2003A&A...401..985M

We present and analyse new optical images of the dust envelope surrounding the high mass-loss carbon star IRC +10216. This envelope is seen due to external illumination by galactic light. Intensity profiles and colors of the nebula were obtained in the UBV bandpasses. The data are compared with the results of a radiative transfer model calculating multiple scattering of interstellar field photons by dust grains with a single radius. The data show that the observed radial shape of the nebula, especially its half maximum radius, does not depend on wavelength (within experimental errors), suggesting that grains scatter in the grey regime, and this is further supported by the plateau colors being close to those of the ISRF as given by Mattila (1980A&AS...39...53M). A grain radius of 0.16µm with envelope parameters as proposed by Groenewegen (1997A&A...317..503G) can reproduce this achromatism of shape and color characteristics. However, there remain substantial discrepancies between model and observations concerning the absolute intensity of the nebula and its radial shape. Some of these discrepancies disappear if one adopts a small grain size (∼0.05µm), or if one assumes a lower dust mass loss rate for the outer layers (θ≥20'', corresponding to 1000 years ago). Within the framework of our simple model, we cannot determine a ``dominant'' grain size. Future more sophisticated models will have to take into account grain size distribution, and also explore complicated issues like the effects of grain porosity and/or asphericity on scattering, the influence of the envelope small-scale structure on the radiative transfer, and the possibility of a field anisotropy. For the same reasons, it is not presently feasible to establish with confidence whether the interstellar radiation field in the visible is significantly different in strength at the location of IRC +10216 compared to the usually adopted one in the solar neighbourhood.