2000 (IPP)
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Item Vanishing of cosmological constant in nonfactorizable geometry(2000-07-20) Padmanabhan, T.; Shankaranarayanan, S.We generalize the results of Randall and Sundrum to a wider class of four-dimensional space- times including the four-dimensional Schwarzschild background and de Sitter universe. We solve the equation for graviton propagation in a general four dimensional background and find an explicit solution for a zero mass bound state of the graviton. We find that this zero mass bound state is normalizable only if the cosmological constant is strictly zero, thereby providing a dynamical reason for the vanishing of cosmological constant within the context of this model. We also show that the results of Randall and Sundrum can be generalized without any modification to the Schwarzschild background.Item Vacuum gaps in pulsars and PSR J2144-3933(2000-12-20) Gil, Janusz; Mitra, D.In this paper we revisit the radio pulsar death line problem within the framework of curvature radiation and/or inverse compton scattering induced vacuum gap model above neutron star polar caps. Our special interest is in the recently detected pulsar PSR J2144−3933 with extremal period 8.5 seconds, which lies far beyond conventional death lines. We argue, that formation of vacuum gaps requires a complicated multi- polar surface magnetic field, with a strenght Bs much higher than the surface dipolar component Bd, and radii of curvature R much smaller than the neutron star radius R = 106 cm. Such a multipolar surface field is also consistent with death lines including the extremal pulsar PSR J2144−3933. Since vacuum gap models produce sparks, our paper naturally supports the spark related models of subpulse drift phenomenon as well as to the spark associated models of coherent pulsar radio emission.Item Using Gravitational Lensing to study damped Lyman - alpha clouds(2000-03-29) Saini, Tarun Deep; Bharadwaj, Somnath; Sethi, Shiv K.We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-α clouds) at high redshift by carrying out deep radio observations in the fields of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magnifies the flux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being restricted to clouds that lie very close to the caustics of the lens. We find that observations at a detection threshold of 50 Jy at 320 MHz (possible with the GMRT) have a greater than 20% probability of detecting an HI cloud in the field of a cluster, provided the clouds have HI masses in the range 5 × 108 M⊙ ≤ MHI ≤ 2.5 × 1010 M⊙. The probability of detecting a cloud in- creases if they have larger HI masses, except in the cases where the number of HI clouds in the cluster field becomes very small. The probability of a detection at 610 MHz and 233 MHz is comparable to that at 320 MHz, though a definitive statement is difficult owing to uncertainties in the HI content at the redshifts corresponding to these fre- quencies. Observations at a detection threshold of 2 Jy (possible in the future with the SKA) are expected to detect a few HI clouds in the field of every cluster provided the clouds have HI masses in the range 2 × 107 M⊙ ≤ MHI ≤ 109 M⊙. Even if such observations do not result in the detection of HI clouds, they will be able to put useful constraints on the HI content of the clouds.Item Tree structure of the percolation universe(2000-01-27) Colombi, S.; Pogosyan, D.; Souradeep, TarunWe present a numerical study of topological descriptors of initially Gaussian and scale-free density perturbations evolving via gravitational instability in an expanding universe. We carefully evaluate and avoid numerical contamination in making accurate measurements on simulated fields on a grid in a finite box. Independent of extent of non linearity, the measured Euler number of the excursion set at the percolation threshold, δc, is positive and nearly equal to the number of isolated components, suggesting that these structures are trees. Our study of critical point counts reconciles the clumpy appearance of the density field at δc with measured filamentary local curvature. In the Gaussian limit, we measure |δc| > σ in contrast to widely held belief that |δc| ∼ σ, where σ2 is the variance of the density field.Item Topology of the Universe(2000-12-25) Roukema, B. F.The Hilbert-Einstein equations are insufficient t describe the geometry of the Universe, as they onl constrain a local geometrical property: curvatur A global knowledge of the geometry of space, if pos sible, would require measurement of the topolog of the Universe. Since the subject was discussed i 1900 by Schwarzschild, observational attempts t measure global topology have been rare for most o this century, but have accelerated in the 1990’s du to the rapidly increasing amount of observations o non-negligible fractions of the observational spher A brief review of basic concepts of cosmic topolog and of the rapidly growing gamut of diverse an complementary observational