Observations of the Near- to Mid-Infrared Unidentified Emission Bands in the Interstellar Medium of the Large Magellanic Cloud

The Astrophysical Journal (Impact Factor: 6.28). 09/2011; 744(1). DOI: 10.1088/0004-637X/744/1/68
Source: arXiv

ABSTRACT We present the results of near- to mid-infrared slit spectroscopic
observations (2.55--13.4 um) of the diffuse emission toward nine positions in
the Large Magellanic Cloud with the Infrared Camera (IRC) 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 um are detected toward all the targets, and ionized gas signatures:
hydrogen recombination lines and ionic forbidden lines 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 photo-dissociation regions, whereas Group B consists of
HII regions. In Group A, the band ratios of I(3.3)/I(11.3), I(6.2)/I(11.3),
I(7.7)/$I(11.3) and $I(8.6)/$I(11.3) 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 um 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)/I(11.3) v.s. $I(7.7)/$I(11.3) as an efficient diagnostic tool
to infer the physical conditions of the interstellar medium.

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