Letter to the Editor

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

¹ European Southern Observatory, Casilla 19001, Santiago 19, Chile e-mail: jlebouqu@eso.org
² LAOG–UMR 5571, CNRS and Université Joseph Fourier, BP 53, 38041 Grenoble, France
³ AIRI/Observatoire de Lyon, France and Jean-Marie Mariotti Center, France
⁴ Instituut voor Sterrenkunde, KULeuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 23 December 2008
Accepted: 4 February 2009

Abstract

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 () 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 , 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.