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Vaidya, S. N.; Karunakaran, C.; Pande, B.M.; Gupta, Nupur; Iyer, Rajashekar; Karweer, S. B

doi:10.1023/a:1018663020449

Cited by 25 publications

(14 citation statements)

References 16 publications

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“…Voshchinnikov (1990) has also found the variation in the "2175 Å" feature with the shape of the grain. In our earlier study on the porous grains (Vaidya et al 1997(Vaidya et al , 1999.e. Papers I and II) we had found the shift in the the "2175 Å" peak as well as the variation in the width of the feature for the graphite grains with porosity.…”

Section: Extinction Properties Of Spheroidal Grainsmentioning

confidence: 95%

“…Draine 1988) to study the extinction properties of the nonspherical inhomogeneous particles (see Borghese et al 2003). We have used the DDA to study the extinction properties for porous and composite particles (Vaidya et al 1997(Vaidya et al , 1999(Vaidya et al , 2001Papers I, II and III hereafter). However, the DDA method required considerable computer time and memory.…”

Section: Introductionmentioning

confidence: 99%

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Interstellar extinction by spheroidal dust grains

Gupta¹,

Mukai

Vaidya³

et al. 2005

A&A

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Abstract.Observations of interstellar extinction and polarization indicate that the interstellar medium consists of aligned nonspherical 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 3.4−0.1 µm. 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 V c , 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 does 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.

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Section: Extinction Properties Of Spheroidal Grainsmentioning

confidence: 95%

Section: Introductionmentioning

confidence: 99%

Interstellar extinction by spheroidal dust grains

Gupta¹,

Mukai

Vaidya³

et al. 2005

A&A

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“…Table 2 shows the best-fit χ 2 minimized values and corresponding temperatures for all the composite grain models with silicate host and graphite as inclusions. For details on χ 2 minimization please refer Vaidya & Gupta (1997, 1999. Table 3 shows the best fit χ 2 minimized values and corresponding temperatures for all the composite grain models with silicate host and voids (porous) as inclusions.…”

Section: Absorption Efficiency Of Composite Spheroidal Grainsmentioning

confidence: 99%

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

Vaidya¹,

Gupta²

2011

A&A

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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.

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“…Mathis (1996) has noted the uncertainty in the use of EMT for treating the composite grains and has suggested that detailed calculations such as the DDA would be necessary for the treatment of voids or silicate/carbon particles for some wavelengths. In the previous work we have used the DDA to study the scattering properties of porous grains (Vaidya & Gupta 1997, 1999, hereafter called as Papers I and II). We have also used the DDA to study the scattering properties of a composite particle made up of a host water sphere with carbon inclusions (Dobbie &Chylek 1998 andChylek et al 1999).…”

Section: Introductionmentioning

confidence: 99%

Interstellar extinction by composite grains

Vaidya

Gupta

Dobbie

et al. 2001

A&A

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Abstract. 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 efficiencies 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 efficiencies of these composite grains, we evaluate the interstellar extinction curve in the wavelength region of 0.55 µm−0.20 µm.

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Cited by 25 publications

References 16 publications

Interstellar extinction by spheroidal dust grains

Interstellar extinction by spheroidal dust grains

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

Interstellar extinction by composite grains

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