2004A&A...423..579M

We present the results of mapping observations with ISO of [OI]63µm, 145µm, [NII]122µm, [CII]158µm, [SiII]35µm, and H₂9.66µm line emissions for the Carina nebula, an active star-forming region in the Galactic plane. The observations were made for the central 40'x20' area of the nebula, including the optically bright HII region and molecular cloud lying in front of the ionized gas. Around the center of the observed area is the interface between the HII region and the molecular cloud which creates a typical photodissociation region (PDR). The [CII]158µm emission shows a good correlation with the [OI]63µm emission and peaks around the HII-molecular region interface. The correlated component has the ratio of [CII]158µm to [OI]63µm of about 2.8. We estimate from the correlation that about 80% of [CII] emission comes from the PDR in the Carina nebula. The photoelectric heating efficiency estimated from the ratio of the ([CII]158µm+[OI]63µm) intensity to the total far-infrared intensity ranges from 0.06 to 1.2%. [OI]145µm is detected marginally at 10 positions. The average ratio of [OI]145µm to [OI]63µm of these positions is about 0.09±0.01 and is larger than model predictions. The observed [CII]158µm to [OI]63µm ratio indicates a relatively low temperature (<500K) of the gas, while the large [OI]145µm to 63µm ratio suggests a high temperature (∼1000K). 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 [OI]63µm by foreground cold gas is taken into account. We suggest that absorption of [CII]158µm together with [OI]63µm by overlapping PDRs, in which the heating via PAHs is suppressed due to the charge-up effect, may resolve the discrepancy. Quite strong [SiII]35µm emission has been detected over the observed area. It shows a good correlation with [NII]122µm, but the correlation with [OI]63µm is very weak, indicating that [SiII]35µm comes mainly from the diffuse ionized gas rather than the PDR. The ratio of [SiII]35µm to [NII]122µm 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.