Multiepoch interferometric study of Mira variables. I. Narrowband diameters of RZ Pegasi and S Lacertae

As part of the long-term monitoring of Mira variables at the Palomar Testbed Interferometer, we report high-resolution narrowband angular sizes of the oxygen-rich Mira S Lac and the carbon-rich Mira RZ Peg in the near-infrared. The data set spans three pulsation cycles for S Lac and two pulsation cycles for RZ Peg (a total of 1070 25 s observations) and represents the first study to correlate multiepoch narrowband interferometric data of Mira variables. When the calibrated visibility data are fitted using a uniform disk brightness model, differences are seen in their angular diameters as a function of wavelength within the K band (2.0-2.4 mum), the source of which is believed to be molecular absorptions in or above the photospheres of the two chemically different Miras. Using visible photometric data provided by the Association Francaise des Observateurs d'Etoiles Variables (AFOEV), the continuum minimum size of RZ Peg lags this by 0.28 +/- 0.02 in pulsation, similar to the phase lag found in Correlation Radial Velocities (CORAVEL) data. However, for S Lac, the continuum minimum size tracks the visual maximum brightness. Based on the mean of the continuum angular diameter cycloids, basic stellar parameters are computed for both RZ Peg and S Lac, with both showing maximum atmospheric extension with respect to the 2.0 and 2.4 mum diameters near phase 0.9. Using the mean value of the fitted cycloids, RZ Peg has a radius R-mean = 377 +/- 111 R. and a mean T-eff = 2706 +/- 36 K; S Lac has a radius R-mean = 292 +/- 73 R and a mean T-eff = 2605 +/- 47 K. The dominant source of error in the radii is the large uncertainty in the distances to these two stars.