Masahiko Mizutani

Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium

Using the mid-infrared spectrometer on board the Infrared Telescope in Space and the low-resolution grating spectrometer (PHT-S) on board the Infrared Space Observatory, we obtained 820 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 μm, 8.6/7.7 μm, 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 incident radiation intensity differs by a factor of 1500. Comparing our results with the polycyclic aromatic hydrocarbons (PAHs) model for the UIR band carriers, we find that PAHs in the DIM have no systematic variation in their size distribution, their degree of dehydrogenation is independent of the strength of UV radiation field, and they are mostly ionized. The finding that PAHs in the DIM with low UV radiation field strength are mostly ionized is incompatible with past theoretical studies, in which a large fraction of neutral PAHs is predicted in this kind of environment. A plausible resolution of this discrepancy is that the recombination coefficients for electron and large PAH positive ion are by at least an order of magnitude less than those adopted in past theoretical studies. Because of the very low population of neutral state molecules, photoelectric emission from interstellar PAHs is probably not the dominant source of heating of the diffuse interstellar gas. The present results imply constant physical and chemical properties of the carriers of the UIR emission bands in the DIM covering the central and disk regions of the Galaxy, which could help in the identification of the carriers.

Origin of diffuse C II 158 micron and Si II 35 micron emission in the Carina nebula

We present the results of mapping observations with ISO of (O  )6 3µm, 145 µm, (N ) 122 µm, (C ) 158 µm, (Si  )3 5µm, and H2 9.66 µm line emissions for the Carina nebula, an active star-forming region in the Galactic plane. The observations were made for the central 40 � × 20 � area of the nebula, including the optically bright H  region and molecular cloud lying in front of the ionized gas. Around the center of the observed area is the interface between the H  region and the molecular cloud which creates a typical photodissociation region (PDR). The (C ) 158 µm emission shows a good correlation with the (O  )6 3µm emission and peaks around the H -molecular region interface. The correlated component has the ratio of (C ) 158 µ mt o (O )6 3µm of about 2.8. We estimate from the correlation that about 80% of (C ) emission comes from the PDR in the Carina nebula. The photoelectric heating efficiency estimated from the ratio of the ((C ) 158 µm + (O  )6 3µm) intensity to the total far-infrared intensity ranges from 0.06 to 1.2%. (O ) 145 µm is detected marginally at 10 positions. The average ratio of (O ) 145 µ mt o (O )6 3µm of these positions is about 0.09 ± 0.01 and is larger than model predictions. The observed (C ) 158 µ mt o (O )6 3µm ratio indicates a relatively low temperature (<500 K) of the gas, while the large (O ) 145 µ mt o 63µm ratio suggests a high temperature (∼1000 K). This discrepancy cannot be accounted for consistently by the latest PDR model with the efficient photoelectric heating via polycyclic aromatic hydrocarbons (PAHs) even if absorption of (O  )6 3µm by foreground cold gas is taken into account. We suggest that absorption of (C ) 158 µm together with (O  )6 3µm by overlapping PDRs, in which the heating via PAHs is suppressed due to the charge-up effect, may resolve the discrepancy. Quite strong (Si  )3 5µm emission has been detected over the observed area. It shows a good correlation with (N ) 122 µm, but the correlation with (O  )6 3µm is very weak, indicating that (Si  )3 5µm comes mainly from the diffuse ionized gas rather than the PDR. The ratio of (Si  )3 5µ mt o (N) 122 µm is about 8 and Si of about 50% of the solar abundance relative to N should be present in the gas phase. The present results suggest that efficient dust destruction takes place and a large fraction of Si returns to the gas in the Carina star-forming region.

Detection of highly-ionized diffuse gas in the Galactic plane

We obtained maps of the central

ISO spectroscopic observations of Carina region

40\arcmin \times 20\arcmin

Diffuse emissions in Carina nebula (Mizutani+, 2004)

region of the Carina nebula, an active star forming region in the southern Milky Way, in various emission lines in the spectral range 43–197 μ m with the LWS on board the ISO. This paper reports on the results of [

The UIR Emission Bands and the FIR Emission Observed by IRTS and ISO

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Characterization of the Unidentified Infrared Emission Bands in the Diffuse Interstellar Medium

] 52, 88 μ m, [

Origin of [Siii] 35 mum and [Cii] 158 mum emission in the Carina nebula

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Unidentified Infrared Emission Bands and Far-Infrared Emission in Various Interstellar Medium

] 57 μ m, and [

Detection of extended low-density ionized gas around the Carina nebula

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