Complex molecule formation around massive young stellar objects

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. Individually these observations provide limited constraints, however, 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. Toward the threeMYSOs, we find multiple complex organic emission peaks characterized by different molecular compositions and temperatures. In particular, CH3CCH and CH3CN seem to always trace a luke-warm (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 complexmolecule hostsmined fromthe 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, as well as provide information about its formation history.