Showing papers by "Charles H. Townes published in 2004"
TL;DR: In this paper, a combination of excellent Fourier coverage at short baselines and high-quality long-baseline fringe data was used to determine the location and clumpiness of the innermost hot dust in the envelopes and to measure the diameters of the underlying stars themselves.
Abstract: We present first results of an experiment to combine data from Keck aperture masking and the Infrared-Optical Telescope Array to image the circumstellar environments of evolved stars with ~20 mas resolution. The unique combination of excellent Fourier coverage at short baselines and high-quality long-baseline fringe data allows us to determine the location and clumpiness of the innermost 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 broadband spectral energy distributions and high-resolution spatial information, without invoking unusual dust properties or multiple distinct shells (from hypothetical "superwinds"). This paper is the first to combine optical interferometry data from multiple facilities for imaging, and we discuss the challenges and potential for the future of this method, given current calibration and software limitations.
138 citations
TL;DR: In this article, the authors present 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.
Abstract: 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 high-quality 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 invoking unusual dust properties or multiple distinct shells (from hypothetical ``superwinds''). This paper is the first to combine optical interferometry data from multiple facilities for imaging, and we discuss the challenges and potential for the future of this method, given current calibration and software limitations.
116 citations
TL;DR: In this paper, the spatial distribution of dust around four late-type stars measured with the University of California, Berkeley, Infrared Spatial Interferometer (ISI) located at Mount Wilson, California, is described.
Abstract: The spatial distributions of dust around four late-type stars measured with the University of California, Berkeley, Infrared Spatial Interferometer (ISI) located at Mount Wilson, California, are described. Used as a heterodyne interferometer at 11.15 μm, the ISI was able to resolve the dust shells around late-type stars U Orionis, χ Cygni, W Aquilae, and IRC +10011 (CIT 3), including their inner radii. Models for dust distribution around these stars have been obtained by fitting the visibility data for both maximum and minimum luminosities and also available mid-infrared spectra of each star. Dust grains are modeled as a mixture of silicates and graphite, and since only two telescopes were used for these observations, the shells are assumed to be spherically symmetric. Visibility curves are shown to change with the luminosity phase of the star, with a larger fraction of the total 11 μm flux emitted from the dust near the star at the maximum than at the minimum phase. Mass-loss rates and estimates of the composition of the dust shell are provided for each star, and dust particle size is determined for IRC +10011 (CIT 3) by comparison of near- and mid-infrared visibilities.
14 citations
TL;DR: In this article, a comparative study of aperture masking on the Keck-I telescope and adaptive optics with the KI telescope is presented, which provides a unique opportunity for a direct comparison of two important and competing techniques for ground-based high resolution imaging.
Abstract: We present a comparative study of aperture-masking on the Keck-I telescope and adaptive optics with the Keck-II telescope. Recent results from an aperture-masking program at the Keck Observatory in the near-infrared amply demonstrate that this method occupies an important niche in achieving diffraction-limited images despite the many advances in adaptive optics technology. Examples of the efficacy of aperture-masking are the images of the persistent dust-producing Wolf-Rayet star WR 104 and the massive young star with IR excess, MWC 349A. Both these objects were resolved, providing fundamental new insights into their nature. Here we present images of these objects made using adaptive optics in the same wavelength band. These provide a unique opportunity for a direct comparison of two important and competing techniques for ground-based high-resolution imaging. From the AO images, we are unable to recover the gross morphology or detail seen in the aperture-masking results. We note that the AO program might have been hindered by less than ideal observing conditions.
6 citations