Complex organic molecules in strongly UV-irradiated gas

Area of Science:

• Astrochemistry
• Interstellar Medium Physics
• Molecular Spectroscopy

Background:

• Interstellar molecular gas exposed to intense ultraviolet (UV) radiation fields, such as in photodissociation regions (PDRs), presents a challenging environment for complex organic molecule (COM) survival and formation.
• Understanding the chemical processes in these regions is crucial for deciphering the origins of organic molecules in the cosmos.

Purpose of the Study:

• To investigate the presence and abundance of COMs and related precursors in a strongly UV-irradiated environment at the edge of the Orion Bar PDR.
• To constrain the rotational temperatures and column densities of detected species.
• To explore potential formation pathways and chemical conditions of COMs in this harsh interstellar environment.

Main Methods:

• Conducted a comprehensive millimetre (mm) line survey using the IRAM 30 m telescope towards the Orion Bar PDR edge.
• Utilized high-resolution maps of H₂CO and C¹⁸O emission at 0.9 mm.
• Analyzed spectral lines to determine rotational temperatures (Trot) and column densities (N) for various COMs and precursors.

Main Results:

• Detected over 250 lines from more than a dozen COMs and related precursors, including H₂CO, CH₃OH, and HCO, despite the extreme UV irradiation.
• Inferred column densities for these species range from 10¹¹ to 10¹³ cm⁻².
• Identified distinct spatial distributions and excitation conditions for different molecules, with H₂CO and CH₃CN surviving in highly irradiated gas, while CH₃OH originates from cooler, shielded regions.

Conclusions:

• The survival and detection of numerous COMs at the PDR edge suggest complex formation mechanisms beyond simple gas-phase reactions or standard models.
• The observed abundance ratios (HCO/H₂CO/CH₃OH ≃ 1/5/3) differ from those found in hot cores and shocks, pointing to unique chemical pathways.
• Potential formation scenarios include hot gas-phase reactions, warm grain-surface chemistry, or the advection of molecules formed in colder, shielded regions into the PDR.