1996A&A...314..896L

We present mid-infrared (10-20µm) photometry obtained on 27 variable oxygen-rich late-type stars, at different epochs during the period 1984-1990. The sample includes representative objects of miras with optical counterparts, as well as type II OH/IR sources; there are also at least 2 supergiants. These measurements are merged with near-infrared ones, already reported and obtained quasi-simultaneously, to derive 72 (1-20µm) broad band energy distributions. These results are complemented also with IRAS data and flux measurements obtained in the sub-millimeter range. The resulting spectra are interpreted in terms of a circumstellar dust shell radiative transfer model applied for each object at different phases of their lightcurves. The modelling appears to be consistent for a large variety of oxygen-rich sources with optical depth at 10µm ranging from ∼0.01 to 10. The temperature of formation of the grains is taken to range from 800 to 950K depending on the pressure at the site of dust formation (P∼10^–7-10^–4dyn.cm^–2). Physical parameters such as inner radius of the circumstellar dust shell, bolometric luminosity and mass-loss rate are evaluated. For the objects in the sample, the mass-loss rate ranges from 2x10^–8 to 10^–4M_☉/yr. In the context of the planned and underway near-infrared surveys we discuss the relations between color indices and optical depth for oxygen-rich sources. Circumstellar shell optical depths and mass-loss rates are shown to be correlated to the [K-12µm] color. Unfortunately, near-infrared data alone seem to be insufficient to determine physical parameters reliably.