Browsing by Author "Vaidya, D.B."

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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.
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
Composite Interstellar Grains
(Mon. Not. R. Astron. Soc, 2008-02-01) 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 efficiencies 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 3.40–0.10μm using the extinction efficiencies 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 find 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.
Composite interstellar grains and the 2175˚A feature
(Organic Matter in Space Proceedings IAU Symposium, 2008-08-14) Vaidya, D.B.; Gupta, Ranjan
We use discrete dipole approximation (DDA) to study the scattering properties of composite grains made up of host silicate spheroids and graphite inclusions. We calculate the extinction cross sections of the composite grains in the wavelength region 0.20–0.55 μm and study the extinction of the composite grains as a function of graphite inclusions. We present the composite grain model and discuss the results.
Infrared Emission from the Composite Grains: Effects of Inclusions and Porosities on the 10 and 18 μm Features
(Astronomy & Astrophysics manuscript, 2011-01-11) Vaidya, D.B.; Gupta, Ranjan
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 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 5.0-25.0μm. 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 10μm and 18μm 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 10μm, 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 5-25μm. The model flux ratio, R=Flux(18μ)/Flux(10μ), compares well with the observed ratio for the circumstellar dust. Conclusions. The results on the composite grains clearly indicate that the silicate feature at 10μm 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.
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.
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.
Interstellar extinction by composite grains
(Astronomy & Astrophysics, 2001-06-24) Vaidya, D.B.; Gupta, R; Dobbie, J.S.; et.al
Recent studies indicate that interstellar grains are composites of very small individual particles of silicates and carbon glued together into an aggregate. Using the discrete dipole approximation (DDA), we calculate the extinction, scattering and absorption e ciencies for composite grains assumed to be made of a host silicate sphere with embedded graphite inclusions. In particular, we study the extinction as a function of inclusion size and volume fraction. Using the extinction e ciencies of these composite grains, we evaluate the interstellar extinction curve in the wavelength region of 0:55 m−0:20 m.
Interstellar extinction by porous grains
(Astron. Astrophys., 1999-04-23) Vaidya, D.B.; Gupta, Ranjan
Interstellar extinction by porous grains
(2015-03-01) Vaidya, D.B.; Gupta, Ranjan
Interstellar Extinction by Spheroidal Dust Grains
(Astronomy & Astrophysics, 2008-02-02) Gupta, Ranjan; Mukai, Tadashi; Vaidya, D.B.; et al.
Observations of interstellar extinction and polarization indicate that the interstellar medium consists of aligned non-spherical dust grains which show variation in the interstellar extinction curve for wavelengths ranging from NIR to UV. To model the extinction and polarization, one cannot use the conventional Mie theory which assumes the grains as solid spheres. We have used a T-matrix based method for computing the extinction efficiencies of spheroidal silicate and graphite grains of different shapes (axial ratios) and sizes and used these efficiencies to evaluate the interstellar extinction curve in the wavelength range 3.4 − 0.1μm. A best fit linear combination of silicate and graphite grains of not very large axial ratio, fits the observed extinction curve reasonably well. We calculate the volume extinction factor Vc, which is an important parameter from the point of view of the cosmic abundance, for the spheroidal grain models that reproduce the interstellar extinction curve. We find that the shape of the grains do not affect the volume extinction factor. Finally we have also studied the extinction and linear polarization efficiencies for aligned spheroids. The results show that the shape of grains affects the linear polarization efficiencies considerably for various orientation angles of the spheroids.
Modeling of silicate emission features in circumstellar dust
(2011-11-22) Gupta, Ranjan; Vaidya, D.B.; Dutta, Rajeshwari
We present a composite grain model to explain the IR emission features at 10 and 18 µm from the circumstellar dust of stars. The IRAS-LRS data from about 700 stars have been analysed and fitted to a set of composite porous dust grains consisting silicate host and graphite/vacuum nclusions. The 10 µm silicate feature shifts with the inclusion volume fraction of graphite grains but not with the changes in porosity. Both 10 and 18 µm features do not broaden with the inclusions of graphite or vacuum. It is also noted that the axial ratio of the dust grain shapes change for various types of circumstellar dust observed in the IRAS data.The model uses a composite fluffy dust grain for ex-plaining most of the observed interstellar extinction curves and also polarization. Another parameter which needs to be constrained by the dust models is the in-terstellar abundances of Carbon and Silicon which is usually overestimated by the solid dust models but our model predicts closer match to the observed ISM abun-dances. Further, our composite dust model also ex-plains the IR emission from circumstellar dust.
Porous and Fluffy Grains in the Regions of Anomalous Extinction
(J. Astrophys. Astr., 2000-01-29) Vaidya, D.B.; Anandarao, B. G.; Desai, J.N; et al.
It has long been established that the ratio of total to selective extinction is anomalously large (³ 5) in certain regions of the interstellar medium. In these regions of anomalous extinction the dust grains are likely to be irregular in shape and fluffy in structure. Using discrete dipole approximation (DDA) we calculate the extinction for porous and fluffy grains. We apply DDA first to solid spheroidal particles assumed to be made of a certain (large) number of dipoles. Then we systematically reduce the number of dipoles to model the porous grains. The aggregates of these particles are suggested to form the fluffy grains. We study the extinction for these particles as a function of grain size, porosity and wavelength. We apply these calculations to interpret the observed extinc tion data in the regions of star formation (e.g. the Orion complex).
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.
Scattering Properties and Composition of Cometary Dust
(Astrophysics and Space Science, 2005-04-11) Gupta, Ranjan; Vaidya, D.B.; Bobbie, 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.
Scattering Properties and Composition of Cometary Dust
(Astrophysics and Space Science, 2005-04-11) Gupta, Ranjan; Vaidya, D.B.; Bobbie, 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