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

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.

Optical extinction of highly porous aerosol following atmospheric freeze drying

“…The DDA is expected to be accurate, as long as the interdipole separation, d , is small compared with the wavelength of the incident radiation [see also Purcell and Pennypacker , ; Vaidya and Gupta , ]. As indicated in Table , in our calculations, d is set to be 0.01 µm, which fits this criterion.…”

Composite circumstellar dust grains

“…It is seen that the absorption efficiencies increase with volume fraction of SiC and there is no appreciable shift in the wavelength of peak absorption at 10µm feature. In our earlier study Vaidya & Gupta (2011), using DDA, we have shown the variation of absorption efficiencies of composite grains with inclusions of graphite and porous silicate grains.…”

“…AR = 1.33 and volume fraction of f=0.1. Please see Table 1 in Vaidya & Gupta (2011) and also Chýlek et al (2000). The DDA code generates a three dimensional matrix, specifying the material type at each dipole site.…”

“…For a detailed description of DDA, see Draine (1988). In an earlier paper by Vaidya & Gupta (2011), the effects of inclusions and porosities on the 10 and 18µm features were studied for the average observed IRAS spectra. In this paper, we use both DDA and EMA-T-Matrix to study the absorption properties of the composite grains consisting of host silicate spheroidal grains and inclusions of graphite or silicon carbide (SiC).…”

Composite Grains: Carriers of the Diffuse Interstellar Bands

“…We use the modified computer code (Vaidya et al 2001, Vaidya & Gupta 2011 to generate the composite grain models. We have studied composite grain models consisting of a host silicate spheroid and inclusions of graphite or other carbonaceous materials (e.g.…”