An extension of the DUSTY radiative transfer code and an application to OH 26.5 and TT Cygni^⋆

Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium
e-mail: marting@oma.be

Received: 6 February 2012
Accepted: 2 May 2012

Abstract

For the best-studied nearby Galactic asymptotic giant branch (AGB) stars a wealth of observational data is typically available in the form of photometry, often extending into the sub-mm, spectra covering a wide wavelength range, and often also visibility curves. Almost 100 AGB stars and red super giants have been imaged as part of the MESS Herschel guaranteed time key program. This does not only add photometry points between 60 and 500 micron, but also intensity profiles at these wavelengths. Dust radiative transfer models are often used to analyse these types of datasets, but there are very many input parameters and therefore it is not straightforward how to derive a best fit. In order to facilitate this, the publicly available one-dimensional (1D) dust radiative transfer code DUSTY was modified and extended, and included as a subroutine in a minimazation code. The code allows a certain parameter set (typically luminosity, dust optical depth, dust temperature at the inner radius and slope of the density law) to be minimised against photometric and spectroscopic data, visibility curves and 1D intensity profiles as constraints. The code is described and first results are presented on the MESS targets OH 26.5 and TT Cyg. For OH26.5 previous findings regarding a two-component wind are confirmed, but with a smaller drop in mass loss (a factor of 5) than previously suggested. For TT Cyg it proved difficult to fit the Herschel intensity profiles and spectral energy distribution simultaneously. The best fits are obtained for density profiles that deviate strongly from r^-2 and are more like r ^{+ 3 −  + 3.5}. This is qualitatively consistent with hydrodynamical models that simulate the interaction of the stellar wind with the interstellar medium.