Investigation of dust properties of the proto-planetary nebula IRAS 18276-1431

TL;DR

This study models the dust properties of the proto-planetary nebula IRAS 18276-1431 using radiative transfer simulations, revealing large dust grains, steep grain size distributions, and a dense torus structure with specific geometric and mass-loss characteristics.

Contribution

It introduces a detailed radiative transfer model of the dust shell in IRAS 18276-1431, constraining dust grain sizes, distributions, and nebula geometry with observational data.

Findings

Polarisation reaches 50-60% with weak wavelength dependence.

Presence of large dust grains indicated by low spectral opacity index beta.

Estimated torus parameters: inner radius 30 AU, outer radius 1000 AU, mass 3 solar masses.

Abstract

We investigate the circumstellar dust properties of the oxygen-rich bipolar proto-planetary nebula 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. The parameters of the dust and the dust shell are constrained by comparing the SED and NIR intensity and polarisation data with the models. The polarisation in the envelope reaches 50 -- 60 % and is nearly constant in the H and K_S bands in the observations. This weak wavelength dependence of the polarisation can be reproduced with a grain size distribution function for the torus: 0.05 micron <= a with n(a)=a^{-(p=5.5)}exp(-a/{a_c=0.3 micron}). The power index p is significantly steeper than that for interstellar dust. Similar results have also been found in some other PPNs and suggest that mechanisms that grind down large particles may also have acted when the dust particles formed. The spectral opacity index beta is found to be 0.6+/-0.5 from the millimeter fluxes. This low value 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 micron <= a <= a_max=5,000.0 micron with n(a)=a^{-(p=2.5)} and the other is 1.0 micron <= a <= a_max=10,000.0 micron with n(a)=a^{-(p=3.5)}. The former has beta of 0.633, but we are not able to find reasonable geometry parameters to fit the SED in the infrared. The latter has beta 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 AU and 1000 AU and the torus mass is 3.0 M_sun. Assuming an expansion velocity of 15 kms^{-1}, the torus formation time and mass-loss rate are found to be \sim300 yrs and \sim10^{-2}M_sun yr^{-1} respectively.