Multi-epoch VLTI/MIDI Observations of the Carbon-rich Mira Star V Oph

The driving mechanism of mass outflows in Mira variables has not yet been fully understood. Recent progress in optical and infrared interferometric techniques has been contributing to studies of the region between the top of the photosphere and the innermost region of the circumstellar dust shell, exactly where mass outflows are expected to be initiated. Infrared interferometric observations of oxygen-rich Mira stars have spatially resolved dense, warm (∼1000–2000 K) molecular layers consisting of H₂O, SiO, and CO and extending to ∼2–3 R _⋆ (e.g., Mennesson et al. 1; Perrin et al. 6; Ohnaka et al. 4). The optically thick emission from the warm molecular layers affect the apparent size of Mira stars: the object appears larger than the star itself at wavelengths where the opacities of the above molecular species are higher. In particular, H₂O has strong spectral features in the mid-infrared, which causes the angular size to increase from the near-infrared to the mid-infrared as observed toward oxygen-rich Miras.