strategies for measu ing the topology of the Universe is provided here.Item Thermally driven outflows from pair-plasma pressure-mediated shock surfaces around schwarzschild black holes(2000-01-23) Das, Tapas K.Introducing a spherical, steady, self-supported pair-plasma pressure-mediated shock surface around a Schwarzschild black hole as the effective physical atmosphere that may be responsible for the generation of astrophysical mass outflows from relativistic quasi- spherical accretion, we calculate the mass outflow rate Rm Ç by simultaneously solving the set of equations governing transonic polytropic accretion and isothermal winds. Rm Ç is computed in terms of only three inflow parameters, which, we believe, has been done for the first time in our work. We then study the dependence of Rm Ç on various inflow as well as shock parameters, and establish the fact that the outflow rate is essentially controlled by the post- shock proton temperature.Item Study of the angular size - redshift relation for models in which decays as the energy density(2000-02-18) Vishwakarma, R. G.By modifying the Chen and Wu ansatz, we have investigate some Friedmann models in which Λ varies as ρ. In order to test the con sistency of the models with observations, we study the angular size - red shift relation for 256 ultracompact radio sources selected by Jackson an Dodgson. The angular sizes of these sources were determined by using ver long-baseline interferometry in order to avoid any evolutionary effects. Th models fit the data very well and require an accelerating universe with a po itive cosmological constant. Open, flat and closed models are almost equall probable, though the open model provides a comparatively better fit to th data. The models are found to have intermediate density and imply the ex istence of dark matter, though not as much as in the canonical Einstein-d Sitter model.Item Structure of the Mg II and damped Lyman- systems along the line of sight to APM 08279+5255(2000-05-24) Petitjean, Patrick; Aracil, Bastien; Srianand, R.; et al.A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is pre-sented.Most of the Mg ii systems in the redshift range z ∼ 1.2– 2.07, although saturated, show large residuals at the bot- tom of the lines. The most likely interpretation is that individual clouds within Mg ii halos do cover only one of the two brightest QSO images. The separation between the two lines of sight decreases from 1.7 to 0.7 h−1 75 kpc (qo = 0.5, zlens = 1) between z = 1.22 and z = 2.07. This reveals that Mg ii halos are made of a collection of clouds of radius smaller than about 1 h−1 75 kpc. Two strong Mg ii absorbers at zabs = 1.062 and 1.181 are studied in detail. This is the first time that the Na iλ3303 doublet is detected in such high redshift systems. To- gether with the detection of the Mg iλ2852 transition, this strongly constrains the physical characteristics of the gas. The N(Na i)/N(Mg i) ratio is found to be larger than unity, implying that the gas is cool and neutral. The Doppler parameters measured in individual and well de- tached components is probably as small as 1 km s−1. The column densities of Na i, Ca ii, Mg i, Ti ii, Mn ii and Fe ii observed at zabs = 1.1801 are very close to that observed along the line of sight towards 23 Ori in our Galaxy. The shape of the QSO continuum is consistent with attenu- ation by dust at z ∼ 1 (AV ∼ 0.5 mag). Altogether it is found that the H i column density at z = 1 is of the order of 1 to 5 1021 cm−2, the corresponding metallicity is in the range 1–0.3 Z⊙, the overall dust-to-metal ratio is about half that in our Galaxy and the relative deple- tion of iron, titanium, manganese and calcium is similar to what is observed in cool gas in the disk of our Galaxy. The objects associated with these two systems could both con-tribute to the lens together with another possible strong system at zabs = 1.1727 and the strong Lyman-α system at zabs = 2.974. The probable damped Lyman-α system at zabs = 2.974 has 19.8 < log N(H i) < 20.3. The transverse dimension of the absorber is larger than 200 h−1 75 pc. Column densities of Al ii, Fe ii, Si ii, C ii and O i indicate abundances rela- tive to solar of −2.31, −2.26, −2.10, −2.35 and −2.37 for, respectively, Fe, Al, Si, C and O (for log N(H i) = 20.3). These surprizingly similar values indicate that the amount of dust in the cloud is very small as are any deviations from relative solar abundances. It seems likely that the upper limits found for the zinc metallicity of several damped Lyman-α systems at z > 3 in previous surveys is indica- tive of a true cosmological evolution of the metallicity in individual systems.Item Spherically symmetric empty space and its dual in general relativity(2000-01-22) Dadhich, NareshItem Simultaneous solution scheme for coupled transonic accretion-wind systems(2000-06-15) Das, Tapas K.We discuss a non-self-similar analytical model capable of explaining the formation of accretion-powered galactic and extra-galactic jets.