Composite circumstellar dust grains

Abstract

We calculate the absorption efficiencies of composite silicate grains with inclusions of graphite and silicon carbide in the spectral range 5-25µm. We study the variation in absorption profiles with volume fractions of inclusions. In particular we study the variation in the wavelength of peak absorption at 10 and 18µm. We also study the variation of the absorption of porous silicate grains. We use the absorption efficiencies to calculate the infrared flux at various dust temperatures and compare with the observed infrared emission flux from the circumstellar dust around some M-Type & AGB stars obtained from IRAS and a few stars from Spitzer satellite. We interpret the observed data in terms of the circumstellar dust grain sizes; shape; composition and dust temperature.

Modelling the mid-infrared polarization in dust around young stars

“…Volume fraction of inclusions: Alonso-Albi et al ( 2009) have made use of a grain mixture comprising of 86% silicate and 14% graphite for protoplanetary disks around Herbig Be stars. In this work, we have used three different fractions by volume, f = 0.1 (10%), 0.2 (20%) and 0.3 (30%) for SiC, graphite as well as porous grain inclusions (in compliance with the findings of Alonso-Albi et al ( 2009) and Gupta et al (2016)). A composite grain model with AR = 1.33, i.e.…”

“…We have used DDSCAT version 6.1 developed by Draine & Flatau (2003) (modified by Gupta et al (2005)) and the T-Matrix code developed by Mishchenko et al (2002) (modified by Vaidya & Gupta (2009)) for our model calculations. The detailed analysis of various composite grain models is provided in Gupta et al (2016). In this work, we have varied the following parameters to obtain the absorption efficiencies and subsequently the linear polarization:…”

“…Hence, the absorption efficiencies have been calculated within the Rayleigh approximation using a grain size, a min = 0.1 µm to a max = 0.5 µm, where 'a' represents the radius of a sphere such that its volume is equal to that of the dust grain spheroid. The details of particle size distribution are provided in Gupta et al (2016).…”

“…In addition, EMA also allows the user to explore the suitability of various dust mixtures [Voshchinnikov et al (2006); Saija et al (2001)]. Both these approximation methods have been used recently in a work by Gupta et al (2016) to model spheroidal composite dust grains made of silicate as the host material with silicon carbide (SiC) and graphite as inclusions. They have studied the effects of change in axial ratio, size, porosity and inclusions to the absorption efficiencies of dust grains in the 5-25 µm range with particular interest towards the 10 µm silicate feature observed in the MIR.…”

“…Various studies have identified dust grains to be porous and fluffy in nature occurring as conglomerates comprising of multiple very tiny grains attached together due to collisions or interaction among the grains and various other processes which are likely to make the dust grains non-spherical and inhomogeneous [Gupta et al (2016); and references therein]. In the absence of a precise theory for the study of composite dust grain properties, we take the help of various approximation techniques, e.g.…”