IRAS low resolution spectrograph spectral class and M and S Miras

Abstract

A large sample of 177 M and S Miras, as revealed by their IRAS LRS spectral class, have been examined to determine the dependence of silicate emission on the visual light curve asymmetry factor, f. It is confirmed that 9.7μm silicate emission feature not only in M but in S Miras also occurs only when f ≦ 0.45. However, not all stars with f ≦ 0.45 show the silicate emission; this non-detection reveals dependence on other parameters like the mean visual light amplitude. Though strong emission feature in M Miras may occur for any value of f, very weak features are absent for small values of f, and the strongest features tend to appear for larger values of f. Infrared excess tends to increase with the strength of the silicate emission as well as with decrease in the value of f. Probability of detection of silicate emission is very high for the visual light curve classes (Ludendorff) α₁, α₂, and α₃, decreases for α₄ and γ₁, and is negligible for the β class.