Probing the Outer Atmosphere of Mira Variables and the Effects of Chemical Composition on the Mid-Infrared Visibility

Abstract

Intermediateand low-mass stars experience slow, but massive mass loss, while moving along the asymptotic giant branch (AGB) during the late stage of their evolution. Mira-type AGB stars are of particular interest, because they exhibit complicated physical and chemical processes such as largeamplitude stellar pulsation, shock waves, grain formation, and momentum transfer from photons to dust grains. Our MIDI observations of the oxygenrich Mira variable RR Sco have revealed that optically thick emission from dense, warm molecular layers (H2O and SiO) as well as dust thermal emission can make the N -band angular size more than twice as large as that in the K band (Ohnaka et al. [1]). Here we present the results of MIDI observations of Mira variables RS Lib and RT Sco. While RS Lib is an oxygen-rich Mira variable, RT Sco is classified as an S-type Mira variable. S stars are characterized by C/O ratios close to unity, which means that neither oxygen-bearing molecules nor carbon-bearing molecules are abundant in their photosphere. Such peculiar chemical composition is expected to affect the structure of the outer atmosphere, where molecule/dust formation takes place. Therefore, a comparative study on Mira variables with different chemical compositions is useful for better understanding the physical properties of the outer atmosphere as well as of the innermost region of the dust shell.