EDP Sciences
Free access
Volume 423, Number 2, August IV 2004
Page(s) 579 - 592
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20041063

A&A 423, 579-592 (2004)
DOI: 10.1051/0004-6361:20041063

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

M. Mizutani1, T. Onaka1 and H. Shibai2, 3

1  Department of Astronomy, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan
    e-mail: onaka@astron.s.u-tokyo.ac.jp
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

(Received 26 August 2003 / Accepted 11 May 2004)

We present the results of mapping observations with ISO of [ $\ion{O}{i}$] 63  $\mu$m, 145  $\mu$m, [ $\ion{N}{ii}$] 122  $\mu$m, [ $\ion{C}{ii}$] 158  $\mu$m, [ $\ion{Si}{ii}$] 35  $\mu$m, and H 2 9.66  $\mu$m line emissions for the Carina nebula, an active star-forming region in the Galactic plane. The observations were made for the central $40\arcmin \times 20\arcmin$ area of the nebula, including the optically bright $\ion{H}{ii}$ region and molecular cloud lying in front of the ionized gas. Around the center of the observed area is the interface between the $\ion{H}{ii}$ region and the molecular cloud which creates a typical photodissociation region (PDR). The [ $\ion{C}{ii}$] 158  $\mu$m emission shows a good correlation with the [ $\ion{O}{i}$] 63  $\mu$m emission and peaks around the $\ion{H}{ii}$-molecular region interface. The correlated component has the ratio of [ $\ion{C}{ii}$] 158  $\mu$m to [ $\ion{O}{i}$] 63  $\mu$m of about 2.8. We estimate from the correlation that about 80% of [ $\ion{C}{ii}$] emission comes from the PDR in the Carina nebula. The photoelectric heating efficiency estimated from the ratio of the ([ $\ion{C}{ii}$] 158  $\mu$m + [ $\ion{O}{i}$] 63  $\mu$m) intensity to the total far-infrared intensity ranges from 0.06 to 1.2%. [ $\ion{O}{i}$] 145  $\mu$m is detected marginally at 10 positions. The average ratio of [ $\ion{O}{i}$] 145  $\mu$m to [ $\ion{O}{i}$] 63  $\mu$m of these positions is about $0.09 \pm 0.01$ and is larger than model predictions. The observed [ $\ion{C}{ii}$] 158  $\mu$m to [ $\ion{O}{i}$] 63  $\mu$m ratio indicates a relatively low temperature ( < 500 K) of the gas, while the large [ $\ion{O}{i}$] 145  $\mu$m to 63  $\mu$m 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 [ $\ion{O}{i}$] 63  $\mu$m by foreground cold gas is taken into account. We suggest that absorption of [ $\ion{C}{ii}$] 158  $\mu$m together with [ $\ion{O}{i}$] 63  $\mu$m by overlapping PDRs, in which the heating via PAHs is suppressed due to the charge-up effect, may resolve the discrepancy. Quite strong [ $\ion{Si}{ii}$] 35  $\mu$m emission has been detected over the observed area. It shows a good correlation with [ $\ion{N}{ii}$] 122  $\mu$m, but the correlation with [ $\ion{O}{i}$] 63  $\mu$m is very weak, indicating that [ $\ion{Si}{ii}$] 35  $\mu$m comes mainly from the diffuse ionized gas rather than the PDR. The ratio of [ $\ion{Si}{ii}$] 35  $\mu$m to [ $\ion{N}{ii}$] 122  $\mu$m 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

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