The following document lists the file abstract/TNAKAGAW_GCFIR.abs
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We propose to make multi-line spectroscopic observations of the selected areas in the Galactic center region on a large scale (+- 5 degrees) to reveal the origins of the conspicuous [CII] 158 micron line deficit in the Galactic center (GC). Our balloon observations revealed that the [CII] line is surprisingly weak toward the GC, and the ratio of the [CII] line emission to far-infrared continuum (I_[CII]/I_FIR) shows a large dip at the central several hundred parsecs of the GC. The results indicate the low abundance of the C+ ions in the GC area. We hypothesized that this low abundance is mainly due to soft UV radiation with fewer C-ionizing photons. This soft radiation field, together with the pervasively high molecular gas density, makes the molecular self-shielding more effective, and raises temperature of molecular gas at the C+/C/CO transition zone. Hence our hypothesis can naturally explain not only the weak [CII] line but also the pervasive existence of warm molecular clouds in the GC. Since our observed [CII] line alone is not sufficient to determine the physical conditions and to check our hypothesis, we propose here multi-line spectroscopic observations. The multi-line spectroscopy by the ISO will not only reveal the origins of this [CII] line deficit and but also to reveal the energy budget of extraordinary interstellar clouds in the GC. We are now making various observations of Photo Dissociation Regions (PDRs) using the ISO. The current program is a natural extension of our systematic study of PDRs. Hence, in this proposal, more time is being requested for TONAKA.WDISM_1 and TONAKA.WDISM_2.