Pre-maximum spectro-imaging of the Mira star T Leporis with AMBER/VLTI

Context. Diffuse envelopes around Mira variables are among the most important sources influencing the chemical evolution of galaxies. However they represent an observational challenge because of their complex spectral features and their rapid temporal variability. Aims. We aimed at constraining the exact brightness distribution of the Mira star T Lep with a model-independent analysis. Methods. We obtained single-epoch interferometric observations with a dataset continuous in the spectral domain (lambda = 1.5-2.4 mu m) and in the spatial domain (interferometric baselines ranging from 11 to 96 m). We performed a model independent image reconstruction for each spectral bin using the MIRA software. We completed the analysis by modeling the data with a simple star+layer model inspired from the images. Results. Reconstructed images confirm the general picture of a central star partially obscured by the surrounding molecular shell of changing opacity. At 1.7 mu m, the shell becomes optically thin, with corresponding emission appearing as a ring circling the star. This is the first direct evidence of the spherical morphology of the molecular shell. Model fitting confirmed a spherical layer of constant size and changing opacity over the wavelengths. Rough modeling points to a continuum opacity within the shell, in addition to the CO and H2O features. Accordingly, it appeared impossible to model the data by a photosphere alone in any of the spectral bins.