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

Abstract

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.