Infrared Spectroscopic Study of a Selection of AGB and Post-AGB Stars

TL;DR

This study uses near-infrared and mid-infrared spectroscopy to analyze AGB and post-AGB stars, revealing correlations between spectral features, mass loss, and evolutionary stages, with evidence of episodic mass loss and spectral feature variations.

Contribution

It provides new insights into spectral signatures, mass loss processes, and evolutionary indicators of AGB and post-AGB stars through combined spectroscopic and SED modeling analysis.

Findings

Moderate correlation between CO band equivalent widths and infrared colors in M and S stars.

Spectral variability observed in some PAGB stars indicating episodic mass loss.

PAH feature strength depends on the spectral type of PAGB stars.

Abstract

We present here near-infrared spectroscopy in the H and K bands of a selection of nearly 80 stars that belong to various 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 colors 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 PAH molecules providing confirmation of the advanced stage of their evolution. Further, we model the 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 colors, 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.