Takao Nakagawa

Interstellar far-ultraviolet radiation (FUV : 912<λ<2000A) dominates the heating and many important aspects of physics and chemistry in some interstellar neutral regions. These regions are called "photodissociation regions." We have made far-infrared spectroscopic observations of many photodissociation regions with a 50cm Balloon-borne Infrared Telescope (BIRT) and a liquid helium cooled Fabry-Perot spectrometer. We also adopted a frequency switching method to observe diffuse compoents efficiently. The spectral lines we observed are [CII] (^2P_<3/2>-^2P_<1/2>. 158μm) and [OI] (^3P_1-^3P_2,63μm), both of which are the most important coolants in photodissociation regions. We have observed many kinds of photodissociation regions, including active star-forming regions (M17 and NGC6334), central regions of our Galaxy, and wide regions along the Galactic plane. Our observations showed that (1) photodissociation regions are very clumpy and that FUV penetrates deep into clouds, (2) photodissociation regions are ubiquitous, and (3) photodissociation regions are not minor interstellar components but their total mass in our Galaxy amounts to 40% of that of molecular gas.

We have made spectrocopic observations of the farinfrared [CII] (^2P_<3/2>&xmap;^2P_<1/2>, λ=157.74μm) line toward an active star-forming complex NGC 6334 with a balloon-borne 50cm telescope and a liquid-Helium cooled Fabry-Perot spectrometer. We have also obtained a two-dimensional map of the NGC 6334 complex by utilizing the newly developed frequency switching method. Strong [CII] emission has been detected throughout the region we have observed. Even a region (source V) with no compact HII region has been found to be a strong emitter of the [CII] line. We propose two hypotheses to explain the emission from source V as follows : (1) photodissociation regions around the source had evolved more rapidly than corresponding HII regions, and (2) the region around the source is very rich in B-type stars, which contribute significantly to [CII] emission but produce negligible HII regions.

Balloon observations of [CII] 158μm and [OI] 63μm from the M17 HII region and the molecular cloud complex are presented. Spatial distribution and energetics of the CII region are discussed. Around the M17 HII region, [OI] 63μm significantly contributes to the cooling of the CII region. Heating due to photoelectric effect of dust grains illuminated by the intence UV radiation can compensate for the cooling due to [CII] and [OI] lines. The CII region extends far beyond the HII region, which indicates the clumpy structure of the CII region. The existence of low-brightness, diffuse [CII] emission component toward M17 SW molecular cloud has been revealed. Distribution of the [CII] emission is quite similar to that of ^<12>CO J=1&xmap;0 emission. Possible energy source of the [CII] emission is discussed. Ratio of the [CII] luminosity to the total far-infrared luminosity of this diffuse component has been found to exceed that of the intence [CII] emission component around the HII region.

Diffuse [C II] line emission at 157.74μm has been observed in an extensive region along the Galactic plane, using a ballon-borne infrared telescope incorporated with a liquid helium cooled Fabry-Perot spectrometer. Strong emission has been ubiquitously detected all over the region between 1=30°and 51°. Most of the emission seems to be diffuse and unassociated with discrete HII regions. The longitude profile of the integrated [C II] intensity distribution is similar to those of IRAS 100μm and ^<12>CO (1-0) but not to that of HI 21cm. The large-scale diffuse [C II] emission probably comes from surfaces of molecular clouds which from diffuse photodissociation regions. The total [C II] luminosity of the Galaxy amounts to 2.6×10^7 L_&ofcir; which corresponds to about 0.35% of the total far-infrared luminosity.

Far-infrared spectroscopic observations have been made using a 50cm balloon-borne infrared telescope, a superfluid helium cooled Fabry-Perot spectrometer, and a frequency switching method, which is a newly developped observational technique. Remarkable two-dimensional [CII] line intensity maps constructed from the data show us that the [CII] 158μm line is strongly emitted not only from the photodissociation regions but also from their surrounding regions and from general region of the Galactic Plane. This fact indicates singly ionized carbon ions and carbon ioinizing UV photons should be abundantly distributed in interstellar clouds of the Galactic Plane. Since the energy emitted in form of the [CII] 158μm line can reach up to 0.5% of the total energy output of the clouds, the [CII] line must be one of the dominant coolants of the interstellar cloud.

Far-infrared H_2O emission spectra of the upper atmosphere have been obtained during a balloon flight in Alice Springs, Australia. Analyses show that the emission features of the telluric H_2O are fitted by a model atmosphere of 245K in mean temperature and of 1.7×10^<18>cm^<-2> in H_2O column density. We have also made an assessment for the telluric H_2O absorption via this model, thus anabling a correction of the observed astronmical spectra.