An infrared photometric and spectroscopic study of post-AGB stars

Abstract

We present here Spitzer mid-infrared (IR) spectra and modelling of the spectral energy distribution (SED) of a selection of post-asymptotic giant branch (PAGB) stars. The mid-IR spectra of the majority of these sources showed spectral features such as polycyclic aromatic hydrocarbons (PAHs) and silicates in emission. Our results from SED modelling showed interesting trends of dependence between the photospheric and circumstellar parameters. A trend of dependence is also noticed between the ratios of equivalent widths (EWs) of various vibrational modes of PAHs and the photospheric temperature T_* and model-derived stellar parameters for the sample stars. The PAGB mass-loss rates derived from the SED models are found to be higher than those for AGB stars. In a few objects, low- and high-excitation fine-structure emission lines were identified, indicating their advanced stage of evolution. Further, IR vibration modes of fullerene (C₆₀) were detected for the first time in the PAGB star IRAS 21546+4721.