The Keck Aperture Masking Experiment : Spectro - Interferometry of 3 Mira Variables from 1

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 symmetric 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 µm 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 H 2 O, 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 µm range. 1. INTRODUCTION Due to their large diameters and high luminosities, Mira variables are a favourite target for optical and near-infrared (NIR) interferometric observations, which have shown dependencies of diameter on wavelength, pulsation phase and pulsation cycle (e. Miras are long period variables (LPVs) of roughly one solar mass which evolve along the Asymptotic Giant Branch (AGB) and are characterized by well-defined pulsation periods and large pulsation amplitudes (up to ∆V ≈ 9). The extended atmospheres of these late-type giants show phase-dependent density and temperature stratifications which, together with complicated intensity profiles and the presence of molecular layers, leave no simple observable quantity to define the structure of the star's envelope. The pulsations also help drive strong winds thus enriching the interstellar medium with molecules such as H 2 O, CO, TiO and SiO (Tsuji et al. 1997; Tej et al. 2003a; Ohnaka 2004). A better understanding of their atmospheric structures will thus further the understanding of these late stages of stellar evolution, and might help shed light on one of the most important sources for the chemical enrichment of the interstellar medium. To fully characterize the structure of a Mira star's atmosphere, one would need the complete intensity profile at all wavelengths and all pulsation phases, recorded over a large number of pulsation cycles. The last decade has …