High-resolution imaging of dust shells using Keck aperture masking and the IOTA Interferometer

We present first results of an experiment to combine data from Keck aperture masking and the Infrared-Optical Telescope Array (IOTA) to image the circumstellar environments of evolved stars with ∼20 milliarcsecond resolution. The unique combination of excellent Fourier coverage at short baselines and highquality long-baseline fringe data allows us to determine the location and clumpiness of the inner-most hot dust in the envelopes, and to measure the diameters of the underlying stars themselves. We find evidence for large-scale inhomogeneities in some dust shells and also significant deviations from uniform brightness for the photospheres of the most evolved M-stars. Deviations from spherically-symmetric mass loss in the red supergiant NML Cyg could be related to recent evidence for dynamically-important magnetic fields and/or stellar rotation. We point out that dust shell asymmetries, like those observed here, can qualitatively explain the difficulty recent workers have had in simultaneously fitting the broad-band spectral energy distributions and high-resolution spatial information, without [email protected]: University of Michigan Astronomy Department, 941 Dennison Bldg, Ann Arbor, MI 48109-1090, USA. Michelson Science Center, California Institute of Technology University of Sydney, Physics Department Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA, 02138, USA Observatoire de Paris-Meudon NASA Goddard Space Flight Center European Southern Observatory Massachusetts Institute of Technology University of Massachusetts, Amherst University of California, Berkeley