The effect of continuum elimination in identifying circumstellar dust around Mira

Abstract

ABSTRACT Asymptotic giant branch (AGB) stars are major contributors of cosmic dust to the universe. Typically, dust around AGB stars is investigated via radiative transfer (RT) modelling, or via simple deconstruction of observed spectra. However, methodologies applied vary. Using archival spectroscopic, photometric, and temporal data for the archetypal dusty star, Mira, we identify its circumstellar silicate dust grains. This is achieved by matching the positions and widths of observed spectral features with laboratory data. To do this comparison properly, it is necessary to account for the continuum emission. Here, we investigate various ways in which a continuum is eliminated from observational spectra and how it affects the interpretation of spectral features. We find that while the precise continuum shapes and temperatures do not have a critical impact on the positions and shapes of dust spectral features, it is important to eliminate continua in a specific way. It is important to understand what contributes to the spectrum in order to remove the continuum in a way that allows comparison with laboratory spectra of candidate dust species. Our methodologies are applicable to optically thin systems, like that of Mira. Higher optical depths will require RT modelling, which cannot include many different potential astrominerals because there is a lack of complex refractive indices. Finally, we found that the classic silicate feature exhibited by Mira is not consistent with a real amorphous silicate alone but may be best explained with a small alumina contribution to match the observed full width at half-maximum of the $\sim 10\, \mu$m feature.