Physical and chemical structure of the Orion Bar

David Jansen
Michiel Hogerheijde
Marco Spaans
Ewine van Dishoeck

Introduction

The Orion Bar is the bright ionization front in between the Orion Molecular Cloud and the H II region around the Trapezium stars. The radiation from these stars creates a photon dominated region (PDR). The layered PDR structure is clearly seen in the Orion Bar since the peak of the emission is seen edge-on.

In order to study this transition zone, several molecular lines were observed on strips across the Bar. The observations are analysed using statistical equilibrium excitation calculations, and geometrical and chemical models for this source will be presented.

Observations

The Orion Bar has been observed in a number of molecular lines on 5 positions across the ionization front (see Figure 1). Also, a couple of maps have been made in millimeter lines of HCO+ and H2CO (Figure 2). Table 1 lists all observed species.

Physical properties

Using a statistical equilibrium excitation code we calculated line ratios for a range in temperatures and densities (Figure 3). Comparison with the observations gives values for temperature and density.

As it turns out, the Orion Bar data cannot be fitted with one component. The best fitting model is one with high density clumps (n(H2)=1E+6 cm-3) embedded in low density interclump gas (n(H2)=3E+4 cm-3). The clumps have a volume filling factor of 0.3 % but they contain 10 % of the gas. Both components are found to have temperatures in the range 85 +/- 30 K.

Using these values of Tkin and n(H2), column densities were derived. Those are listed in Table 1.

Geometry

The variation in H2 column density (inferred from C18O) is most easily interpreted as a variation in line-of-sight length through the PDR since there is no evidence for variation in any of the other physical parameters. However, since this whole column exhibits PDR characteristics, the most likely explanation is a geometrical one : at the extreme positions, the PDR is seen face-on, whereas at the peak position it is seen edge-on (Figure 4).

Chemistry

Chemical calculations have been performed using a one-dimensional code with approx. 1200 reactions (see Jansen et al. 1995a). These one-dimensional models are well suited for the face-on off-peak positions (see Figure 5).

For the peak position, these model results were convolved with the derived geometry. This simple approarch already yields a quite remarkable fit to the observations (see Table 1 and Figure 6).

To check this model, two-dimensional calculations were performed using the actual geometry of the source. These models also included the clump distribution.

References