Department of Astronomy, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan e-mail: firstname.lastname@example.org
2 Division of Particle and Astrophysical Science, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
3 Institute of Advanced Research (IAR), Nagoya University, Nagoya 464-8602, Japan
Accepted: 11 May 2004
We present the results of mapping observations with ISO of  63 μm, 145 μm,  122 μm,  158 μm,  35 μm, 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 area of the nebula, including the optically bright region and molecular cloud lying in front of the ionized gas. Around the center of the observed area is the interface between the region and the molecular cloud which creates a typical photodissociation region (PDR). The  158 μm emission shows a good correlation with the  63 μm emission and peaks around the -molecular region interface. The correlated component has the ratio of  158 μm to  63 μm of about 2.8. We estimate from the correlation that about 80% of  emission comes from the PDR in the Carina nebula. The photoelectric heating efficiency estimated from the ratio of the ( 158 μm +  63 μm) intensity to the total far-infrared intensity ranges from 0.06 to 1.2%.  145 μm is detected marginally at 10 positions. The average ratio of  145 μm to  63 μm of these positions is about and is larger than model predictions. The observed  158 μm to  63 μm ratio indicates a relatively low temperature (<500 K) of the gas, while the large  145 μm to 63 μm ratio suggests a high temperature (~ 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  63 μm by foreground cold gas is taken into account. We suggest that absorption of  158 μm together with  63 μm by overlapping PDRs, in which the heating via PAHs is suppressed due to the charge-up effect, may resolve the discrepancy. Quite strong  35 μm emission has been detected over the observed area. It shows a good correlation with  122 μm, but the correlation with  63 μm is very weak, indicating that  35 μm comes mainly from the diffuse ionized gas rather than the PDR. The ratio of  35 μm to  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.
Key words: infrared: ISM / ISM: abundances / ISM: individual objects: Carina nebula / ISM: lines and bands / ISM: dust, extinction
Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the United Kingdom) and with the participation of ISAS and NASA.
© ESO, 2004