High Spectral Resolution SOFIA/EXES Observations of C2H2 towards Orion-IRc2

TL;DR

This study uses high-resolution SOFIA/EXES observations to analyze C2H2 and other molecules in Orion IRc2, revealing new detections, isotopic ratios, and insights into the physical conditions and history of the hot core.

Contribution

First high-resolution mid-infrared spectra of C2H2 in Orion IRc2, detecting all rovibrational lines and measuring isotopic ratios and temperature differences.

Findings

All ten C2H2 rovibrational lines detected with high S/N.

Isotopic ratio 12C/13C measured as 14 +/- 1.

Distinct temperatures for ortho and para C2H2, indicating non-equilibrium conditions.

Abstract

We present high-spectral resolution observations from 12.96 - 13.33 microns towards Orion IRc2 using the mid-infrared spectrograph, EXES, on SOFIA. These observations probe the physical and chemical conditions of the Orion hot core, which is sampled by a bright, compact, mid-infrared background continuum source in the region, IRc2. All ten of the rovibrational C2H2 transitions expected in our spectral coverage, are detected with high S/N, yielding continuous coverage of the R-branch lines from J=9-8 to J=18-17, including both ortho and para species. Eight of these rovibrational transitions are newly reported detections. The isotopologue, 13CCH2, is clearly detected with high signal-to-noise. This enabled a direct measurement of the 12C/13C isotopic ratio for the Orion hot core of 14 +/- 1 and an estimated maximum value of 21. We also detected several HCN rovibrational lines. The ortho and para C2H2 ladders are clearly separate and tracing two different temperatures, 226 K and 164 K, respectively, with a non-equilibrium ortho to para ratio (OPR) of 1.7 +\- 0.1. Additionally, the ortho and para V_LSR values differ by about 1.8 +/- 0.2 km/s, while, the mean line widths differ by 0.7 +/- 0.2 km/s, suggesting that these species are not uniformly mixed along the line of sight to IRc2. We propose that the abnormally low C2H2 OPR could be a remnant from an earlier, colder phase, before the density enhancement (now the hot core) was impacted by shocks generated from an explosive event 500 yrs ago.