Infrared Spatial Interferometer (ISI)
Operating in the mid-IR, ISI studies circumstellar dust around evolved stars and finds that half emit dust episodically in outbursts separated by decades or centuries. These include aOri, a Sco, and a Her, with dust at a mean distance of 38 R*, and Mira variables, with dust at < 6 R•. ISI is now monitoring some 30 stars for changes in their surrounding dust from which the stellar distances can be found.
ISI found the diameter of Mira to be 47.8 ± 0.5 mas, at pulsation phase 0.9, substantially larger than previously expected, with a systematic variation of ±13% over its ~ 330 day period. With its three telescopes, ISI can measure “closure phase” to check for departures from circular symmetry. While such stars as a Her and a Sco were found to be symmetric sources, at least seven Miras show varying degrees of asymmetry.