Infrared imaging and spectroscopy of the Luminous Blue Variables Wra 751 and AG Car

Abstract

We present ground-based infrared imaging and ISO spectroscopy of the luminous blue variables Wra 751 and AG Car. The images show in both cases a detached shell with a roughly circular distribution of emission. The infrared images of AG Car coincide very well with the optical images. The optical (Hα ) image of Wra 751 is different from the infrared image; the Hα nebula is suggested to be a scattering nebula containing cold dust particles. Fitting both the images and the spectra consistently with a 1-D radiative transfer model, we derive properties of their dust shells. Wra 751 is surrounded by a dust shell with inner and outer radii of 0.17 and 0.34 pc respectively and a dust mass of 0.017 M_sun . The dust shell of AG Car has inner and outer radii of 0.37 and 0.81 pc respectively and a total dust mass of 0.25 M_sun . Dust mass-loss rates during the formation of the shells are 2.7x 10^-6 and 3.4x 10^-5 M_sun yr^-1, respectively. The total dust mass and hence the derived dust mass-loss rates are uncertain by at least a factor of two. For AG Car, the derived dust mass and mass-loss rate are higher than previous estimates. This is mainly caused by the fact that a contribution of very large grains (> 10 mu m) is needed to explain the flux levels at longer wavelengths. Dust models for both objects fail to explain the flux shortward of 15 to 20 mu m: a population of small warm grains, not in thermal equilibrium with the central star is necessary to explain this excess. Similarities between dust shells around Wolf-Rayet stars and Wra 751 and AG Car (mass, grain size population, morphology) suggest a similar formation history and imply an evolutionary connection. A similar connection with red supergiants is suggested on the basis of the dust composition and derived time-averaged mass-loss rates. based on observations obtained with ISO, an ESA project with instruments funded by ESA Member states (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. Based on observations obtained at ESO, La Silla, Chile