Morphological effects on IR band profiles: Experimental spectroscopic analysis with application to observed spectra of oxygen-rich AGB stars

TL;DR

This study investigates how dust grain morphology influences infrared spectral features in oxygen-rich AGB star environments, identifying anatase TiO2 particles as key contributors to specific emission bands.

Contribution

It provides new experimental aerosol spectra of various oxide dusts, highlighting morphological effects on IR features and linking specific dust shapes to observed stellar spectra.

Findings

Anatase TiO2 particles with rounded edges match key emission features.

Nano-sized spinel and Al2TiO5 grains may enhance the 13 μm feature.

Extinction spectra data are publicly available for further research.

Abstract

To trace the source of the unique 13, 19.5, and 28 $\mu$m emission features in the spectra of oxygen-rich circumstellar shells around AGB stars, we have compared dust extinction spectra obtained by aerosol measurements. We have measured the extinction spectra for 19 oxide powder samples of eight different types, such as Ti-compounds (TiO, TiO$_2$, Ti$_2$O$_3$, Ti$_3$O$_5$, Al$_2$TiO$_5$, CaTiO$_3$), $\alpha$-, $\gamma$-, $\chi$-$\delta$-$\kappa$-Al$_2$O$_3$, and MgAl$_2$O$_4$ in the infrared region (10 - 50 $\mu$m) paying special attention to the morphological (size, shape, and agglomeration) effects and the differences in crystal structure. Anatase (TiO$_2$) particles with rounded edges are the possible 13, 19.5 and 28 $\mu$m band carriers as the main contributor in the spectra of AGB stars, and spherically shaped nano-sized spinel and Al$_2$TiO$_5$ dust grains are possibly associated with the anatase, enhancing the prominence of the 13 $\mu$m feature and providing additional features at 28 $\mu$m. The extinction data sets obtained by the aerosol and CsI pellet measurements have been made available for public use at http://elbe.astro.uni-jena.de