Interstellar extinction by spheroidal dust grains

Gupta, Ranjan; Mukai, T.; Vaidya, D. B.; Sen, A. K.; Okada, Yasuhiko

doi:10.1051/0004-6361:20052707

Cited by 25 publications

(31 citation statements)

References 24 publications

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“…However, the required range of values for a max is probably unrealistic, given the observational constraints from the extinction curve. The interstellar extinction curve of regions of low extinctions is well reproduced by an MRN grain size distribution of spheroidal grains with a max ¼ 0:25 m (Gupta et al 2005). If this is the value of a max in regions of relatively low density, Figure 6 shows that E(H À K ) can increase with density only by less than a factor of 2 as the maximum grain size grows from a max ¼ 0:25 to 0.6 m. In other words, neglecting the process of grain growth can cause errors only of less than a factor of 2 in the dust column density based on E(H À K ), which is insufficient to explain the difference between the gas and the dust power spectra.…”

Section: Grain Growth and Nir Extinction Efficiencymentioning

confidence: 83%

“…We cannot entirely rule out this explanation, given the still limited knowledge of dust grain properties (different grain models can satisfy the same observational constraints, and most grain models are not realistic enough, as they overestimate the abundance of carbon and silicon relative to interstellar medium values in order to explain the extinction curve; Gupta et al 2005). However, based on available models, the UV extinction curve is best fit by a value of a max ¼ 0:25 m in regions of relatively low extinction.…”

Section: Discussionmentioning

confidence: 99%

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Can We Trust the Dust? Evidence of Dust Segregation in Molecular Clouds

Padoan

Cambrésy

Juvela

et al. 2006

ApJ

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Maps of estimated dust column density in molecular clouds are usually assumed to reliably trace the total gas column density structure. In this work we present results showing a clear discrepancy between the dust and the gas distribution in the Taurus molecular cloud complex. We compute the power spectrum of a 2MASS extinction map of the Taurus region and find that it is much shallower than the power spectrum of a 13 CO map of the same region that was previously analyzed. This discrepancy could be explained as the effect of grain growth on the grain extinction efficiency. However, this would require a wide range of maximum grain sizes, which is ruled out based on constraints from the extinction curve and the available grain models. We show that major effects due to CO formation and depletion are also ruled out. Our result may therefore suggest the existence of intrinsic spatial fluctuations of the dustto-gas ratio, with amplitude increasing toward smaller scales. Preliminary results of numerical simulations of trajectories of inertial particles in turbulent flows illustrate how the process of clustering of dust grains by the cloud turbulence may lead to observable effects. However, these results cannot be directly applied to large-scale supersonic and magnetized turbulence at present.

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Section: Grain Growth and Nir Extinction Efficiencymentioning

confidence: 83%

Section: Discussionmentioning

confidence: 99%

Can We Trust the Dust? Evidence of Dust Segregation in Molecular Clouds

Padoan

Cambrésy

Juvela

et al. 2006

ApJ

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“…This figure also shows the inclusions embedded in the host oblate spheroid. Oblate spheroids were selected based on the numerous results of previous studies (Greenberg & Hong 1975;Henning & Stognienko 1993;O'Donnell 1994;Gupta et al 2005) that showed that oblate spheroids represent properties of circumstellar dust particles better; specifically, this model provides a good fit to the observed polarization across the 10 μm feature (Lee & Draine 1985).…”

Section: Composite Grains and Ddamentioning

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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“…The influence of the type of spheroidal grain on the polarisation is discussed by Voshchinnikov (2004). Dust models considering spheroidal particles that fit the average Galactic extinction and polarisation curves were presented by Gupta et al (2005); Draine & Allaf-Akbari (2006); and Draine & Fraisse (2009). The observed interstellar extinction and polarisation curves towards particular sight lines have been modelled by Voshchinnikov & Das (2008) and Das et al (2010).…”

Section: Introductionmentioning

confidence: 99%

Dust in the diffuse interstellar medium

Siebenmorgen

Вощинников

Bagnulo

2014

A&A

112

123

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We present a model for the diffuse interstellar dust that explains the observed wavelength-dependence of extinction, emission, and the linear and circular polarisation of light. The model is set up with a small number of parameters. It consists of a mixture of amorphous carbon and silicate grains with sizes from the molecular domain of 0.5 up to about 500 nm. Dust grains with radii larger than 6 nm are spheroids. Spheroidal dust particles have a factor 1.5-3 greater absorption cross section in the far-infrared than spherical grains of the same volume do. Mass estimates derived from submillimetre observations that ignore this effect are overestimated by the same amount. In the presence of a magnetic field, spheroids may be partly aligned and polarise light. We find that polarisation spectra help to determine the upper particle radius of the otherwise rather unconstrained dust size distribution. Stochastically heated small grains of graphite, silicates, and polycyclic aromatic hydrocarbons (PAHs) are included. We tabulate parameters for PAH emission bands in various environments. They show a trend with the hardness of the radiation field that can be explained by the ionisation state or hydrogenation coverage of the molecules. For each dust component its relative weight is specified so that absolute element abundances are not direct input parameters. The model is compared to the average properties of the Milky Way, which seem to represent dust in the solar neighbourhood. It is then applied to specific sight lines towards four particular stars, with one of them located in the reflection nebula NGC 2023. For these sight lines, we present ultra-high signal-to-noise linear and circular spectro-polarimetric observations obtained with FORS at the VLT. Using prolate rather than oblate grains gives a better fit to observed spectra; the axial ratio of the spheroids is typically two and aligned silicates are the dominant contributors to the polarisation.

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Interstellar extinction by spheroidal dust grains

Cited by 25 publications

References 24 publications

Can We Trust the Dust? Evidence of Dust Segregation in Molecular Clouds

Can We Trust the Dust? Evidence of Dust Segregation in Molecular Clouds

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

Dust in the diffuse interstellar medium

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