Complex organic molecules in organic-poor massive young stellar objects

TL;DR

This study investigates complex organic molecules in a less-explored class of massive young stellar objects with weak hot core emissions, revealing that hot cores are not essential for their formation and that initial ice composition influences their distribution.

Contribution

It demonstrates that complex organic molecules are present in organic-poor MYSOs without luminous hot cores, highlighting the roles of temperature and initial ice conditions in chemical composition.

Findings

Complex molecules detected in all three sources at comparable abundances to hot cores.

N-bearing molecules are concentrated toward source centers, while O- and C-bearing molecules are more widespread.

Gas-phase HNCO/CH3OH ratios correlate with NH3/CH3OH ice ratios, supporting gas-grain model predictions.

Abstract

Massive young stellar objects (MYSOs) with hot cores are classic sources of complex organic molecules. The origins of these molecules in such sources, as well as the small- and large-scale differentiation between nitrogen- and oxygen-bearing complex species, are poorly understood. We aim to use complex molecule abundances toward a chemically less explored class of MYSOs with weak hot organic emission lines to constrain the impact of hot molecular cores and initial ice conditions on the chemical composition toward MYSOs. We use the IRAM 30m and the Submillimeter Array to search for complex organic molecules over 8-16 GHz in the 1~mm atmospheric window toward three MYSOs with known ice abundances, but without luminous molecular hot cores. Complex molecules are detected toward all three sources at comparable abundances with respect to CH$_3$OH to classical hot core sources. The relative importance of CH$_3$CHO, CH$_3$CCH, CH$_3$OCH$_3$, CH$_3$CN, and HNCO differ between the organic-poor MYSOs and hot cores, however. Furthermore, the N-bearing molecules are generally concentrated toward the source centers, while most O- and C-bearing molecules are present both in the center and in the colder envelope. Gas-phase HNCO/CH$_3$OH ratios are tentatively correlated with the ratios of NH$_3$ ice over CH$_3$OH ice in the same lines of sight, which is consistent with new gas-grain model predictions. Hot cores are not required to form complex organic molecules, and source temperature and initial ice composition both seem to affect complex organic distributions toward MYSOs. To quantify the relative impact of temperature and initial conditions requires, however, a larger spatially resolved survey of MYSOs with ice detections.