The derivation of pulsation velocities from Doppler line profiles in M-type Mira variables

Model atmospheres for pulsating Mira variables have been used to examine the correction factor that should be used to convert radial velocities derived from the Doppler profiles of observed infrared lines into the velocities immediately above and below the strong shock in the atmosphere. The correction factor varies by a moderate amount depending on the lines selected. Correction factors of ∼ 1.25 and 1.4 are appropriate for the conversion of the observed velocities of the outflowing, post-shock and infalling pre-shock material, respectively, when the lines are strong. This is the case for the low excitation CO 1st vibrational overtone lines in the K band. A correction factor of 1.6 or more is appropriate for the weaker lines of OH and the 2nd vibrational overtone of CO in the H band. These results mean that the true pulsation velocity amplitude of a typical Mira variable is ∼34 km s^-1. Such large pulsation velocity amplitudes are inconsistent with current models for Mira variables pulsating in the first overtone mode but are consistent with pulsation in the fundamental mode.