Assessment of the polycyclic aromatic hydrocarbon-diffuse interstellar band proposal

Area of Science:

• Astrochemistry
• Interstellar Medium Physics

Background:

• Diffuse interstellar bands (DIBs) are observed spectral features in interstellar space.
• Polycyclic aromatic hydrocarbons (PAHs) are candidate molecules for DIB carriers.
• The physical and chemical properties of PAHs in interstellar conditions are crucial for understanding DIBs.

Purpose of the Study:

• To examine the potential link between neutral and ionized polycyclic aromatic hydrocarbons (PAHs) and diffuse interstellar band (DIB) carriers.
• To assess the contribution of PAHs to DIBs based on their properties in the diffuse interstellar medium.
• To derive molecular size and structure constraints for interstellar PAHs by comparing DIB characteristics with PAH spectroscopic properties.

Main Methods:

• Studying the general physical and chemical properties of PAHs.
• Analyzing the charge distribution and spectral signatures of PAHs (neutral, cationic, anionic).
• Comparing spectroscopic properties of PAHs with known DIB characteristics.

Main Results:

• PAHs exist as neutral molecules and positive/negative ions in the diffuse interstellar medium.
• Compact PAH ions correlate with strong, broad DIBs (Type A bands), while noncompact PAH ions correlate with weak, narrow DIBs (Type I bands).
• Constraints suggest neutral PAHs >30 carbons and compact PAH ions <250 carbons contribute to DIBs; DIB distribution aligns with compact/noncompact PAH ion distribution.

Conclusions:

• Small PAH ions are highly promising DIB carriers if a finite number of species dominate.
• DIB families likely reflect local cloud conditions influencing PAH charge states (cations, anions, neutrals).
• Further laboratory and astronomical ultraviolet investigations are essential to validate the PAH-DIB hypothesis.