Physical parameters for Orion KL from modelling its ISO high resolution far-IR CO line spectrum

TL;DR

This study models the high-resolution far-IR CO spectrum of Orion KL using advanced chemical and radiative transfer models, revealing the origin of specific CO transitions from a hot core rather than the plateau.

Contribution

It introduces a combined chemical and radiative transfer modeling approach that predicts molecular abundances dynamically, improving upon previous assumptions.

Findings

CO lines J=18 to J=25 originate mainly from a hot core

The hot core has a diameter of 0.02 pc and density of 10^7 cm^-3

The modeling differentiates the hot core from the plateau in CO emission sources.

Abstract

As part of the first high resolution far-IR spectral survey of the Orion KL region (Lerate et al. 2006), we observed 20 CO emission lines with Jup=16 to Jup=39 (upper levels from approx 752 K to 4294 K above the ground state). Observations were taken using the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory (ISO), in its high resolution Fabry-Perot (FP) mode (approx 33 km s$^{-1}$). We present here an analysis of the final calibrated CO data, performed with a more sophisticated modelling technique than hitherto, including a detailed analysis of the chemistry, and discuss similarities and differences with previous results. The inclusion of chemical modelling implies that atomic and molecular abundances are time-predicted by the chemistry. This provides one of the main differences with previous studies in which chemical abundances needed to be assumed as initial condition. The chemistry of the region is studied by simulating the conditions of the different known components of the KL region: chemical models for a hot core, a plateau and a ridge are coupled with an accelerated Lambda-iteration (ALI)radiative transfer model to predict line fluxes and profiles. We conclude that the CO transitions with 18<Jup<25 mainly arise from a hot core of diameter 0.02 pc and a density of 10$^{7}$ cm$^{-3}$ rather from the plateau as previous studies had indicated.