2007A&A...470..259L

We determine the dominant dust grain size and dust density distribution in the outer envelope of the high mass-loss carbon star IRC +10216 and estimate the interstellar radiation field (ISRF) intensity and colours near IRC +10216. We use Monte Carlo radiative transfer simulations to calculate the surface brightness distribution of the envelope due to scattered Galactic light. Calculations are made with several dust models and cloud density structures using realistic anisotropic interstellar radiation field. The results of the calculations are compared with measurements reported in the literature. The shape of the brightness profile of the cloud at different optical wavelengths implies that the dominant dust grain radius in the outer envelope of IRC +10216 is at least 0.1µm. The shape of the brightness profiles at large offsets shows that the cloud density structure is steeper than r^–2, indicating that the mass-loss rate has increased with time. The peak brightness and colours of the nebula cannot be reproduced simultaneously with the observed cloud shape with any of the dust models tested, if commonly used values for interstellar radiation field intensity are adopted. The best fit to the data is obtained with large grains (grain radius a>0.25µm); our simulations then imply a ∼30% lower interstellar radiation field intensity in B band than given by Galaxy models. However, it is possible that detailed modelling including, e.g., porous grains and realistic dust size distributions could eliminate the discrepancy.