We present results from a spectro-interferometric study of the Miras o Cet, R Leo and W Hya obtained with the Keck Aperture Masking Experiment from 1998 Sep to 2002 Jul. The spectrally dispersed visibility data permit fitting with circularly symmetri
c brightness profiles such as a simple uniform disk. The stellar angular diameter obtained over up to ~ 450 spectral channels spaning the region 1.1-3.8 microns is presented. Use of a simple uniform disk brightness model facilitates comparison between epochs and with existing data and theoretical models. Strong size variations with wavelength were recorded for all stars, probing zones of H2O, CO, OH, and dust formation. Comparison with contemporaneous spectra extracted from our data show a strong anti-correlation between the observed angular diameter and flux. These variations consolidate the notion of a complex stellar atmosphere consisting of molecular shells with time-dependent densities and temperatures. Our findings are compared with existing data and pulsation models. The models were found to reproduce the functional form of the wavelength vs. angular diameter curve well, although some departures are noted in the 2.8-3.5 micron range.
The angular diameters of six oxygen rich Mira-type long-period variables have been measured at various near-infrared (NIR) wavelengths using the aperture masking technique in an extensive observing program from 1997 Jan to 2004 Sep. These data sets s
pan many pulsation cycles of the observed objects and represent the largest study of multi-wavelength, multi-epoch interferometric angular diameter measurements on Mira stars to date. The calibrated visibility data of o Cet, R Leo, R Cas, W Hya, chi Cyg and R Hya are fitted using a uniform disk brightness distribution model to facilitate comparison between epochs, wavelengths and with existing data and theoretical models. The variation of angular diameter as a function of wavelength and time are studied, and cyclic diameter variations are detected for all objects in our sample. These variations are believed to stem from time-dependent changes of density and temperature (and hence varying molecular opacities) in different layers of these stars. The similarities and differences in behaviour between these objects are analyzed and discussed in the context of existing theoretical models. Furthermore, we present time-dependent 3.08 micron angular diameter measurements, probing for the first time these zones of probable dust formation, which show unforeseen sizes and are consistently out of phase with other NIR layers shown in this study. The S-type Mira chi Cyg exhibits significantly different behaviour compared to the M-type Miras in this study.
