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Item The photometric study of light scattering from the surface of alumina powder and interpretations by Hapke formula(Sciencedirect, 2011-08-21) Deb, D; Sen, A.K.; Das, H.S.; et al.A laboratory experiment helps to understand the light scattering property of regolith like samples with known compositions and other physical parameters. The laboratory data so obtained can be compared with the existing in situ data on celestial objects like asteroids. Further, it may be analyzed with the help of various theoretical models to understand the light scattering processes from regolith more clearly. In this work we have performed laboratory based photometry of the light scattered from the surfaces of powdered alumina (Al2O3) at various tilt angles of the sample and at large phase angles, with the particles having diameter 0.3 lm. The wavelength of observation was 632.8 nm. These data have been fitted by a surface scattering model originally suggested by Hapke. Instead of using empirical Henyey–Greenstein phase function to fix the values of albedo and phase function to be used within Hapke formula, we have used Mie theory for the same. This approach helped us to determine the single particle properties such as particle diameter and complex refractive index from surface scattering phase curve alone. Mie theory depends only on the size parameter X(=2p(radius/wavelength)) and complex refractive index (n, k) of the material. Since the absorption coefficient (k) for alumina is known to be very low but not exactly zero, the best fit to the experimental data was obtained by least square technique with k as a free parameter, as the other parameters are known. Finally, we compare our results with other published results and discussItem 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.Item 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.Item Porous and Fluffy Grains in the Regions of Anomalous Extinction(J. Astrophys. Astr., 2000-01-29) Vaidya, D.B.; Anandarao, B. G.; Desai, J.N; et al.It has long been established that the ratio of total to selective extinction is anomalously large (³ 5) in certain regions of the interstellar medium. In these regions of anomalous extinction the dust grains are likely to be irregular in shape and fluffy in structure. Using discrete dipole approximation (DDA) we calculate the extinction for porous and fluffy grains. We apply DDA first to solid spheroidal particles assumed to be made of a certain (large) number of dipoles. Then we systematically reduce the number of dipoles to model the porous grains. The aggregates of these particles are suggested to form the fluffy grains. We study the extinction for these particles as a function of grain size, porosity and wavelength. We apply these calculations to interpret the observed extinc tion data in the regions of star formation (e.g. the Orion complex).Item 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.Item SDSS J092712.64+294344.0: recoiling black hole or merging galaxies?(2009-08-31) Vivek, M; Srianand, R.; Noterdaeme, P; et al.We report long-slit spectroscopic observations of SDSS J092712+294344 carried-out at the recently commissioned 2m telescope in IUCAA Girawali Observatory, India. This AGN-like source is known to feature three sets of emission lines at zem = 0.6972, 0.7020 and 0.7128. Different scenarios such as a recoiling black hole after asymmetric emission of gravitational waves, binary black holes and possible merging systems are proposed for this object. We test these scenarios by comparing our spectra with that fromthe Sloan Digital Sky Survey (SDSS), obtained 4 years prior to our observations. Comparing the redshifts of [Oiii]λλ4960,5008 we put a 3σ limit on the relative acceleration to be less than 32 km s−1 yr−1 between different emitting regions. Using the 2D spectra obtained at different position angles we show that the [Oiii]λ5008 line from the zem = 0.7128 component is extended beyond the spectral point spread function.We infer the linear extent of this line emitting region is ∼ 8 kpc.We also find a tentative evidence for an offset between the centroid of the [Oiii]λ5008 line at zem = 0.7128 and the QSO trace when the slit is aligned at a position angle of 299◦. This corresponds to the zem = 0.7128 system being at an impact parameter of ∼1 kpc with respect to the zem = 0.6972 in the north west direction. Based on our observations we conclude that the binary black hole model is most unlikely. The spatial extent and the sizes are consistent with both black hole recoil and merging scenarios.Item A diffuse bubble-like radio-halo source MRC 0116+111: imprint of AGN feedback in a low-mass cluster of galaxies(2009-07-09) Bagchi, Joydeep; Jacob, Joe; Gopal, Krishna; et al.We present detailed observations of MRC 0116+111, revealing a luminous, mini radiohalo of 240 kpc diameter located at the centre of a cluster of galaxies at redshift z = 0.131. Our optical and multi-wavelength GMRT and VLA radio observations reveal a highly unusual radio source: showing a pair of giant ( 100 kpc diameter) bubble-like diffuse structures, that are about three times larger than the analogous extended radio emission observed in M87 - the dominant central radio galaxy in the Virgo Cluster. However, in MRC 0116+111 we do not detect any ongoing Active Galactic Nucleus (AGN) activity, such as a compact core or active radio jets feeding the plasma bubbles. The radio emitting relativistic particles and magnetic fields were probably seeded in the past by a pair of radio-jets originating in the AGN of the central cD galaxy. The extremely steep high-frequency radio spectrum of the north-western bubble, located 100 kpc from cluster centre, indicates radiation losses, possibly because having detached, it is rising buoyantly and moving away into the putative hot intra-cluster medium. The other bubble, closer to the cluster centre, shows signs of ongoing particle re-acceleration.We estimate that the radio jets which inflated these two bubbles might have also fed enough energy into the intra-cluster medium to create an enormous system of cavities and shock fronts, and to drive a massive outflow from the AGN, which could counter-balance and even quench a cooling flow. Therefore, this source presents an excellent opportunity to understand the energetics and the dynamical evolution of radio-jet inflated plasma bubbles in the hot cluster atmosphere.Item Multi-wavelength observations of afterglow of GRB080319B and the modeling constraints(2009-04-11) Pandey, S.B.; Castro-Tirado, A. J.; Jelinek, M.; et al.Context. We present observations of the afterglow of GRB 080319B at optical, mm and radio frequencies from a few hours to 67 days after the burst. Aims. To understand the nature of this brightest explosion based on the observed properties and it’s comparison with the afterglow models. Methods. Present observations along with other published multi-wavelength data have been used to study the light-curves and spectral energy distributions of the burst afterglow. Results. Our results show that the observed features of the afterglow fits equally good with the Inter Stellar Matter and the Stellar Wind density profiles of the circum-burst medium. In case of both density profiles, location of the maximum synchrotron frequency νm is below optical and the value of cooling break frequency νc is below X−rays, ∼ 104 s after the burst. Also, the derived value of the Lorentz factor at the time of naked eye brightness is ∼ 300 with the corresponding blast wave size of ∼ 1018 cm. Conclusions. The numericalItem Janibacter hoylei sp. nov., Bacillus isronensis sp. nov. and Bacillus aryabhattai sp. nov., isolated from cryotubes used for collecting air from the upper atmosphere(Society for General Microbiology, 2009-05-15) Narlikar, J. V.; Shivaji, S.; Chaturvedi, Preeti; et al.Item On searches for gravitational waves from mini-creation events by laser interferometric detectors(Wiley-Blackwell, 2006-02-27) Sarmah, B. P.; Banerjee, S.K.; Dhurandhar, Sanjeev; et al.As an alternative view to the standard big bang cosmology, the quasi-steady-state cosmology argues that the Universe was not created in a single great explosion: it did not have a beginning, nor will it ever come to an end. The creation of new matter in the Universe is a regular feature occurring through finite explosive events. Each creation event is called a mini-bang, or a mini-creation event. Gravitational waves are expected to be generated as a result of any anisotropy present in this process of creation. A mini-creation event that ejects matter in two oppositely directed jets is thus a source of gravitational waves, which can in principle be detected by laser interferometric detectors. In the present work we consider the gravitational waveforms propagated by linear jets and then estimate the response of laser interferometric detectors such as LIGO and LISA.