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 I] 63 μm, 145 μm, [N II] 122 μm, [C II] 158 μm, [Si II] 35 μm, and H_2 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 arcmin × 20 arcmin area of the nebula, including the optically bright 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 H II region and the molecular cloud which creates a typical photodissociation region (PDR). The [C II] 158 μm emission shows a good correlation with the [O I] 63 μm emission and peaks around the H II-molecular region interface. The correlated component has the ratio of [C II] 158 μm to [O I] 63 μm of about 2.8. We estimate from the correlation that about 80% of [C II] emission comes from the PDR in the Carina nebula. The photoelectric heating efficiency estimated from the ratio of the ([C II] 158 μm + [O I] 63 μm) intensity to the total far-infrared intensity ranges from 0.06 to 1.2%. [O I] 145 μm is detected marginally at 10 positions. The average ratio of [O I] 145 μm to [O I] 63 μm of these positions is about 0.09 ± 0.01 and is larger than model predictions. The observed [C II] 158 μm to [O I] 63 μm ratio indicates a relatively low temperature ( <500 K) of the gas, while the large [O I] 145 μm to 63 μm 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 I] 63 μm by foreground cold gas is taken into account. We suggest that absorption of [C II] 158 μm together with [O I] 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 [Si II] 35 μm emission has been detected over the observed area. It shows a good correlation with [N II] 122 μm, but the correlation with [O I] 63 μm is very weak, indicating that [Si II] 35 μm comes mainly from the diffuse ionized gas rather than the PDR. The ratio of [Si II] 35 μm to [N II] 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.

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.

Table 2 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/579