Infrared emission from the composite grains: effects of inclusions and porosities on the 10 and 18 μm features

¹ ICCSIR, 380009 Ahmedabad, India
e-mail: deepak.vaidya@iccsir.org
² IUCAA, Post Bag 4, Ganeshkhind, 411007 Pune, India
e-mail: rag@iucaa.ernet.in

Received: 30 April 2010
Accepted: 6 January 2011

Abstract

Aims. In this paper we study the effects of inclusions and porosities on the emission properties of silicate grains and compare the model curves with the observed infrared emission from circumstellar dust.

Methods. We calculated the absorption efficiency of the composite grain, made up of a host silicate oblate spheroid and inclusions of ice/graphite/or voids, in the spectral region 5.0–25.0 μm. The absorption efficiencies of the composite spheroidal oblate grains for three axial ratios were computed using the discrete dipole approximation (DDA). We studied the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we studied the variation in the 10   μm and 18   μm emission features with the volume fraction of the inclusions and porosities. We then calculated the infrared fluxes for these composite grains at several dust temperatures (T = 200−350 K) and compared the model curves with the average observed IRAS-LRS curve, obtained for circumstellar dust shells around oxygen rich M-type stars. The model curves were also compared with two other individual stars.

Results. The results for the composite grains show variation in the absorption efficiencies with the variation in the inclusions and porosities. In particular, it is found that the wavelength of peak absorption at 10   μm shifts towards longer wavelengths with variation in the volume fraction of the graphite inclusions. The spheroidal composite grains with axial ratio ~1.33; volume fraction of f = 0.1, and dust temperature between 210–340 K, fit the observed infrared emission from circumstellar dust reasonably well in the wavelength range 5–25 μm. The model flux ratio, R = Flux(18 μ)/Flux(10 μ), compares well with the observed ratio for the circumstellar dust.

Conclusions. The results on the composite grains clearly indicate that the silicate feature at 10 μm shifts with the volume fraction of graphite inclusions. The feature does not shift with the porosity. Both the features do not show any broadening with the inclusions or with porosity. The absorption efficiencies of the composite grains calculated using DDA and effective medium approximation (EMA) do not agree. The composite grain models presented in this study need to be compared with the observed IR emission from the circumstellar dust around a few more stars.