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Author: search.filters.author.Gupta, Ranjan

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    Imaging polarimetry of some selected dark clouds
    (2015-03-01) Sen, A.K.; Gupta, Ranjan; Ramprakash, A. N.; Tandon, S. N.
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    Interstellar extinction by porous grains
    (2015-03-01) Vaidya, D.B.; Gupta, Ranjan
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    Comparative performance of artificial neural networks for UV spectral classification
    (2015-02-07) Mukherjee, Soma; Bhattacharya, Ujwal; Parui, S.K; Gupta, Ranjan; Gulati, R.K
    In this paper we present an application of an artificial neural network model based on a multi-layered back propagation algorithm for spectral classification of UV data from the International Ultraviolet Explorer (IUE) low dispersion spectra reference atlas. The model used is similar to that of von Rippel et al. (1994), and is found to reduce the classification error as compared to .the recently reported results on the same data set (Gulati et al. 1994b ). The improved version of the network is much simpler in structure and the training time is reduced by a factor of almost 20. Such networks will prove very useful in efficient classification of large databases
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    Indo-U.S. Library of Coude Feed Stellar Spectra
    (2011-07-06) Valdes, Francisco; Gupta, Ranjan; Rose, James A.; et al.
    We have obtained spectra for 1273 stars using the 0.9m Coud´e Feed telescope at Kitt Peak National Observatory. This telescope feeds the coud´e spectrograph of the 2.1m telescope. The spectra have been obtained with the #5 camera of the coud´e spectrograph and a Loral 3K X 1K CCD. Two gratings have been used to provide spectral coverage from 3460 ˚ A to 9464 ˚ A, at a resolution of ∼1˚ A FWHM and at an original dispersion of 0.44 ˚ A/pixel. For 885 stars we have complete spectra over the entire 3460 ˚ A to 9464 ˚ A wavelength region (neglecting small gaps of < 50 ˚ A), and partial spectral coverage for the remaining stars. The 1273 stars have been selected to provide broad coverage of the atmospheric parameters Teff , log g, and [Fe/H], as well as spectral type. The goal of the project is to provide a comprehensive library of stellar spectra for use in the automated classification of stellar and galaxy spectra and in galaxy population synthesis. In this paper we discuss the characteristics of the spectral library, viz., details of the observations, data reduction procedures, and selection of stars. We also present a few illustrations of the quality and information available in the spectra. The first version of the complete spectral library is now publicly available from the National Optical Astronomy Observatory (NOAO) via FTP and HTTP.
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    Automated Classification of 2000 Bright IRAS Sources
    (2011-07-06) Gupta, Ranjan; Singh, Harinder P.; Volk, K.; et al.
    An Artificial Neural Network (ANN) scheme has been employed that uses a supervised back-propagation algorithm to classify 2000 bright sources from the Calgary database of IRAS (Infrared Astronomical Satellite) spectra in the region 8µm to 23µm. The database has been classified into 17 predefined classes based on the spectral morphology. We have been able to classify over 80 percent of the sources correctly in the first instance. The speed and robustness of the scheme will allow us to classify the whole of the LRS database, containing more that 50,000 sources, in the near future.
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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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    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