Bispectrum speckle interferometry observations and radiative transfer modelling of the red supergiant NML Cyg - Multiple dust-shell structures evidencing previous superwind phases

NML Cyg is a highly evolved OH/IR supergiant, one of the most prominent inf rared objects due to its strong obscuration by dust, and supposed to be amo ng the most luminous supergiants in the galaxy. We present the first diffra ction-limited 2.13 mum observations of NML Cyg with 73 mas resolution. The speckle interferograms were obtained with the 6 m telescope at the Special Astrophysical Observatory, and the image reconstruction is based on the bis pectrum speckle-interferometry method. The visibility function declines tow ards the diffraction limit to similar to0.6. Radiative transfer calculation s have been carried out to model the spectral energy distribution, given by ground-based photometry and ISO spectroscopy, and our 2.13 mum visibility function. Additionally, midinfrared visibility functions at 11 mum were con sidered. The observed dust shell properties do not appear to be in accordan ce with standard single-shell (uniform outflow) models but seem to require multiple components. Considering previous periods of enhanced mass-loss, va rious density enhancements in the dust shell were taken into account. An ex tensive grid of models was calculated for different locations and strenghts of such superwind I regions in the dust shell. To match the observations f rom the optical to the sub-mm domain requires at least two superwind region s embedded in the shell. The best model includes a dust shell with a temper ature of 1000 K at its inner radius of 6.2R(*). a close embedded superwind shell extending from 15.5R(*) to 21.7R(*) with an amplitude (factor of dens ity enhancement) of 10, and a Ear-out density enhancement at 186R(*) with a n amplitude of 5. The angular diameters of the central star and of the inne r rim of the dust shell amount to 16.2 mas and 105 mas, resp. The diameter of tile embedded close superwind region extends from 263 mas to 368 mas, an d the inner boundary of the distant superwind region has a diameter of 3." 15. In the near-infrared the dust condensation zone is limb-brightened lead ing to a corresponding ring-like intensity distribution. The grain sizes, a , were found to be in accordance with a standard distribution function, n(a ) similar to a(-3.5), with a ranging between a(min) = 0.005 mum and a(max) = 0.15 mum The bolometric flux amounts to F-bol = 3.63 10(-9) Wm(-2) corres ponding to a central-star luminosity of L/L. = 1.13 10(5) (d/kpc)(2). Withi n the various parts of the dust shell, 1/r(2) density distributions could b e maintained differing only in their amplitude A. A slight improvement of t he far-infrared properties can be obtained if a shallower density distribut ion of rho similar to 1/r(1.7) is considered in the distant superwind regio n. The present-day mass-loss rate was determined to be M = 1.2 10(-4) M./yr . The inner embedded superwind shell corresponds to a phase of enhanced mas s-loss (with amplitude 10) in the immediate history of NML Cyg which began 59 yr ago and lasted for similar to 18 yr. Correspondingly, the outer super wind region is due to to a high mass-loss period (amplitude 5) which termin ated 529 yr ago.