2008MNRAS.385.1076V

We present here near-infrared spectroscopy in the H and K bands of a selection of nearly 80 stars that belong to various asymptotic giant branch (AGB) types, namely S type, M type and SR type. This sample also includes 16 post-AGB (PAGB) stars. From these spectra, we seek correlations between the equivalent widths of some important spectral signatures and the infrared colours that are indicative of mass-loss. Repeated spectroscopic observations were made on some PAGB stars to look for spectral variations. We also analyse archival Spitzer mid-infrared spectra on a few PAGB stars to identify spectral features due to polycyclic aromatic hydrocarbon (PAH) molecules providing confirmation of the advanced stage of their evolution. Further, we model the spectral energy distributions (SEDs) of the stars (compiled from archival data) and compare circumstellar dust parameters and mass-loss rates in different types.

Our near-infrared spectra show that in the case of M- and S type stars, the equivalent widths of the CO(3-0) band are moderately correlated with infrared colours, suggesting a possible relationship with mass-loss processes. A few PAGB stars revealed short-term variability in their spectra, indicating episodic mass-loss: the cooler stars showed in CO first overtone bands and the hotter ones showed in HI Brackett lines. Our spectra on IRAS 19399+2312 suggest that it is a transition object. From the Spitzer spectra, there seems to be a dependence between the spectral type of the PAGB stars and the strength of the PAH features. Modelling of SEDs showed among the M and PAGB stars that the higher the mass-loss rates, the higher the [K - 12] colour in our sample.