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Investigation of Interstellar Polycyclic Aromatic Hydrocarbon (PAH) molecules as carriers for mid-infrared emission bands

Monday, August 30, 2021 - 03:00
Online on Zoom
Dr. Mridusmita Buragohain
Department of Astronomy, The University of Tokyo

Interstellar Polycyclic Aromatic Hydrocarbon (PAH) molecules exist in diverse forms depending on the local physical environment of the Interstellar Medium (ISM). Formation of ionized PAHs is favorable in the extreme condition of the ISM. Besides its pure form, PAHs are likely to exist in substituted forms, for example, PAHs with functional groups, nitrogenated PAHs, protonated and deuterated PAHs, etc. These PAHs may convert into alternate forms as a result of ongoing chemical processes in the ISM. The spectral evidence of PAH molecules and its variants in the ISM are observed via the mid-infrared emission bands, particularly at 3.3, 6.2, 7.7, 8.6, 11.2 and 12.7 μm. These bands, also known as ‘Aromatic Infrared Bands (AIBs)’ are widely observed towards a varied range of astronomical sources and arise from the vibrational relaxation of PAH molecules on absorption of background UV photons. However, the exact form of PAH molecules that are responsible for the AIBs is still ambiguous. Here, we discuss a few of the possible forms of interstellar PAH molecules (for example: deuteronated, nitrogenated and aliphatic PAHs etc.) as carriers for AIBs. Density Functional Theory (DFT) calculation on several classes of PAHs is employed to study its spectral characteristics in infrared which is compared with the observed bands in quest of any similarity that establishes its presence in the ISM.

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