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MPIfR Optical & Infrared Interferometry Group

Hofmann, K.-H., Woodruff, H.C., and Schertl, D.

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

Cool Stars, Stellar Systems and the Sun
Annual Scientific Meeting of the Astronomische Gesellschaft, July 5-9, 2004, Hamburg, Germany

ESA Special Publications Series (ESA-SP), v.560, p.651-655 (2005)

We present accurate K'-band visibility and angular diameter measurements of Mira stars, obtained with the VLT Interferometer Commissioning Instrument (VINCI) at the Very Large Telescope Interferometer in the commissioning period between April 2001 and February 2003. A total of 49 VINCI observations were carried out from Oct. 2001 to Jan. 2003 and publicly released. For several Mira variables the observed visibility functions (modulus of the Fourier transform of the intensity distribution of the object) and diameters change noticeably with phase.

For example, the FWHM Gauss fit diameter of o Ceti increased from 19.23 ± 0.2mas at phase 0.13 to 23.02 ± 0.2mas at phase 0.4. Angular stellar filter radii and Rosseland radii were derived from the measured visibilities by fitting theoretical center-to-limb intensity variations (CLVs) of different Mira star models from Bessell, Scholz & Wood (1996, BSW), Hofmann, Scholz & Wood (1998, HSW), and Tej et al. (2003, TLSW). When available, HIPPARCOS parallaxes allowed us to determine linear radii. From the measured angular diameters and bolometric fluxes we calculated the effective temperatures. The shapes of the measured visibility curves (within the baseline length range of 11m to 16m) of o Ceti and the determined linear Rosseland radius of 328 ± 37 solar radii agree well with the near-maximum fundamental mode P model of HSW and TLSW (R = 318 solar radii). On the other hand, comparison of the data with the first- overtone pulsation models suggests that these models are not a good representation of o Ceti.

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