Research Papers (RG)

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A 3D Automated Classification Scheme for the TAUVEX data pipeline
(Mon. Not. R. Astron. Soc., 2007-02-02) Bora, Archana; Gupta, Ranjan; Singh, Harinder P; et.al
In order to develop a pipeline for automated classification of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an artificial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250°A to 3220°A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classified 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two different simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classified the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classification scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.
An analysis of the distribution of background star
(Mon. Not. R. Astron. Soc, 2002-02-02) 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. In order to study the spatial distribution of the degree of polarization and position angles across the different parts of the cloud a simple model is proposed, where the cloud has been assumed to be a simple dichroic polarizing sphere and the light from the background star first passes through the IS medium and then through the cloud, before reaching the observer. One finds this simple model can explain to a reasonable extent the observed spatial (radial) dependence of the value of p for two of the clouds (CB25 , CB39), but for rest of the clouds the model fails. However, through this model one can explain why the polarization (p) need not always increase with total extinction Av as one moves in the deeper interior part of the cloud.
AUTOMATED CLASSIFICATION OF 2000 BRIGHT IRAS SOURCES
(The Astrophysical Journal Supplement Series, 2004-06-25) 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 Infrared Astronomical Satellite (IRAS) spectra in the region 8–23 m. The database has been classified into 17 predefined classes based on the spectral morphology. We have been able to classify over 80% of the sources correctly in the first instance. The speed and robustness of the scheme will allow us to classify the whole of the Low Resolution Spectrometer database, containing more than 50,000 sources, in the near future. Subject headings: infrared: galaxies — methods: data analysis
Automated classification of sloan digital sky survey (SDSS) stellar spectra using artificial neural networks
(Astrophys Space Sci, 2008-04-21) Bazarghan, Mahdi; Gupta, Ranjan
Automated techniques have been developed to automate the process of classification of objects or their analysis. The large datasets provided by upcoming spectroscopic surveys with dedicated telescopes urges scientists to use these automated techniques for analysis of such large datasets which are now available to the community. Sloan Digital Sky Survey (SDSS) is one of such surveys releasing massive datasets. We use Probabilistic Neural Network (PNN) for automatic classification of about 5000 SDSS spectra into 158 spectral type of a reference library ranging from O type to M type stars.
Automated star–galaxy segregation using spectral and integrated band data for TAUVEX/ASTROSAT satellite data pipeline
(New Astronomy, 2009-10-13) Bora, Archana; Gupta, Ranjan; Singh, Harinder P; et.al
We employ an Artificial Neural Network (ANN) based technique to develop a pipeline for automated segregation of stars from the galaxies to be observed by Tel-Aviv University Ultra-Violet Experiment (TAUVEX). We use synthetic spectra of stars from UVBLUE library and selected International Ultraviolet Explorer (IUE) low-resolution spectra for galaxies in the ultraviolet (UV) region from 1250 to 3220 Å as the training set and IUE low-resolution spectra for both the stars and the galaxies as the test set. All the data sets have been pre-processed to get band integrated fluxes so as to mimic the observations of the TAUVEX UV imager. We also perform the ANN based segregation scheme using the full length spectral features (which will also be useful for the ASTROSAT mission). Our results suggest that, in the case of the non-availability of full spectral features, the limited band integrated features can be used to segregate the two classes of objects; although the band data classification is less accurate than the full spectral data classification.
A CCD photometric study of the late type contact binary EK Comae Berenices
(New Astronomy, 2010-04-30) Deb, Sukanta; Singh, Harinder P; Seshadri, T.R; et.al
We present CCD photometric observations of the W UMa type contact bi- nary EK Comae Berenices using the 2 metre telescope of IUCAA Girawali Observatory, India. The star was classified as a W UMa type binary of subtype-W by Samec et al. (1996). The new V band photometric observa- tions of the star reveal that shape of the light curve has changed significantly from the one observed by Samec et al. (1996). A detailed analysis of the light curve obtained from the high-precision CCD photometric observations of the star indicates that EK Comae Berenices is not a W-type but an A- type totally eclipsing W UMa contact binary. The photometric mass ratio is determined to be 0.349 ± 0.005. A temperature difference of T = 141±10 K between the components and an orbital inclination of i[o] = 89.800±0.075 were obtained for the binary system. Absolute values of masses, radii and luminosities are estimated by means of the standard mass-luminosity relation for zero age main-sequence stars. The star shows O’Connell effect, asymmetries in the light curve shape around the primary and secondary maximum. The observed O’Connell effect is explained by the presence of a hot spot on the primary component.
A CCD photometric study of the newly discovered contact binary ASAS 134738+0410.1
(Bull. Astr. Soc. India, 2010-05-20) Deb, Sukanta; Singh, Harinder P.; Seshadri, T. R
We present a CCD photometric study of the star with ASAS ID 134738+ 0410.1 using V band observations obtained from the IUCAA Girawali Observatory (IGO) 2-metre telescope, India. The star was selected from the Scuti database of All Sky Automated Survey (ASAS) (Pojmanski 2002). Our analysis reveals that the star is not a Scuti variable but is in fact a W UMa type contact binary with an orbital period of 0.2853067 day. Two new times of primary and secondary minima were determined from the observed data. A preliminary solution obtained using the Wilson-Devinney light curve modelling technique indicates that the star is more likely a partially-eclipsingWUMa type contact binary. However, the determination of actual subtype of this binary is quite impossible from the photometry alone, as the observed light curve can be fitted for both A- and W-type solutions. The exact classification of this binary needs to be determined from high resolution spectroscopy.
