Professor Ranjan Gupta
Permanent URI for this communityhttp://localhost:4000/handle/11007/1767
Browse
Item 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.alIn 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.Item AUTOMATED CLASSIFICATION OF 2000 BRIGHT IRAS SOURCES(The Astrophysical Journal Supplement Series, 2004-06-25) Gupta, Ranjan; Singh, Harinder P.; Volk, K.; et.alAn 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 analysisItem 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.alWe 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.Item 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.alWe 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.Item An Imaging Polarimeter(IMPOL) for multi-wavelength observations(Astron. Astrophys. Suppl. Ser, 2008-02-02) Ramaprakash, A.N; Gupta, Ranjan; Sen, A.K.; et.alTaking 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.Item Interstellar extinction by composite grains(Astronomy & Astrophysics, 2001-06-24) Vaidya, D.B.; Gupta, R; Dobbie, J.S.; et.alRecent 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.Item 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.alThe 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.Item Optical Spectroscopy of Candidates of Young Stellar Objects in NGC 1333(2001-01-01) Itoh, Yoichi; Gupta, Ranjan; Oasa, Yumiko; et.alWe carried out low-resolution optical spectroscopy of 14 low-luminosity young stellar object (YSO) candidates in the NGC 1333 cluster. These objects were previously identified by the near-infrared imaging survey. Eleven objects were confirmed as YSOs by the H line emission. Strengths of the H emission are correlated with the near-infrared excesses of the objects. Spectral types of all YSOs are estimated to be M-type, indicative of low-mass. Comparisons of the results of our spectroscopic observations and the previous photometric observations with evolutionary tracks on the HR diagram suggest two objects to be young brown dwarfs.Item Photopolarimetric study of the star-forming clouds CB3, CB25, and CB39(Astronomy & Astrophysics, 2010-09-07) Sen, A.K.; Polcaro, V. F; Dey, I.; et.alThe 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 2 m H.C. Telescope, India, to determine the colour excess E(B − V) of the background stars by following a technique adopted by Bernabei & Polacaro (2001, A&A, 371, 123). These three clouds were selected from a set of eight clouds previously observed by us in optical polarimetry (Sen et al. 2000, A&AS, 141, 175). Further independent spectroscopic measurements of a few selected sample stars were recently carried out during February and March 2010 from 1.52 m 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.Item Recent polarimetric observations of comet 67 P/Churyumov-Gerasimenko(Icy Bodies of the Solar System Proceedings IAU Symposium, 2009-11-24) Anny-Chantal Levasseur-Regourd1; Hadamcik, Edith; Sen, A.K.; et.alRemote observations of solar light scattered by dust in comet 67P/Churyumov- Gerasimenko coma are of major importance to assess the properties of the dust and thus to prepare the rendezvous of the Rosetta spacecraft with comet 67P/Churyumov-Gerasimenko. We present polarimetric data obtained from India in December 2008 and France in March 2009. Compared with previous observations of this comet and of other Jupiter family comets, they confirm that it is dust-poor, although it may exhibit outbursts leading to the ejection of dust particles from its subsurface, especially after its perihelion passage.Item Scattering Properties and Composition of Cometary Dust(Astrophysics and Space Science, 2005-04-11) Gupta, Ranjan; Vaidya, D.B.; Bobbie, J.S; et.alComposition 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.Item Scattering Properties and Composition of Cometary Dust(Astrophysics and Space Science, 2005-04-11) Gupta, Ranjan; Vaidya, D.B.; Bobbie, J.S; et.alComposition 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 compositeItem ULTRAVIOLET STELLAR SPECTRAL CLASSIFICATION USING A MULTILEVEL TREE NEURAL NETWORK(Elsevier Science Ltd, 1995-01-25) Gulati, R.K; Gupta, R; Gothoskar, P; et.alHere we present a pattern classification technique based on an Artificial Neural Network (ANN) in a multi-level tree configuration to classify ultraviolet stellar spectra from the IUE Low-Dispersion Spectra Reference Atlas. Preliminary results of this technique show that 94% of the spectra have been classified correctly with an accuracy of one sub-class. A conventional 9~2 minimization scheme has also been applied to the data to compare the classification obtained from these schemes with that of the IUE catalog classification.