An observational program using Berkeley's unique Infrared Spatial Interferometer (ISI) with three movable 60 inch telescopes will be carried out to obtain high-resolution images of evolved late-type stars. This will include interferometric measurement of the diameters of old stars in narrow spectral regions at mid-infrared (11 micron) wavelengths chosen to avoid spectral lines due to gases in the immediate atmospheres of stars, careful examination of the shapes of stars using these baselines in different directions and with phase closure, and narrow-band measurements directly on spectral lines to determine the spatial distribution of such gases around the stars. Some mapping of dust emitted by stars using multiple baselines and phase also may be made, but measurement of stellar disks and shapes will be emphasized.
Baselines available to the ISI range from four to approximately 75 meters and allow accurate measurements of old stars as well as the dust and gas distributions around them. Measurements in the mid-IR and with narrow bandwidths minimize the distortions in apparent size due to limb darkening or to spectra of surrounding gas. The ISI system uses heterodyne detection which has a bandwidth of about 0.2/cm; furthermore the resulting signals can be filtered to obtain arbitrarily narrow spectral widths. Stellar size measurements to a precision of 1% or better have been demonstrated. Phase closure measurements are made to an accuracy of one degree, allowing nearly complete Fourier analysis of intensity patterns, including asymmetries. The observing campaign will emphasize the examination of possible deviations from sphericity of Miras and red giants, and changes in stellar size with time and phase of stellar oscillation.
Previous ISI measurements have shown movements of dust clouds surrounding late-type stars as well as deviations of the clouds from spherical symmetry. During the coming year focus will be placed on measurements of the stars themselves. The third movable telescope recently installed on Mt.Wilson functions well, and increases the visibility measurements by a factor of three as well as allowing accurate phase closure measurements. This allows interferometry with three baselines of various orientations to be measured simultaneously, along with phase closure. Asymmetries and irregular timing in emission of material by old stars appear common, but the asymmetries of stars themselves are not yet known.
The overall goals of this project are to provide a substantial increase in our knowledge of the characteristics of old stars through the detailed new measurements which are made possible via ISI and the development of interferometric technology. Opportunities will be provided for the training of young scientists (from undergraduates to postdocs) in optics, electronics, computer control of systems, telescope operation, data analysis, and scientific interpretation.
