Nonuniform Dust Outflow Observed around Infrared Object Nml Cygni

Measurements by the University of California Berkeley Infrared Spatial Interferometer at 11.15 km have yielded strong evidence for multiple dust shells and/or signiÐcant asymmetric dust emission around NML Cyg. New observations reported also include multiple 8È13 km spectra taken from 1994È1995 and N-band (10.2 km) photometry from 1980È1992. These and past measurements are analyzed and Ðtted to a model of the dust distribution around NML Cyg. No spherically symmetric single dust shell model is found consistent with both nearand mid-infrared observations. However, a circularly symmetric maximum entropy reconstruction of the 11 km brightness distribution suggests a double-shell model for the dust distribution. Such a model, consisting of a geometrically thin shell of intermediate optical depth plus an outer shell is consistent not only with the 11 km visibility data but (q11 km D 1.9) (q11 kmD 0.33), also with near-infrared speckle measurements, the broadband spectrum, and the 9.7 km silicate feature. The outer shell, or large-scale structure, is revealed only by long-baseline interferometry at 11 km, being too cold (D400 K) to contribute in the near-infrared and having no unambiguous spectral signature in the mid-infrared. The optical constants of Ossenkopf, Henning, & Mathis proved superior to the Draine & Lee (1984) constants in Ðtting the detailed shape of the silicate feature and broadband spectrum for this object. Recent observations of maser emission around NML Cyg by Richards, Yates, & Cohen 2 (1996) are consistent with the location of the two dust shells and provide further evidence for the twoshell model. Subject headings : circumstellar matter È dust, extinction È infrared : ISM: continuum È ISM: jets and outÑows È stars : AGB and post-AGB È stars : individual (NML Cyg)