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Unidentified infrared bands, polycyclic aromatic hydrocarbons and TU tau

Boersma, C. (2005) Unidentified infrared bands, polycyclic aromatic hydrocarbons and TU tau. Master's Thesis / Essay, Astronomy.

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Large Polycyclic Aromatic Hydrocarbon molecules (pahs) are thought to be formed as the byproduct of the soot formation process in the outflows of carbon-rich Asymptotic Giant Branch (agb) stars. Carbon-rich agb stars feed the universe with these complex species. However, the observational evidence for these molecules in the outflows of agb stars is scarce. Generally, these types of species are detected in interstellar and circumstellar environments through their ir fluorescence spectra, pumped by fuv photon absorption. However, agb stars are exceedingly cool, and so without appreciable fuv photons. An exception is the carbon-rich agb star TU Tau, where a companion star provides the fuv photons necessary to excite pah molecules. Fifty ISO sws (1) spectra of warm carbon-rich agb stars have been investigated for residual uir band emission to constrain the pah evolution scheme. A binary star, TU Tau, shows interesting spectral structure in the appropriate wavelength regions. The ir spectra of agb stars show a multitude of molecular absorption bands and dust emission features. Hence, the choice of the continuum is crucial to the identification and characterization of 'excess' spectral structure. Stars similar to TU Tau have been selected for comparing uir bands. Their continua and optical depths have been matched with TU Tau. Blackbody fits to the spectra showed that local continua should be used and that optical depth corrections are necessary. The residual uir band emission was obtained by subtracting the corrected spectra from TU Tau. The derived uir band profiles have been compared to uir band profiles from the ism, pne and carbonrich post agb stars. The profiles of TU Tau are shown to have the most resemblance with those from pne. Uncertainties in the deduced residual uir band profiles exist, however comparisons to several stars give similar results, which strengthens the confidence put in the derived profiles. Integrated band flux ratios have also been determined and compared to object type flux ratio correlations found in other studies. Here no definite match was found. The influence of the nearby companion star on the pah ionization state and pah formation rate has also been established. The analysis indicates that the contribution of the companion star to these parameters can be significant, depending on position in the outflow. Future modeling on the stellar outflow, including fuv photon processing, should reveal if the uir band profiles from TU Tau are characteristic for 'common' carbon-rich agb stars. The match of the band profiles from TU Tau with those from pne indicates that pahs are formed in the cse of carbon-rich agb stars and make it largely unmodified into the pn phase. The variations in the band strength ratios between the difierent objects has been linked to the ionization state of pahs and reflects the different physical environments within these objects. This is an indication that the differences between pne and ism pahs is largely due to modifications during the ism phase. (1)Based on observations with ISO, an ESA project with instruments funded by esa member states (especially the pi countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of ISAS and NASA.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Astronomy
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 07:28
Last Modified: 15 Feb 2018 07:28

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