The Origin of Unidentified Infrared Emission Lines in Planetary Nebulae Environments

Faculty Sponsor

Stan Zygmunt

College

Arts and Sciences

Department/Program

Physics and Astronomy

Presentation Type

Poster Presentation

Symposium Date

Summer 7-24-2024

Abstract

Planetary nebulae emit unidentified infrared emission (UIE) lines that come from an unknown source. A proposed source for these UIE lines is polycyclic aromatic hydrocarbons (PAHs) which are composed of fused carbon rings. We have calculated the absorption spectra from various PAHs and compare them to observed UIE lines to try to determine where the UIEs come from. Our goal is to observe how ionization and nitrogen or oxygen heteroatom substitutions affect the infrared (IR) spectrum. We used the Gaussian 09 program to calculate the structures and IR absorption spectra for about ninety different PAHs. These include linear and non-linear fused ring structures and nitrogen and oxygen substitutions, both in neutral and ionized states. We determined the vibrational modes associated with each of the peaks observed in the IR spectra. These modes involve C-H stretching, C-C stretching and both in-plane and out-of-plane C-H bending modes. Oxygen and nitrogen substitutions tend to increase the intensity of of the C-C stretching and C-H in-plane bending modes. We also determined that PAH molecules must have hydrogen atoms with specific numbers of neighboring hydrogen atoms in order to match the observed UIE bands. While none of the molecules we have studied match the UIE bands exactly, our results limit possible candidates for PAHs that can reproduced the major UIE peaks in planetary nebulae spectra.

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