1 Introduction

The Orion-IRc2/KL region ( $d\approx 450$ pc) has traditionally been the prime source for studies in astrochemistry because of its extraordinarily rich spectra. Millimeter and submillimeter single-dish surveys show thousands of lines of nearly a hundred different molecules (e.g., Blake et al. 1987; Sutton et al. 1995; Schilke et al. 1997, 2001), whereas interferometer studies reveal intriguing chemical differentiation over scales of less than 2000 AU (e.g., Wright et al. 1996; Blake et al. 1996). In spite of this wealth of data, molecules such as CO₂ and C₂H₂, which are symmetric and thus lack a dipole moment, cannot be observed through rotational transitions at millimeter wavelengths. Moreover, CO₂ cannot be observed from Earth due to its high abundance in our atmosphere. Evans et al. (1991) have shown that important complimentary information can be obtained from vibration-rotation absorption lines toward bright mid-infrared sources. We present here spectra in the 13.5-15.5 $\mu$m range toward three positions in the core of the Orion molecular cloud, taken with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO), which are unhindered by the Earth's atmosphere. Absorption and emission features of CO₂, C₂H₂ and HCN are detected, which can be used to constrain the physical structure of this complex region and study the different chemistry of these molecules.

Millimeter studies have revealed a number of different physical and kinematic components in a 30'' region around the infrared source IRc2 (see Genzel & Stutzki 1989 for an overview). A clumpy hot core is located immediately adjacent to IRc2, with the clump exteriors currently being evaporated and/or ablated by the winds from the embedded massive young stellar object(s) (YSOs). This hot core is contained in a cavity, surrounded by a torus of dense, quiescent gas (the extended ridge) in the NE-SW direction. To the NW and SE, two shocked regions - called Peak 1 and Peak 2 - are revealed by bright H₂ 2 $\mu$m emission, indicating the positions where the high-velocity plateau or outflow runs into the ambient molecular cloud. Peak 1 is located $\sim$25 $^{\prime\prime}$NW and Peak  $2 \sim 24^{\prime\prime}$ SE of IRc2 (Beckwith et al. 1978). A cartoon of the core of the Orion molecular cloud indicating these different physical components is shown in Fig. 1.

The ISO-SWS beam ranges from $14''~\times~ 20''$ to $20''~ \times~ 27''$, so that these different regions can be separated spatially with the SWS. The 2.4-45.2 $\mu$m ISO-SWS spectrum toward IRc2 has been presented by van Dishoeck et al. (1998) and shows many features including emission lines of ionized species, PAHs, H₂, as well as absorption by interstellar ices and gas-phase species (see also Wright et al. 2000; González-Alfonso et al. 1998; Harwit et al. 1998). The full SWS spectrum toward Peak 1 has been presented by Rosenthal et al. (2000), whilst that toward Peak 2 is broadly similar (Wright 2000 and priv. comm. 2002). González-Alfonso et al. (1998) discuss the CO and H₂O vibrational emission bands toward Peak 1 and 2. Because of the weaker continuum, the lines are more prominent at these positions than toward IRc2, especially the vibration-rotation and pure-rotational lines of H₂.

In this paper, we focus on the ro-vibrational bands of gas-phase CO₂, C₂H₂, and HCN along the lines of sight toward IRc2, Peak 1 and Peak 2. CO₂ is predicted to be one of the more abundant carbon- and oxygen-bearing species and is detected ubiquitously in interstellar ices, with abundances of $\sim$15% with respect to H₂O ice, or $\sim$ 10^-5 - 10^-6 with respect to H₂(e.g. Gerakines et al. 1999). In contrast, the gas-phase CO₂abundance is surprisingly low, $\sim$10^-7, toward massive YSOs (van Dishoeck et al. 1996; van Dishoeck 1998; Dartois et al. 1998; Boonman et al. 2000). Since the abundances of many gas-phase species are enhanced toward IRc2, in particular those of species involved in the gas-grain chemistry (e.g., Blake et al. 1987; Charnley et al. 1992), it is important to investigate whether the CO₂ chemistry follows this trend. Observations of C₂H₂ and HCN are interesting because they are both significant in the carbon- and nitrogen chemistry, and because their excitation provides information on the physical conditions (Lahuis & van Dishoeck 2000). For HCN, rotational transitions in the submillimeter and ro-vibrational transitions in the infrared can be observed. In a number of massive YSOs the HCN abundance derived from submillimeter observations is a factor of $\sim$100 lower than that derived from infrared observations, suggesting a jump in its abundance in high temperature regions (Lahuis & van Dishoeck 2000; van der Tak et al. 1999, 2000; Boonman et al. 2001). There is still considerable debate whether such abundance jumps are mainly due to evaporation of ices, to quiescent high-temperature chemistry at a few hundred K or to shock chemistry at a few thousand K. The comparison of the Orion IRc2 and the shocked Peak 1 and Peak 2 results can provide constraints on the different models.

In Sect. 2, the ISO-SWS data reduction methods are discussed. Section 3 will present models for the HCN, C₂H₂, and CO₂absorption toward Orion IRc2. The inferred abundances are compared with those found toward other sources. In Sect. 4, the observations toward the shock positions Peak 1 and 2 are presented, and the excitation of the molecules is analyzed. Section 5 will compare the results for the three different positions and the conclusions are presented in Sect. 6.

Figure 1: Cartoon of the core of the Orion molecular cloud. The figure represents a cross section in the plane of the sky. The size and orientation of the ISO-SWS beam around 15 $\mu$m at the approximate positions of IRc2, Peak 1 and Peak 2 are indicated by the rectangular boxes (adapted from van Dishoeck et al. 1998).