Observatoire de la Côte d'Azur, Département FRESNEL, CNRS UMR 6528, 2130, route de l'Observatoire, Caussols, 06460 St Vallier de Thiey, France

We report the first spectrally-resolved observations of the Be star  Cas in the He I 6678 and H emission lines using the Grand Interféromètre à 2 Télescopes in the southern France. Milliarcsecond angular resolution measurements were carried in both lines and their neighbouring continuum during October and November 1993. The He I 6678 and H maximum emissions correspond respectively to 1.05 and 1.5 of the local continuum level. The interferometric baselines ranged from 15m to 51m on the sky which correspond to angular resolutions of 9 to 3 mas at He I 6678 and 6.5 to 2 mas at H wavelengths. We compare these values to predicted extents of H and He I 6678 components of the circumstellar gas from models of radiative transfer in these lines. We conclude that the emitting region must be smaller than 8.5 stellar radius in H and close to 2.3 stellar radius in He I 6678 which is, for He I 6678, smaller than the nearby continuum extent. These results confirm  Cas basic parameters for this star obtained by Stee et al. on 1993 from their model constrained by GI2T observations in H line.

1998, A&A, 332, 268