Precision Analysis of $\mathrm{^{12}C / ^{13}C}$ Ratios in Orion IRc2 Acetylene Isotopologues via $\chi^2$ Fitting

TL;DR

This study employs a robust $^2$ fitting method on high-resolution spectra to accurately determine $^{12}$C/$^{13}$C ratios in Orion IRc2, revealing discrepancies with traditional methods and providing more precise isotopic measurements.

Contribution

It introduces a novel $^2$ fitting approach combined with quantum chemical calculations and a new Python tool, TOPSEGI, for improved isotopic ratio analysis in astrophysical spectra.

Findings

Achieved more precise $^{12}$C/$^{13}$C ratios with reduced uncertainties.

Discovered significant discrepancies with traditional rotational diagram methods.

Extended isotopologue analysis to molecules not covered in HITRAN.

Abstract

We present a detailed analysis of acetylene (C$_2$H$_2$) and its isotopologues in the Orion IRc2 region, focusing on the determination of $^{12}$C/$^{13}$C isotopic ratios using high-resolution infrared spectra from SOFIA. By employing a robust $\chi^2$ fitting method, we simultaneously determined temperature and column density, achieving a $^{12}$C/$^{13}$C ratio of $18.72^{+1.54}_{-1.46}$ for the blue clump and $15.07^{+1.61}_{-1.60}$ for the red clump. These results revealed significant discrepancies with the traditional rotational diagram method, which overestimated the ratios by 12.1% and 23.9%, respectively. Our $\chi^2$ approach also reduced uncertainties by up to 75%, providing more precise and reliable isotopic ratios. Additionally, we extended the analysis to isotopologues not covered in HITRAN, calculating vibrational and rotational constants through quantum chemical calculations. This allowed us to model subtle isotopic shifts induced by $^{13}$C and deuterium substitution, enabling accurate isotopologue detection in astrophysical environments. The Python package (TOPSEGI) developed in this study facilitates efficient $\chi^2$ fitting and isotopic ratio analysis, making it a valuable tool for future high-resolution observations. This work highlights the critical role of advanced spectral models and fitting techniques in understanding isotopic fractionation and the chemical evolution of interstellar matter.