Interferometric observations of the Mira star o Ceti with the VLTI/VINCI instrument in the near-infrared

We present K -band commissioning observations of the Mira star prototype o Cet obtained at the ESO Very Large Telescope Interferometer (VLTI) with the VLT INterferometer Commissioning Instrument (VINCI) and two siderostats. The observations were carried out between 2001 October and December, in 2002 January and December, and in 2003 January. Rosseland angular radii are derived from the measured visibilities by fitting theoretical visibility functions obtained from center-tolimb intensity variations (CLVs) of Mira star models. Using the derived Rosseland angular radii and the spectral energy distributions (SEDs) reconstructed from available photometric and spectrophotometric data, we find effective temperatures ranging from Teff = 3192± 200 K at phase Φ = 0.13 to 2918± 183 K at Φ = 0.26. Comparison of these Rosseland radii, effective temperatures, and the shape of the observed visibility functions with model predictions suggests that o Cet is a fundamental mode pulsator. Furthermore, we investigated the variation of visibility function and diameter with phase. The Rosseland angular diameter of o Cet increased from 28.9 ± 0.3 mas (corresponding to a Rosseland radius of 332±38 R for a distance ofD = 107±12 pc) at Φ = 0.13 to 34.9± 0.4 mas (402± 46 R ) at Φ = 0.4. The observational error of the Rosseland linear radius almost entirely results from the error of the parallax, since the error of the angular diameter is only approximately 1%. Introduction Mira stars are long-period variables which evolve along the asymptotic giant branch (AGB) with well-defined pulsation periods (80-1000 days). Because of their large visual light curve amplitudes, they are easily identified and classified. Fig. 1 shows the visual light curve of o Ceti together with the dates at which the VINCI observations were carried out. The change of CLV and spectral type with phase and cycle, as well as the brief time these stars remain in the variable stage, pose interesting problems for observation and modeling.