The Spatial Distribution of Organics toward the High-Mass YSO NGC 7538 IRS9

TL;DR

This study investigates the spatial distribution of organic molecules around the high-mass YSO NGC 7538 IRS9, revealing temperature-dependent chemical processes and the presence of a weak hot core influencing molecular composition.

Contribution

It combines IRAM 30m and SMA observations to map organic molecules around NGC 7538 IRS9, providing new insights into their spatial distribution and chemical evolution.

Findings

Complex organics form at all radii but vary with temperature.

Emission increases at specific radii suggest hot core presence.

Molecular composition changes dramatically with temperature.

Abstract

Based on astrochemical theory, the complex molecular composition around high-mass YSOs should evolve from the outer envelope in toward the central hot region as a sequence of temperature dependent chemical pathways are activated in ices and in the gas-phase. The resulting complex molecules have been broadly classified into three generations dependent on the temperature (<25, >25, and >100 K) required for formation. We combine IRAM 30m and Submillimeter Array observations to explore the spatial distribution of organic molecules around the high-mass young stellar object NGC 7538 IRS9, whose weak complex molecule emission previously escaped detection, quantifying the emission and abundance profiles of key organic molecules as a function of distance from the central protostar. We find that emission from N-bearing organics and saturated O-bearing organics present large increases in emission around 8000 AU and R<3000 AU, while O-bearing molecules and hydrocarbons do not. The increase in flux from some complex molecules in the envelope, around 8000 AU or 25 K, is consistent with recent model predictions of an onset of complex ice chemistry at 20-30 K. The emission increase for some molecules at R<3000 AU suggests the presence of a weak hot core, where thermal ice evaporation and hot gas-phase chemistry drives the chemistry. Complex organics thus form at all radii and temperatures around this protostar, but the composition changes dramatically as the temperature increases, which is used to constrain the chemical generation(s) to which different classes of molecule belong.