2010 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/152

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    Perturbed power-law parameters from WMAP7
    (2010-11-19) Minu, Joy; Souradeep, Tarun
    We present a perturbative approach for studying inflation models with soft departures from scale free spectra of the power law model. In the perturbed power law (PPL) approach one obtains at the leading order both the scalar and tensor power spectra with the running of their spectral indices, in contrast to the widely used slow roll expansion. The PPL spectrum is confronted data and we show that the PPL parameters are well estimated from WMAP-7 data.
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    Optical spectroscopy of candidates of young stellar objects in NGC 1333
    (2010-12-31) Itoh, Yoichi; et al.; Gupta, Ranjan; Oasa, Yumiko
    We 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.
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    Odd-parity CMB bispectrum
    (2010-10-22) Kamionkowski, Marc; Souradeep, Tarun
    Measurement of the cosmic microwave background (CMB) bispectrum, or three-point correlation function, has now become one of the principle efforts in early-Universe cosmology. Here we show that there is a odd-parity component of the CMB bispectrum that has been hitherto unexplored. We argue that odd-parity temperature-polarization bispectra can arise, in principle, through weak lensing of the CMB by chiral gravitational waves or through cosmological birefringence, although the signals will be small even in the best-case scenarios. Measurement of these bispectra requires only modest modifications to the usual data-analysis algorithms. They may be useful as a consistency test in searches for the usual bispectrum and to search for surprises in the data.
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    Non-adiabatic gravitational collapse of a superdense star
    (2010-06-01) Sarwe, S. B.; Tikekar, R.
    The relativistic equations governing the non-adiabatic shear-free collapse of massive superdense stars in the presence of dissipative forces producing heat flow in the background of space–times of the Vaidya–Tikekar ansatz with associated physical three-spaces that have the three-spheroidal geometry are formulated. It is shown how the system can be used to examine the development and progress of the collapse during subsequent epochs until the radiating star becomes a black hole.
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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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    Turbulence modeling and the physics of the Intra-cluster medium
    (2010-07-11) Niemeyer, Jens C.; Paul, Surajit; et al.; Iapichino, L.
    The effective modeling of the stirring and development of turbulent flows in grid-based hydrodynamical simulations is computationally challenging. Here we present two possible ways to tackle the problem: first, we consider the use of the adaptive mesh refinement (AMR), applying novel refinement criteria which are optimized to follow the evolution of a turbulent flow. In a second step, the AMR is combined with a subgrid scale (SGS) model for the unresolved turbulence, thus resulting in a new numerical technique called FEARLESS (Fluid mEchanics with Adaptively Refined Large Eddy SimulationS). FEARLESS performs both the adaptive refinement of the regions where turbulent flows develop and a consistent coupling of the SGS turbulence with the resolved scales, and is argued to be a suitable tool in simulations of turbulent clumped flows. The results of galaxy cluster simulations, performed with the new tool, give rise to several interesting implications with regard to the physics of these objects, and to the numerical methods employed for their exploration in computational cosmology.
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    Time delay interferometry for LISA with one arm dysfunctional
    (2010-01-27) Dhurandhar, Sanjeev; Nayak, K. R.; Vinet, J-Y.
    In order to attain the requisite sensitivity for LISA - a joint space mission of the ESA and NASA- the laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. By combining six appropriately time-delayed data streams containing fractional Doppler shifts - a technique called time delay interferometry (TDI) - the laser frequency noise may be adequately suppressed. We consider the general model of LISA where the armlengths vary with time, so that second generation TDI are relevant. However, we must envisage the possibility, that not all the optical links of LISA will be operating at all times, and therefore, we here consider the case of LISA operating with two arms only. As shown earlier in the literature, obtaining even approximate solutions of TDI to the general problem is very difficult. Since here only four optical links are relevant, the algebraic problem simplifies considerably. We are then able to exhibit a large number of solutions (from mathematical point of view an infinite number) and further present an algorithm to generate these solutions.
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    Statistical isotropy violation of the CMB brightness fluctuations
    (2010-03-22) Aich, Moumita; Souradeep, Tarun
    Certain anomalies at large angular scales in the cosmic microwave background measured by WMAP have been suggested as possible evidence of breakdown of statistical isotropy(SI). SI violation of cosmological perturbations is a generic feature of ultra large scale structure of the cosmos and breakdown of global symmetries. Most CMB photons free-stream to the present from the surface of last scattering. It is thus reasonable to expect statistical isotropy violation in the CMB photon distribution observed now to have originated from SI violation in the baryon-photon fluid at last scattering, in addition to anisotropy of the primordial power spectrum studied earlier in literature. We consider the generalized anisotropic brightness distribution fluctuations, ∆(~ k, ˆ n, τ ) (at con-formal time τ ) in contrast to the SI case where it is simply a function of |~ k| and ˆ k ˆ n. The brightness fluctuations expanded in Bipolar Spherical Harmonic (BipoSH) series, can then be written as ∆LM ℓ1ℓ2 (k, τ ) where L > 0 terms encode deviations from statistical isotropy. Violation of SI encoded in the present off-diagonal elements of the harmonic space correlation haℓmaℓ′m′ i, equivalently, the BipoSH coefficients ALM ℓℓ′ , are then related to the generalized BipoSH brightness fluctuation terms at present. We study the evolution of ∆LM ℓ1ℓ2 (k, τ ) from non-zero terms ∆LM ℓ3ℓ4 (k, τs) at last scattering, in the free streaming regime. We show that the terms with given BipoSH multipole, LM, evolve independently. Moreover, similar to the SI case, power at small spherical harmonic (SH) multipoles of ∆LM ℓ3ℓ4 (k, τs) at the last scattering, is transferred to ∆LM ℓ1ℓ2 (k, τ ) at larger SH multipoles. The structural similarity is more apparent in the asymptotic expression for large values of the final SH multipoles. This formalism allows an elegant identification of any SI violation observed today to a possible origin in SI violating physics present in the baryon-photon fluid. This is illustrated for the known result of SI violating angular correlations due to the presence of a homogeneous magnetic field in the baryon-photon fluid.
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    Size of the longest filament in the luminous red galaxy distribution
    (2010-09-12) Souradeep, Tarun
    Filaments are one of the most prominent features visible in the galaxy distribution. Considering the Luminous Red Galaxies (LRGs) in the Sloan Digital Sky Survey Data Release Seven (SDSS DR7), we have analyzed the filamentarity in 11 nearly two dimensional (2D) sections through a volume limited subsample of this data. The galaxy distribution, we find, has excess filamentarity in comparison to a random distribution of points. We use a statistical technique “Shuffle” to determine LMAX, the largest length-scale at which we have statistically significant filaments. We find that LMAX varies in the range 100−130 h−1Mpc across the 11 slices, with a mean value LMAX = 110 ± 12 h−1Mpc. Longer filaments, though possibly present in our data, are not statistically significant and are the outcome of chance alignments.