Complex molecule formation around massive young stellar objects.

Karin I Öberg,John B Reiter,Claudia Cyganowski,Edith C Fayolle

doi:10.1039/c3fd00146f

Abstract

Interstellar complex organic molecules were first identified in the hot inner regions of massive young stellar objects (MYSOs), but have more recently been found in many colder sources, indicating that complex molecules can form at a range of temperatures. However, individually these observations provide limited constraints on how complex molecules form, and whether the same formation pathways dominate in cold, warm and hot environments. To address these questions, we use spatially resolved observations from the Submillimeter Array of three MYSOs together with mostly unresolved literature data to explore how molecular ratios depend on environmental parameters, especially temperature. Towards the three MYSOs, we find multiple complex organic emission peaks characterized by different molecular compositions and temperatures. In particular, CH3CCH and CH3CN seem to always trace a lukewarm (T = 60 K) and a hot (T > 100 K) complex chemistry, respectively. These spatial trends are consistent with abundance-temperature correlations of four representative complex organics--CH3CCH, CH3CN, CH3OCH3 and CH3CHO--in a large sample of complex molecule hosts mined from the literature. Together, these results indicate a general chemical evolution with temperature, i.e. that new complex molecule formation pathways are activated as a MYSO heats up. This is qualitatively consistent with model predictions. Furthermore, these results suggest that ratios of complex molecules may be developed into a powerful probe of the evolutionary stage of a MYSO, and may provide information about its formation history.

Highlights

This existence of low-temperature formation pathways of Complex Organic Molecules (COMs) does not exclude, that additional formation pathways of COMs are activated as the source temperature is increased
There is evidence for significantly different COM compositions both between different kinds of sources (Fig. 1) 11 and across a single massive young stellar object (MYSO) where the COM abundance profiles were resolved across a range of temperatures12
The molecular images display the spectrally integrated, spatially resolved emission profiles of 7 organic emission lines, including lines from the COMs HNCO, CH3CN, and CH3CCH detected in all sources

Summary

Introduction

This existence of low-temperature formation pathways of COMs does not exclude, that additional formation pathways of COMs are activated as the source temperature is increased. The precursors of COMs, or the zeroth generation, form in cold molecular clouds, where low temperatures (∼10 K) and moderately high densities (∼ 105 cm−3) result in efficient freeze-out of all elements and molecules heavier than H and He onto interstellar dust grains. While H atoms do not form permanent ice layers, they can still reside long enough on grain and ice surfaces to be an important reaction partner at low temperatures. At 10 K, H is orders of magnitude more mobile than heavier elements and molecules, such as C and CO, and this stage is characterized by hydrogenation-reactions on grain surfaces to form e.g. CH3OH from CO. In the gas-phase this stage is characterized by ion-neutral reactions, which may be responsible for some COMs, especially hydrocarbons

Results

Discussion

Conclusion