IUCAA Preprints

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Author: search.filters.author.Vaidya, D.B.Has files: Yes

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    Interstellar extinction by porous grains
    (2015-03-01) Vaidya, D.B.; Gupta, Ranjan
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    Scattering properties and composition of cometary dust
    (2005-04-01) Gupta, Ranjan; Vaidya, D.B.; Dobbie, J.S.; et al.
    Composition of the Comet dust obtained by the dust impact analyzer on the Halley probes indicated that the comet dust is a mixture of silicate and carbonaceous material. The collected interplanetary dust particles (IDP’s) are fluffy and composite, having grains of several different types stuck together. Using Discrete Dipole Approximation (DDA) we study the scattering properties of composite grains. In particular, we study the angular distribution of the scattered intensity and linear polarization of composite grains. We assume that the composite grains are made up of a host silicate sphere/spheroid with the inclusions of graphite. Results of our calculations on the composite grains show that the angle of maximum polarization shifts, and the degree of polarization varies with the volume fraction of the inclusions. We use these results on the composite grains to interpret the observed scattering in cometary dust.
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    Composite Interstellar Grains
    (2007-01-20) Vaidya, D.B.; Gupta, Ranjan; Snow, T. P.
    A composite dust grain model which is consistent with the observed interstellar extinction and linear polarization is presented. The composite grain is made up of a host silicate spheroid and graphite inclusions. The extinction e ciencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA). The interstellar extinction curve is evaluated in the spectral region 3.40{0.10 musing the extinction e ciencies of the composite spheroidal grains. The model extinction curves are then compared with the average observed interstellar extinction curve.We also calculate the linear polarization for the spheroidal composite grains at three orientation angles and nd the wavelength of maximum polarization. Further, we estimate the volume extinction factor, an important parameter from the point of view of cosmic abundance, for the composite grain models that reproduce the average observed interstellar extinction. The estimated abundances derived from the composite grain models for both carbon and silicon are found to be lower than that are predicted by the bare silicate/graphite grain models but these values are still higher than that are implied from the recent ISM values
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    Infrared emission from the composite grains: Effects of inclusions and porosities on the 10 and 18 µm features
    (2011-01-11) Vaidya, D.B.; Gupta, Ranjan
    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 calculate 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 m. The absorption efficiencies of the composite spheroidal oblate grains for three axial ratios are computed using the discrete dipole approximation (DDA). We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10 m and 18 m emission features with the volume fraction of the inclusions and porosities. We then calculate the infrared fluxes for these composite grains at several dust temperatures (T=200-350K) and compare the model curves with the average observed IRAS-LRS curve, obtained for circumstellar dust shells around oxygen rich M-type stars. The model curves are also compared with two other individual stars. Results. The results on 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 m, shifts towards longer wavelengths with variation in the volume fraction of the inclusions of graphite. The spheroidal composite grains with axial ratio ∼ 1.33; volume fraction of f=0.1 and dust temperature between 210-340K, fit the observed infra-red emission from circumstellar dust reasonably well in the wavelength range 5-25 m. The model flux ratio, R=Flux(18 )/Flux(10 ), 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 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.
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    Composite grains: Effects of porosity and inclusions on the 10 um silicate feature
    (2009-03-01) Vaidya, D.B.; Gupta, Ranjan
    We calculate the absorption efficiency of the composite grains, made up of host silicate spheroids and inclusions of ices/graphites/or voids, in the spectral region 7.0− 14.0µm The absorption efficiencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA) as well as using the effective medium approximation & T-Matrix (EMT-Tmatrix) approach. We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10.0µm feature with the volume fraction of the inclusions and porosity. We then calculate the infrared fluxes for these composite grains and compare the model curves with the average observed IRAS-LRS curve, obtained for several circumstellar dust shells around stars. These results on the composite grains show that the wavelength of the peak absorption shifts and the width of the 10.0µm feature varies with the variation in the volume fraction of the inclusions. The model curves for composite grains with axial ratios not very large (AR∼1.3) and volume fractions of inclusions with f=0.20, and dust temperature of about 250-300◦K , fit the observed emission curves reasonably well.
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    Interstellar dust models towards some IUE stars
    (2010-11-24) Katyal, Nisha; Gupta, Ranjan; Vaidya, D.B.
    We present a study of interstellar extinction towards certain directions in our galaxy defined by about 59 stars which were observed by the IUE satellite. The observed UV extinction curves have been modeled by making light scattering calculations based on Discrete Dipole Approximation (DDA) for extinction efficiencies of composite dust grains which is made up of a host silicate spheroid and graphite inclusions. The study indicates that smaller dust grains are more efficient for the directions in the galaxy where observed Rv values are found to be low.