A Study of Dust Condensation Around M Mira Variables Based on IRAS LRS Observations

Abstract

We have studied Low-Resolution Spectrograph (LRS) spectra obtained by IRAS for a sample of about 100 M Mira variables with known light curves. Our earlier discovery that Mira variables with symmetric light curves do not show the 9.7 00b5m silicate feature while Miras with asymmetric light curves do, is confirmed on the basis of this much larger sample. The broad weak 12 µm feature in the spectra of Miras with symmetric light curves is identified with an emission feature in the spectrum of aluminum oxide. The observed spectra as well as the 60 µm IRAS point source fluxes are reproduced quite satisfactorily by optically-thin spherical shell models containing mixtures of silicate and aluminum oxide dust particles. From this modelling we find that the silicate dust has temperatures in the range 200-500 K at the inner boundary of the silicate dust shell and that the appearance or absence of the 9.7 µm silicate feature depends on the magnitude of the inner dust shell temperature. Possible explanations of these results are discussed in terms of the physics of the condensation process.KeywordsAluminum Oxide ParticleLight CurfDust ShellDust TemperatureMira VariableThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.