Time series infrared spectroscopy of the Mira variable Chi Cygni

Abstract

An analysis is made of a time series of 32 high-resolution infrared spectra spanning more than three cycles of the S-type Mira variable Chi Cygni. Most of the spectra are of the 1.6-2.5 micron region; five are of a narrow region at 4.6 microns. Using information derived from the delta v = 1, 2, and 3 vibration-rotation bands of CO, it is shown that Chi Cygni is characterized by a regularly pulsating photosphere and a complex, circumstellar envelope. Velocities, excitation temperatures, and column densities derived from the CO delta v = 3 bands around 1.6 microns reveal that the photometric variability is associated with an outwardly propagating wave. Probably driven by a general stellar oscillation, this wave traverses the photosphere during the premaximum and maximum phases. The structure of the circumstellar envelope is seen as dominated by a stationary layer that is inferred from its 800 K temperature to be at a distance of the order of 10 stellar radii from the photosphere.