Time series infrared spectroscopy of Mira variables. II - CO (second overtone) eight Mira variables and one SRa variable

Abstract

The CO second overtone lines have been analyzed in time series spectra of nine long-period variables. The CO lines sample deep photospheric layers. Velocity, temperature, and column density derived from these lines are presented for each star as a function of phase. These data show that the photometric and spectroscopic variability of Mira variables is associated with an outwardly propagating wave, driven by a general stellar oscillation. This wave traverses the deep photosphere as a shock. The program stars have periods ranging from 293 to 445 days and visual amplitudes from 2 to 8 mag. Eight stars are Mira-type variables. Among the Mira variables, the behavior of velocity, temperature, and column density with phase are similar, yet differences do exist from star to star, demonstrating that a family of objects has been sampled. One of the program stars, X Oph, is a semiregular (SRa) long-period variable. This star is undergoing stellar oscillations similar to those of the Mira variables, but with a lower amplitude. Implications of the observations for the origin of the SiO maser are discussed.