Nonuniform Dust Outtow Observed around Infrared Object Nml Cygni

Measurements by the University of California Berkeley Infrared Spatial Interferome-ter at 11.15 m have yielded strong evidence for multiple dust shells and/or signiicant asymmetric dust emission around NML Cyg. New observations reported also include multiple 8-13 m spectra taken from 1994-1995 and N band (10.2 m) photometry from 1980-1992. These and past measurements are analyzed and tted to a model of the dust distribution around NML Cyg. No spherically symmetric single dust shell model is found consistent with both near-and mid-infrared observations. However, a circularly symmetric maximum entropy reconstruction of the 11 m brightness distribution suggests a double shell model for the dust distribution. Such a model, consisting of a geometrically thin shell of intermediate optical depth (11m 1:9) plus an outer shell (11m 0:33), is consistent not only with the 11 m visibility data, but also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 m silicate feature. The outer shell, or large scale structure, is revealed only by long-baseline in-terferometry at 11 m, being too cold (400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Ossenkopf, Henning, & Mathis (1992) proved superior to the Draine & Lee (1984) constants in tting the detailed shape of the silicate feature and broadband spectrum for this object. Recent observations of H 2 O maser emission around NML Cyg by Richards, Yates, & Cohen (1996) are consistent with the location of the two dust shells and provide further evidence for the two-shell model.