2013MNRAS.430.3112M

We investigate the circumstellar dust properties of the oxygen-rich bipolar proto-planetary nebula (PPN) IRAS 18276-1431 by means of two-dimensional radiative transfer simulations of the circumstellar dust shell. The model geometry is assumed to have a torus and an envelope which consists of a pair of bipolar lobes and a spherical asymptotic giant branch shell. The parameters of the dust and the dust shell are constrained by comparing the spectral energy distribution (SED) and near-infrared intensity and polarization data with the models. The polarization in the envelope reaches 50-60percent and is nearly constant in the H and KS bands in the observations. This weak wavelength dependence of the polarization can be reproduced with a grain-size distribution function for the torus: 0.05µm ≤ a with n(a)∝a- (p = 5.5)exp( - a/ac = 0.3µm). The power index p is significantly steeper than that for interstellar dust (p ∼ 3). Similar results have also been found in some other PPNs and suggest that mechanisms that grind down large particles, such as sputtering, may also have acted when the dust particles formed. The spectral opacity index β is found to be 0.6±0.5 from the 760µm to 2.6 mm fluxes, which is characterized by the dust in the torus. This low value (<2) indicates the presence of large dust grains in the torus. We discuss two possible dust models for the torus. One has a size distribution function of 1.0 ≤ a ≤ amax = 5000.0µm with n(a) ∝ a-(p = 2.5) and the other is 1.0µm ≤ a ≤ amax = 10000.0µm with n(a) ∝ a-(p = 3.5). The former has β of 0.633, but we are not able to find reasonable geometry parameters to fit the SED in the infrared. The latter has β of 1.12, but reproduces the SED better over a wide wavelength range. With this dust model, the geometric parameters are estimated as follows: the inner and outer radii are 30 and 1000au and the torus mass is 3.0M☉. Given that the torii are generally not found to be rotating, a large fraction of the torus material is likely to be expanding. Assuming an expansion velocity of 15kms-1, the torus formation time and mass-loss rate are found to be ∼ 300 yr and ∼ 10-2M☉ yr-1, respectively.