Astron. Astrophys. 358, 708-716 (2000)

2. Observations and data reduction

Imaging spectrophotometry was performed with the 3232 element mid-IR camera (CAM) on board the ISO satellite, using the Circular Variable Filters (CVFs) (see Cesarsky et al. 1996a for a complete description). The observations employed the 6" per pixel field-of-view of CAM. Full scans of the two CVFs in the long-wave channel of the camera were performed with both increasing and decreasing wavelength. The results of these two scans are almost identical, showing that the transient response of the detector was only a minor problem for these observations. The total wavelength range covered is 5.15 to 16.5 µm  and the wavelength resolution . 100.28 s exposures were added for each step of the CVF, and 7 more at the first step in order to limit the effect of the transient response of the detectors. The total observing time was about 1 hour. The raw data were processed as described in Cesarsky et al.  (1996b), with improvements described by Starck et al.  (1998) using the CIA software ¹. The new transient correction described by Coulais & Abergel (1998) has been applied but the corrections introduced are minimal, as mentioned above. The bright star  Ori A is visible in the maps of several spectral components and has been used to re-position the data cube. This involved a shift of only 2". The final positions are likely to be good to 3" (half a pixel).

All the maps presented here were obtained from the CVF data cube and have approximately the same resolution: namely, 6" pixels at the short wavelengths increasing to about 8" at 15 µm ; see Appendix C for more details.

In several of these maps a faint emission can be seen on the south-east part of the ISOCAM field of view. This feature does not correspond to anything conspicuous in published images of the region, in particular in the near-IR images of Marconi et al.  (1998). It is a spurious feature due to multiple reflections of the strong Trapezium between the detector and the CVF filter wheel, as shown by the ISOCAM ray tracing studies of Okumura (1999).

The complete SWS scan (2.4-46 µm) was reduced with the latest version of SWS-IA running at the Institut d'Astrophysique Spatiale. Calibration files version CAL-030 were used.

Fig. 3 presents the SWS spectrum () obtained inside the H II  region at the position indicated on Fig. 2. Fig. 3 also shows the comparison of the SWS spectrum with that of the CAM-CVF pixels averaged in the SWS aperture. The agreement between these spectra is excellent, well within 20 percent for the continuum.

Fig. 3. The SWS spectrum (full line) compared to the CAM-CVF spectrum (dotted line). In this latter case, all the CAM pixels falling in the SWS aperture have been co-added.

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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