2012ApJ...744...68M

We present the results of near- to mid-infrared slit spectroscopic observations (2.55-13.4 µm) of the diffuse emission toward nine positions in the Large Magellanic Cloud with the infrared camera on board AKARI. The target positions are selected to cover a wide range of the intensity of the incident radiation field. The unidentified infrared bands at 3.3, 6.2, 7.7, 8.6, and 11.3 µm are detected toward all the targets and ionized gas signatures; hydrogen recombination lines and ionic forbidden lines are detected toward three of them. We classify the targets into two groups: those without the ionized gas signatures (Group A) and those with the ionized signatures (Group B). Group A includes molecular clouds and photodissociation regions, whereas Group B consists of H II regions. In Group A, the band ratios of I_3.3 µm_/I 11.3 µm_, I_6.2 µm_/I11.3 µm_, I_7.7 µm_/I11.3 µm_, and I_8.6 µm_/I11.3 µm_ show positive correlation with the IRAS and AKARI colors, but those of Group B do not follow the correlation. We discuss the results in terms of the polycyclic aromatic hydrocarbon (PAH) model and attribute the difference to the destruction of small PAHs and an increase in the recombination due to the high electron density in Group B. In the present study, the 3.3 µm band provides crucial information on the size distribution and/or the excitation conditions of PAHs and plays a key role in the distinction of Group A from B. The results suggest the possibility of the diagram of I_3.3 µm_/I11.3 µm_ versus I_7.7 µm_/I11.3 µm_ as an efficient diagnostic tool to infer the physical conditions of the interstellar medium.