2004 (IPP)

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Spherical and non-spherical gravitational collapse in Hussain spacetime
(2011-06-06) Patil, K. D.; Thool, U. S.
We investigate the nature of singularities arising in Husain solution. We analyze both spherical and non-spherical gravitational collapse in Husain spacetime. An interesting feature that emerges is that gravitational collapse of spherical cosmological Husain solution lead to the formation of naked singularities, while non-spherical cosmological collapse proceeds to form a black hole. Further strength of naked singularities arising in these spacetimes has been analyzed. It is found that these naked singularities are strong in Tipler’s sense.
Tracking pulsar dispersion measures using the GMRT
(2011-07-06) Ahuja, A. L.; Gupta, Y.; Mitra, D.; et al.
Relativistic Gravitational mass in Tolman-VI type solution
(2011-07-06) Ray, Saibal; Das, Basanti
Some known solutions for static charged ﬂuid spheres of Tolman-VI type in general relativity are reexamined. The gravitational mass that appears in the Pant and Sah [1] and Tikekar [2] solutions is shown to be of electromagnetic origin in the sense that the gravitational mass, along with all other physical quantities, depends on the electromag- netic ﬁeld alone. The energy condition, singularity and stability problems of the models are discussed thoroughly. VI
Aspects of scalar field dynamics in Gauss-Bonnet Brane Worlds
(2011-07-06) Sami, M.; Savchenko, N.; Toporensky, A.
The Einstein-Gauss-Bonnet equations projected from the bulk to brane lead to a complicated Friedmann equation which simpliﬁes to H2 ∼ ρq in the asymptotic regimes. The Randall-Sundrum (RS) scenario corresponds to q = 2 whereas q = 2/3 & q = 1 give rise to high energy Gauss-Bonnet (GB) regime and the standard GR respectively. Amazingly, while evolving from RS regime to high energy GB limit, one passes through a GR like region which has important implications for brane world inﬂation. For tachyon GB inﬂation with potentials V (φ) ∼ φp investigated in this paper, the scalar to tensor ratio of perturbations R is maximum around the RS region and is generally suppressed in the high energy regime for the positive values of p. The ratio is very low for p > 0 at all energy scales relative to GB inﬂation with ordinary scalar ﬁeld. The models based upon tachyon inﬂation with polynomial type of potentials with generic positive values of p turn out to be in the 1σ observational contour bound at all energy scales varying from GR to high energy GB limit. The spectral index nS improves for the lower values of p and approaches its scale invariant limit for p = −2 in the high energy GB regime. The ratio R also remains small for large negative values of p, however, diﬀerence arises for models close to scale invariance limit. In this case, the tensor to scale ratio is large in the GB regime whereas it is suppressed in the intermediate region between RS and GB. Within the frame work of patch cosmologies governed by H2 ∼ ρq , the behavior of ordinary scalar ﬁeld near cosmological singularity and the nature of scaling solutions are distinguished for the values of q < 1 and q > 1. The tachyon dynamics, on the other hand, exhibits stable scaling solutions ∀q if the adiabatic
Implementing an observatory control system-I. A Generic approach
(2011-07-06) Sunu, Engineer
An architectural framework for implementing a distributed obser- vatory control system is presented here. It has been partially realized and tested in the 2m optical and infrared Observatory at Pune, India.
Nonsingular spherical models with a variable cosmological term
(2011-07-06) Pradhan, Anirudh; Srivastava, Kashika; Lal, Amrit
Exact solutions of the Einstein’s ﬁeld equations describing a spheri- cally symmetric cosmological model without a big bang or any other kind of singularity recently obtained by Dadhich and Patel (2000) are revis- ited. The matter content of the model is a shear-free perfect ﬂuid with isotropic pressure and a radial heat ﬂux. Three diﬀerent exact solutions are obtained for both perfect ﬂuid and ