2 Observations and reduction

The spectra toward Orion IRc2, Peak 1 and Peak 2 from 12.0-16.5 $\mu$m were made with the ISO-SWS grating. IRc2 was observed on September 6 1997 (revolution 660) using the SWS06 observing mode centered at $\alpha$(1950 $)= 05^{\rm h} 32^{\rm m} 46.8^{\rm s}$, $\delta$(1950 $)= -05^{\circ} 24^\prime 25^{\prime\prime}$, which is about 1 $^{\prime\prime}$ S and 3 $^{\prime\prime}$ W of the IRc2 position listed by Gezari (1992). The Peak 1 spectrum was taken at the position $\alpha$(1950 $)= 05^{\rm h} 32^{\rm m} 46.3^{\rm s}$, $\delta$(1950 $)= -05^{\circ} 24^\prime 2^{\prime\prime}$ in the SWS01 speed 4 observing mode, on October 3, 1997 (revolution 687). Finally, on February 25 1998 (revolution 833) the spectrum toward Peak 2 was taken using the SWS01 speed 4 observing mode at the position $\alpha$(1950 $)= 05^{\rm h} 32^{\rm m} 48.4^{\rm s}$, $\delta$(1950 $)= -05^{\circ} 24^\prime 34^{\prime\prime}$, $\sim$1 $^{\prime\prime}$ E of the position listed by Beckwith et al. (1978). The beam size in this wavelength range is 14 $^{\prime\prime} \times 27^{\prime\prime}$, and was oriented $\sim$7$^{\circ}$ and $\sim$11$^{\circ}$ in NW-SE direction for IRc2 and Peak 2 respectively. These two beams do not overlap; however, the infrared source BN falls within the IRc2 beam (see sketch of region in Fig. 1). The Peak 1 beam, which has the same size and is orientated $\sim$6$^{\circ}$ in NE-SW direction, partially overlaps with the IRc2 beam, but does not contain the BN object at the observed wavelength range.

Data reduction was done within the ISO-SWS Interactive Analysis System SIA using the ISO Off-line Processing (OLP version 10) software modules and calibration files (see Lahuis et al. 1998 and Wieprecht et al. 2001 for a description of the SIA system and its relation to the ISO OLP system). For the SWS01 observations the Standard Processed Data (SPD) were re-derived to create spectra at full grating resolution with some loss in signal to noise. This software has been developed at the Dutch ISO Data Analysis Centre (DIDAC) and is based on the OLP software and calibration. It will become available within SIA and OSIA for general use. Instrumental fringes have been minimized when applying the Instrumental Spectral Response Function (RSRF) by RSRF matching to allow for offsets in the wavelength calibration and differences in resolution between the data and the RSRF. The remaining fringe residuals after the RSRF calibration were removed using a robust iterative sine fitting method based on an approximated Fabry-Pérot model (see Lahuis & van Dishoeck 2000 and Kester et al. 2001). All spectra have been rebinned to a spectral resolving power of 3500, twice the instrumental resolution. The final spectra have a typical S/N ratio on the continuum of 50-100.

Figure 2: Comparison of the 13.5-14.3 $\mu$m spectra toward the massive young stellar objects Orion-IRc2 and AFGL 2591. The HCN and C2H2 Q-branches toward Orion-IRc2 are much narrower than those toward AFGL 2591, and the absence of the hot bands at 13.7 and 14.0 $\mu$m indicates that less warm gas is probed ( $T_{{\rm ex}}<400$ K). The AFGL 2591 spectrum is adapted from Lahuis & van Dishoeck (2000).

  

Table 1: Model parameters for the absorption toward Orion-IRc2^a.

^a

Filling in by emission has not been included, but it is only important for HCN. For CO₂
and C₂H₂ the effect is $\la$25%, which is within the error bars (see text).

^b

With 62% BN continuum subtracted (see text).