Herschel observations of EXtra-Ordinary Sources: Analysis of the HIFI 1.2 THz Wide Spectral Survey Toward Orion KL II. Chemical Implications

TL;DR

This study analyzes Herschel/HIFI spectral data of Orion KL to understand the chemistry of complex organics, revealing temperature-dependent formation pathways and the potential difficulty of removing nitrogen-bearing species from grain surfaces.

Contribution

It introduces a novel technique to identify molecules emitting in hot gas and compares observed molecular abundances and temperatures with chemical models, providing new insights into organic molecule formation in star-forming regions.

Findings

Nitrogen-bearing organics trace hotter gas than oxygen-bearing ones.

Most complex organics' abundances match chemical models over >10^5 years.

Sulfur-oxygen molecules probe the hottest gas, indicating high-temperature formation pathways.

Abstract

We present chemical implications arising from spectral models fit to the Herschel/HIFI spectral survey toward the Orion Kleinmann-Low nebula (Orion KL). We focus our discussion on the eight complex organics detected within the HIFI survey utilizing a novel technique to identify those molecules emitting in the hottest gas. In particular, we find the complex nitrogen bearing species CH$_{3}$CN, C$_{2}$H$_{3}$CN, C$_{2}$H$_{5}$CN, and NH$_{2}$CHO systematically trace hotter gas than the oxygen bearing organics CH$_{3}$OH, C$_{2}$H$_{5}$OH, CH$_{3}$OCH$_{3}$, and CH$_{3}$OCHO, which do not contain nitrogen. If these complex species form predominantly on grain surfaces, this may indicate N-bearing organics are more difficult to remove from grain surfaces than O-bearing species. Another possibility is that hot (T$_{\rm kin}$$\sim$300 K) gas phase chemistry naturally produces higher complex cyanide abundances while suppressing the formation of O-bearing complex organics. We compare our derived rotation temperatures and molecular abundances to chemical models, which include gas-phase and grain surface pathways. Abundances for a majority of the detected complex organics can be reproduced over timescales $\gtrsim$ 10$^{5}$ years, with several species being under predicted by less than 3$\sigma$. Derived rotation temperatures for most organics, furthermore, agree reasonably well with the predicted temperatures at peak abundance. We also find that sulfur bearing molecules which also contain oxygen (i.e. SO, SO$_{2}$, and OCS) tend to probe the hottest gas toward Orion KL indicating the formation pathways for these species are most efficient at high temperatures.