High‐Resolution Imaging of Dust Shells by Using Keck Aperture Masking and the IOTA Interferometer

We present first results of an experiment to combine data from Keck aperturemasking and the Infrared-Optical Telescope Array (IOTA) to image thecircumstellar environments of evolved stars with ~20 milliarcsecond resolution.The unique combination of excellent Fourier coverage at short baselines andhigh-quality long-baseline fringe data allows us to determine the location andclumpiness of the inner-most hot dust in the envelopes, and to measure thediameters of the underlying stars themselves. We find evidence for large-scaleinhomogeneities in some dust shells and also significant deviations fromuniform brightness for the photospheres of the most evolved M-stars. Deviationsfrom spherically-symmetric mass loss in the red supergiant NML Cyg could berelated to recent evidence for dynamically-important magnetic fields and/orstellar rotation. We point out that dust shell asymmetries, like those observedhere, can qualitatively explain the difficulty recent workers have had insimultaneously fitting the broad-band spectral energy distributions andhigh-resolution spatial information, without invoking unusual dust propertiesor multiple distinct shells (from hypothetical ``superwinds''). This paper isthe first to combine optical interferometry data from multiple facilities forimaging, and we discuss the challenges and potential for the future of thismethod, given current calibration and software limitations.Comment: To appear in ApJ (61 pages: 4 tables, 23 figures). Image resolution degrade