa Takao
Okuda Haruyuki
Okumura Ken'ichi
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