The Anomalous Behavior of Aluminum Oxide Bands in Mira Variables

Abstract

In spectra of normal M giants, absorption bands of AlO occur with an intensity that is well correlated with the spectral type as defined by the TiO bands. In Mira variables, however, the AlO band can vary greatly from cycle to cycle and can be abnormally strong or weak for the spectral type. This behavior is not shared by other molecules, such as ScO and VO, which vary predictably with spectral type. In some Mira variables the AlO bands have been observed to go over into emission. This occurred in Mira in 1924 and 1964, R Ser in 1960, R Psc in 1966, R Cry and R Cas in 1967, and RT Lib in 1968. From a survey of all spectrograms of Mira itself available at Mount Wilson and Palomar Observato- ries, we find that the AlO bands tend to be strong in absorption during "strong line" cycles and tend to be weak or reversed in "weak line" cycles