Showing papers by "Charles H. Townes published in 2006"

First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars

Abstract: We have measured nonzero closure phases for about 29% of our sample of 56 nearby asymptotic giant branch (AGB) stars, using the three-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 mas. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

Sparse-Aperture Adaptive Optics

Abstract: Aperture masking interferometry and Adaptive Optics (AO) are two of the competing technologies attemptingto recover diraction-limited performance from ground-based telescopes. However, there are good argumentsthat these techniques should be viewed as complementar y, not competitive. Masking has been shown to deliversuperior PSF calibration, rejection of atmospheric noise and robust recoveryof phase information through the useofclosure phases. However, this comes at the penalty of lossof ”ux at the mask, restricting the technique to brighttargets. Adaptive optics, on the othe r hand, can reach a fainter class of objects but suers from the dicultyof calibration of the PSF which can vary with observational parameters such as seeing, airmass and sourcebrightness. Here we present results from a fusion of these two techniques: placing an aperture mask downstreamof an AO system. The precision characterization of the PSF enabled by sparse-aperture interferometry can nowbe applied to deconvolution of AO images, recovering structure from the traditionally-dicult regime within thecore of the AO-corrected transfer function. Results of this program from the Palomar and Keck adaptive opticalsystems are presented.Keywords: Optical Interferometry, Adaptive Optics

First Surface-resolved Results with the IOTA Imaging Interferometer: Detection of Asymmetries in AGB stars

Abstract: We have measured non-zero closure phases for about 29% of our sample of 56 nearby Asymptotic Giant Branch (AGB) stars, using the 3-telescope Infrared Optical Telescope Array (IOTA) interferometer at near-infrared wavelengths (H band) and with angular resolutions in the range 5-10 milliarcseconds. These nonzero closure phases can only be generated by asymmetric brightness distributions of the target stars or their surroundings. We discuss how these results were obtained, and how they might be interpreted in terms of structures on or near the target stars. We also report measured angular sizes and hypothesize that most Mira stars would show detectable asymmetry if observed with adequate angular resolution.

Characterization of Dust Shell Dynamics and Asymmetry for Six Mira-Type Stars

Abstract: Interferometric observations of six Mira-type stars: R Aqr, CIT 3, χ Cyg, W Aql, R Leo, and U Ori are reported. All measurements were made by UC Berkeley's Infrared Spatial Interferometer (ISI), which is comprised of three 1.65 m telescopes using a heterodyne detection system currently operating at 11.15 μm. All data were taken in a nonredundant east-west linear configuration of telescopes, at a variety of spatial frequencies, with baselines of up to 12 m. By fitting a smooth curve to the closure phase data, as a function of the shortest baseline, the phases of individual visibility measurements can be determined. With curves of the visibility and phase, one-dimensional images are then constructed by an inverse Fourier transform. These images show significant changes in the stars and surrounding dust between the years 2003 and 2004 indicating nonconstant gas emission. They also show significant and varied types of asymmetry, including asymmetries that may be caused by companions, asymmetric stars, or asymmetric dust emission.

The Asymmetric dust environment of IK tauri

Abstract: Mid-infrared observations of IK Tau have been made at 11.15 μm with the three-telescope Infrared Spatial Interferometer on Mount Wilson and also using individual segments of the Keck telescope for multiple-aperture interferometry on the Keck telescope at 10.7 μm. Both experiments provided closure phase and show temporal variations and asymmetries in the surrounding dust, with a difference of about 15% in intensity between two sides of the star. Asymmetries have been previously observed in the distribution of SiO masers closely surrounding the star. Comparison with earlier interferometric measurements shows substantial reduction in dust surrounding the star over the last decade. Several asymmetric dust models are investigated and simple images constructed.

Changes in Apparent Size of Giant Stars with Wavelength due to Electron-Hydrogen Collisions

Abstract: Interferometric measurements of stellar sizes in frequency bands ranging from the near-infrared to longer wavelengthsgivedifferentresults.Variousexplanationshavebeenproposedtoaccountforthesevariationsinapparentsize withwavelength,butnonehave beenentirely consistent.Weproposethat thermal ionizationinthestellar atmosphere and resulting opacity, primarily due to free-free electron-hydrogen collisions, play a significant role. Such an opacity has a quadratic dependence on photon wavelength and produces variations in the opacity of the atmosphere with wavelength, consistent with pertinent measurements. This may be particularly important for Mira-type stars, and two examples, o Ceti and W Hya, are analyzed as examples. For stars that are much smaller or with more concentrated mass, it is not likely to be significant. Subject headingg stars: atmospheres — stars: fundamental parameters — stars: variables: other — techniques: interferometric