Mid‐Infrared Interferometry on Dust Shells around Four Late‐Type Stars

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.