A comparison of synthetic and observed spectra for G-K dwarfs using arti
(Astron. Astrophys, 1967-01-18) Gulati, R.K; Gupta, Ranjan; Rao, N.K
A library of synthetic spectra, based on Kurucz model atmospheres, has been used to compare the spectroscopic observations in thewavelength range 4850-5384 A by using statistical and supervised arti cial neural network methods. The effective temperatures assigned by these methods for G-K dwarfs are compared with those given in Gray and Corbally (1994) and found to be closely matching their calibration curve. This result provides a promising new technique for determination of fundamental stellar atmospheric parameters on the basis of comparison between the model generated synthetic spectra and observed stellar spectra.
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.
Cross-checking reliability of some available stellar spectral libraries using artificial neural networks
(Stellar Populations as Building Blocks of Galaxies Proceedings IAU Symposium, 2006-06-25) Gupta, Ranjan; Singh, S. Jotin; Singh, Harinder P
Cross-checking the reliability of various stellar spectral databases is an important and desirable exercise. Since number of stars in various databases have no known spectral types and some of the libraries do not have complete coverage resulting in gaps. We use an automated classification scheme based on Artificial Neural Networks (ANN) to cross-classify stars in the Indo-US stellar spectral library (Valdes et al. 2004), JHC (Jacoby, Hunter & Christian 1984), ELODIE spectra (Moultaka et al. 2004) and STELIB (Le Borgne et al. 2003). We have also examined the effects of over-training and over-fitting on the classification efficiency of a Neural Network. It is hoped that such a automated data analysis and validation technique will be useful in the future.
Filling Gaps in Indo-US Stellar Spectral Library using Principal Component Analysis
(Stellar Populations as Building Blocks of Galaxies Proceedings IAU Symposium, 2006-07-12) Singh, Harinder P; Singh, S. Jotin; Gupta, Ranjan; et
The Indo-US coud´e feed stellar spectral library (CFLIB) published recently by Valdes et al. (2004) contains spectra of 1273 stars in the spectral region 3460 to 9464 ˚A at a resolution of 1 ˚A. About 500 stars in this database have gaps ranging from a few ˚A to several tens of ˚A in this wavelength range. We use a variation of Principal Component Analysis (PCA) technique to fill gaps of up to 5˚A in a subset of spectra from the CFLIB. We hope to exploit the full potential of the scheme and attempt to fill larger gaps in stellar spectra in a subsequent study.
Frequency andsizedistributiondependenceofvisibleandinfrared extinctionforastronomicalsilicateandgraphitegrains
(JournalofQuantitativeSpectroscopy& RadiativeTransfer, 2011-10-09) Roy, Ashim K; Sharma, Subodh K; Gupta, Ranjan
In arecentpaper,thefrequencyandsizedistributiondependenceofextinctionspectra for astronomicalsilicateandgraphitegrainswasanalyzedinthecontextofMRNtype interstellardustmodelsinthefarultravioletandultravioletregions.Thesegrains were takentobehomogeneousspheresfollowingapowerlawsizedistribution.Inthe present workweextendtheanalysisfurthertocoverthevisibleaswellastheinfrared part oftheelectromagneticspectrum.Theanalyticformulaspresentedherealongwith thosegivenintheearlierpaperwouldenableonetoevaluateextinctionforthesegrains withinawiderwavelengthrange1000–22,500 ˚A andanalyzetheobservational interstellarextinctiondatainfargreaterdetails
An Imaging Polarimeter(IMPOL) for multi-wavelength observations
(Astron. Astrophys. Suppl. Ser, 2008-02-02) Ramaprakash, A.N; Gupta, Ranjan; Sen, A.K.; et.al
Taking advantage of the advances in array detector technology, an imaging polarimeter (IMPOL) has been constructed for measuring linear polarization in the wavelength band from 400-800 nm. It makes use of a Wollaston prism as the analyser to measure simultaneously the two orthogonal polarization components that define a Stoke’s parameter. An achromatic half-wave plate is used to rotate the plane of polarization with respect to the axis of the analyser so that the second Stoke’s parameter also can be determined. With a field of view correponding to about 30 × 30 mm2 for a / 1.2 m, f/13 telescope, a sensitive, liquid-N2 cooled CCD camera as the detector and a built-in acquisition and guidance unit, the instrument can be used for studying stellar fields or extended objects with an angular resolution of ∼2′′ . The instrumental polarization is less than 0.05% and the accuracies of measurement are primarily limited by photon noise for typical observations.
Imaging polarimetry of some selected dark clouds?
(Astron. Astrophys. Suppl. Ser., 2000-01-11) Sen, A.K.; Gupta, Ranjan; Ramaprakash, A.N; et al.
A set of eight Bok Globules CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 were observed polarimetrically in white light, using our Imaging Polarimeter (IMPOL), from the 1.2 m IR telescope at Mount Abu, India. The observations were carried out on di erent nights during the period December 1997 and April 1998. The CCD images obtained from the instrument (IMPOL) were analyzed, to produce polarization map of the Bok Globules. The stars in the eld, which are mostly background to the cloud show typically 2% of linear polarization. Clouds which are less dynamic (having 12CO line widths V < 2:5 km s−1), in general show slightly better alignment of polarization vectors with the projected direction of galactic plane. On the other hand the more dynamic group of clouds, has the polarization vectors more scattered and poorly aligned with the projected direction of the galactic plane. However one of the clouds observed CB58 does not follow this trend very well.
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.
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 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.
IUCAA 2 meter telescope and its first light instrument IFOSC
(Bull. Astr. Soc. India, 2002-05-05) Gupta, Ranjan; Burse, Mahesh P.; Das, H.K.; et.al
The various features of the IUCAA 2-meter telescope, its site parameters and considerations made for preserving the local seeing in terms of ventilation, thermal emissivity of the dome etc are described. The first light back-end instrument for this telescope, i.e. Iucaa Faint Object Spectrometer and Camera (IFOSC), too is described in some detail.