Model-Independent Continuum Removal and the Limits of the Blackbody Approximation in Circumstellar Dust Spectroscopy

Accurately characterizing dust around optically thin, oxygen-rich evolved stars requires separating the stellar and dust contributions in their mid-infrared spectra. This study applies a quantitative, model-independent continuum-elimination technique to six Infrared Space Observatory (ISO) Short Wavelength Spectrometer (SWS) observations of the Mira variable Z Cyg, spanning its pulsation cycle. The method isolates dust-grain emission bands from 2.4–45 µm without full radiative-transfer modeling and enables direct comparison of spectra across epochs. The validity of the blackbody approximation for the underlying stellar continuum was tested by comparing results derived from idealized blackbody fits with those obtained using empirical M- and S-star spectra.

Analysis of the continuum-removed spectra reveals phase-dependent variations in the 10 µm and 18 µm silicate features, including subtle shifts in peak position, asymmetry, and equivalent width correlated with pulsation phase and stellar luminosity. These results trace how dust heating, cooling, and potential grain growth evolve over time within the circumstellar envelope. By establishing a consistent, empirically anchored framework for multi-epoch analysis, this study provides new insight into the temporal coupling between stellar variability and dust formation processes in oxygen-rich outflows. The findings have broader relevance for infrared spectroscopy, data-driven radiative-transfer approximations, and laboratory-astrophysics comparisons of silicate and oxide dust analogs.