ar X iv : 0 70 9 . 38 78 v 1 [ as tr o - ph ] 2 5 Se p 20 07 The Keck Aperture Masking Experiment : Multi - wavelength observations of 6 Mira Variables

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 span 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, χ 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 3.08μm 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. NIR light-curves were recovered, and show the distinctive phase lag in the maxima of ≈ 0.2 (compared to the visual maximum), similar to the lag found by, e.g. Smith et al. (2002) and Nadzhip et al. (2001). The S-type Mira χ Cyg exhibits significantly different behaviour compared to the M-type Miras in this study, both in its NIR light-curves and its diameter pulsation signature. Our data show that the NIR diameters predicted by current models are too small and need to incorporate additional and/or enhanced opacity mechanisms. Also, new tailored models are needed to explain the behaviour of the S-type Mira χ Cyg. Subject headings: instrumentation: interferometers – techniques: interferometric – stars: late-type – stars: AGB and post-AGB – stars: fundamental parameters – stars: individual: Mira stars: individual: R Hya stars: individual: Chi Cyg stars: individual: W Hya stars: individual: R Leo stars: individual: R Cas 1School of Physics, University of Sydney, NSW 2006, Australia 2University of Michigan at Ann Arbor, Department of Astronomy, 500 Church Street, Ann Arbor, MI 48109-1090, USA 3Planetary Science, Caltech, 1200 E. California Blvd, Pasadena CA 91125 USA 4NASA Goddard Space Flight Center, Infrared Astrophysics, Code 685, Greenbelt, MD 20771, USA 5Institut für Theoretische Astrophysik der Universität Heidelberg, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany