NONUNIFORM DUST OUTFLOW OBSERVED AROUND INFRARED OBJECT NML CYGNI

Measurements by the University of California Berkeley Infrared Spatial Interferometer at 11.15 mu m have yielded strong evidence for multipl e dust shells and/or significant asymmetric dust emission around NML C yg. New observations reported also include multiple 8-13 mu m spectra taken from 1994-1995 and N-band (10.2 mu m) photometry from 1980-1992. These and past measurements are analysed and fitted to a model of the dust distribution around NML Cyg. No spherically symmetric single dus t shell model is found consistent with both near- and mid-infrared obs ervations. However, a circularly symmetric maximum entropy reconstruct ion of the 11 mu m brightness distribution suggests a double-shell mod el for the dust distribution. Such a model, consisting of a geometrica lly thin shell of intermediate optical depth (tau(11 mu m) similar to 1.9) plus an outer shell (tau(11 mu m) similar to 0.33), is consistent not only with the 11 mu m visibility data but also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 mu m silica te feature. The outer shell, or large-scale structure, is revealed onl y by long-baseline interferometry at 11 mu m, being too cold (similar to 400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Osse nkopf, Henning, & Mathis proved superior to the Draine & Lee (1984) co nstants in fitting the detailed shape of the silicate feature and broa dband spectrum for this object. Recent observations of H2O maser emiss ion around NML Cyg by Richards, Yates, & Cohen (1996) are consistent w ith the location of the two dust shells and provide further evidence f or the two-shell model.