UIR bands in the ISO SWS spectrum of the carbon star TU Tauri

Abstract

Polycyclic Aromatic Hydrocarbon (PAH) molecules are thought to form in the ejecta of carbon-rich Asymptotic Giant Branch (AGB) stars. Fifty ISO SWS spectra of warm carbon-rich AGB stars have been investigated for the Unidentified IR (UIR) emission due to fluorescence by PAH molecules. In this sample the binary star TU Tau, which has a blue companion, shows interesting spectral structure in the appropriate wavelength regions. The profiles of the UIR bands in TU Tau have been derived by comparing to suitable carbon-star spectra. The presence of the UIR bands in TU Tau is attributed to UV photons originating from the A2 companion star which are necessary to excite PAH molecules. The absence of the UIR bands in the remainder of the sample is ascribed to the lack of UV ( or visible) photons in their environment. Hence, the absence of UIR bands does not necessarily imply the absence of circumstellar PAHs in these sources. The derived UIR band profiles have been compared to UIR band profiles from Reflection Nebulae, Planetary Nebulae (PNe), HII regions, Young Stellar Objects, evolved stars and Galaxies. The profiles of TU Tau are shown to have the most resemblance to those from PNe. 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. TU Tau is the only binary with a blue companion in this sample of AGB stars. In line with earlier studies, we suggest that the blue photons provided by this companion are required for efficient excitation of PAH molecules in AGB ejecta. In addition, we argue that these blue photons may promote complex chemistry in the ejecta of TU Tau. The similarity in the peak profiles observed in the spectrum of TU Tau with those in the spectra of PNe indicates that PAHs are formed in the circumstellar envelope of carbon-rich AGB stars and make it largely unmodified into the PNe phase. The variations in the band strength ratios between the different objects has been linked to the ionization state of PAHs and reflects the different physical environments between this AGB star and PNe. In contrast, the variation in UIR band profiles between stellar ejecta and the Interstellar Medium ( ISM) are largely attributed to chemical modifications during the ISM phase.