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Ahmadābād, India

Katyal N.,Inter-University Center for Astronomy and Astrophysics | Katyal N.,Jawaharlal Nehru University | Gupta R.,Inter-University Center for Astronomy and Astrophysics | Vaidya D.B.,ICCSIR
Publications of the Astronomical Society of the Pacific | Year: 2013

We study the extinction properties of composite dust grains, consisting of host silicate spheroids and graphite as inclusions, using discrete dipole approximation (DDA). We calculate the extinction cross sections of the composite grains in the ultraviolet spectral region, 1200-3200 Å, and study the variation in extinction as a function of the volume fraction of the inclusions. We compare the model extinction curves with the observed interstellar extinction curves obtained from the data given by the International Ultraviolet Explorer (IUE) satellite. Our results for the composite grains show a distinct variation in the extinction efficiencies with the variation in the volume fraction of the inclusions. In particular, it is found that the wavelength of peak absorption at 2175 Å shifts towards the longer wavelength with the variation in the volume fraction of inclusions. We find that the composite grain models with the axial ratios viz. 1.33 and 2.0 fit the observed extinction reasonably well with a grain size distribution, a = 0.005-0.250 μm. Moreover, our results on the composite grains clearly indicate that the inhomogeneity in the grain structure, composition, and the surrounding media modify the extinction properties of the grains. © 2013. The Astronomical Society of the Pacific. All rights reserved. Source


Vaidya D.B.,ICCSIR | Gupta R.,Inter-University Center for Astronomy and Astrophysics
Astronomy and Astrophysics | Year: 2011

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. © 2011 ESO. Source


Vaidya D.B.,ICCSIR | Gupta R.,Inter-University Center for Astronomy and Astrophysics
AAPP Atti della Accademia Peloritana dei Pericolanti, Classe di Scienze Fisiche, Matematiche e Naturali | Year: 2011

Using the discrete dipole approximation (DDA) we calculate the absorption effciency of the composite grain, made up of a host silicate spheroid and inclusions of graphite, in the spectral region 5.0-25.0μm. We study the absorption as a function of the voulume fraction of the inclusions. In particular, we study the variation in the 10.0μm and 18.0μm emission features with the volume fraction of the inclusions. Using the extinction effciencies, of the composite grains we calculate the infrared fuxes at several dust temperatures and compare the model curves with the observed infrared emission curves (IRAS-LRS), obtained for circumstellar dust shells around oxygen rich M-type stars. © 2011 by the Author(s); licensee Accademia Peloritana dei Pericolanti, Messina, Italy. Source


Katyal N.,Inter-University Center for Astronomy and Astrophysics | Gupta R.,Inter-University Center for Astronomy and Astrophysics | Vaidya D.B.,ICCSIR
Earth, Planets and Space | Year: 2011

A composite dust grain model which simultaneously explains the observed interstellar extinction, polarization, IR emission and the abundance constraints, is required. We present a composite grain model, which is made up of a host silicate oblate spheroid and graphite inclusions. The interstellar extinction curve is evaluated in the spectral region 3.4-0.1 μm using the extinction efficiencies of composite spheroidal grains for three axial ratios. Extinction curves are computed using the discrete dipole approximation (DDA). The model curves are subsequently compared with the average observed interstellar extinction curve and with an extinction curve derived from the IUE catalogue data. Copyright © The Society of Geomagnetism and Earth Planetary and Space Sciences (SGEPSS). Source

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