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    Interstellar Extinction by Spheroidal Dust Grains
    (2005-06-01) Gupta, Ranjan
    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 efficien- cies 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 m. 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.
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    Analysis of the distribution of background star polarization in dark clouds
    (2005-04-01) Sen, A.K.; Mukai, T.; Gupta, Ranjan; et al.
    The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization maps in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et al 2000). With a view to understand the origin of this polarization, in the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures (Td and Tg) in the cloud as expected from Davis-Greenstein grain alignment mechanism (Davis & Greenstein 1952). However, the average observed polarization (pav) appears to be related to the turbulence ∆V (measured by 12CO line width) by the mathematical relation pav = 2.95 exp(−0.24∆V ). The possible relation between the direction of polarization vector and other physical parameters are also discussed. For this analysis in addition to the data on above eight dark clouds, the data on CB4 (Kane et al. 1995) are also included for comparison.
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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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    Scattering and Extinction Properties of Nanodiamonds
    (2009-11-01) Rai, Rakesh K.; Rastogi, Shantanu
    The study of scattering and extinction properties of possible nanodiamond grains in the ISM are reported. Calculations using Discrete Dipole Approximation (DDA) for varying ellipsoidal shapes and sizes from 2.5 to 10 nm are considered. Nanodiamonds show negligible extinction from IR to near-UV and very sharp far-UV rise. Comparison with observations rule out possibility of independent nanodiamond dust but point towards possibility of nanodiamonds as a component in the ISM. Radiation induced transformations may lead to carbonaceous grains with different core and mantles. So calculations are also performed for a core-mantle target model with nanodiamond core in graphite mantles. The graphite extinction features get modified with the peak at 2175 ˚ A being lowered, broadened, blue shifted and accompanied by enhanced extinction in the far-UV. Such variations in the 2175 ˚ A band and simultaneous far-UV rise are observed along some sources. A three component dust model incorporating silicate, graphite and graphite with nanodiamond core is also considered. The model extinction compares very well with the average galactic extinction in the complete range from 0.2 to 10 m−1. The best fit requires small size and small number of nanodiamonds
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    Modelling interstellar extinction and polarization with spheroidal grains
    (2008-01) Voshchinnikov, N. V.; Das, H.K.
    We calculate the wavelength dependence of the ratio of the linear polarization degree to extinction (polarizing efficiency) P(λ)/A(λ) from the ultraviolet to ear-infrared. The prolate and oblate particles with aspect ratios from a/b = 1.1 up to 10 are assumed to be rotating and partially aligned with the mechanism of paramagnetic relaxation (Davis-Greenstein). Size/shape/orientation effects are analyzed. It is found that the wavelength dependence of P(λ)/A(λ) is mainly determined by the particle composition and size whereas the values of P(λ)/A(λ) depend on the particle shape, degree and direction of alignment.
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    Interstellar extinction and polarization - A spheroidal dust grain approach perspective
    (2010-01-10) Das, H.K.; Voshchinnikov, N. V.; Il'in, V.B.
    We extend and investigate the spheroidal model of interstellar dust grains used to simultaneously interpret the observed interstellar extinction and polarization curves. We compare our model with similar models recently suggested by other authors, study its properties and apply it to fit the normalized extinction A(λ)/AV and the polarizing efficiency P(λ)/A(λ) measured in the near IR to far UV region for several stars seen through one large cloud. We conclude that the model parameter Ω being the angle between the line of sight and the magnetic field direction can be more or less reliably determined from comparison of the theory and observations. This opens a way to study the spatial structure of interstellar magnetic fields by using multi-wavelength photometric and polarimetric observations.
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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.
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    Photopolarimetric study of the star-forming clouds CB3,CB25,and CB39
    (2010-07-22) Sen, A.K.; et al.; Polcaro, V. F.; Dey, I.
    The small compact isolated dark clouds also known as ’Bok globules’ are believed to be ideal sites for low-mass star formation. Some of these clouds are undergoing gravitational collapse, and the ambient magnetic field plays a key role in collapse dynamics. The background star polarimetry is generally accepted as a good tool to map the magnetic field, which is responsible for the alignment of dichroic grains that produce polarization. Aims. The background star polarization when studied together with extinction is expected to help us to understand various grain properties and the role of polarimetry as a tracer of magnetic field in these star-forming clouds. With this idea, polarization and colour excess E(B − V ) values for a set of background stars have been studied together to understand various astrophysical process in some star-forming dark clouds. Methods. Optical photometric observations of the three clouds CB3, CB25, and CB39 were carried out at the 2m H.C. Telescope, India, to determine the colour excess E(B −V ) of the background stars by following a technique adopted by Barnei and Polacaro (2001). These three clouds were selected from a set of eight clouds previously observed by us in optical polarimetry ( Sen et al. 2000). Further independent spectroscopic measurements of a few selected sample stars were recently carried out during February and March 2010 from 1.52m Cassini Telescope, Loinao, Italy, to confirm the correctness of estimated E(B − V ) values obtained by this photometric technique. Results. The colour excess E(B−V ) values so obtained were compared with optical polarization values obtained for the same set of stars. It was found that the measured extinction values increase with the increase in percentage polarization for the cloud CB39 and to some extent for CB25. However, for cloud CB31 no such correlation was observed. It is normally expected that the grains causing extinction should also cause polarization of the light from background stars. Any possible deviation from this under different circumstances here has been discussed in the light of the ongoing physical processes in the star-forming clouds.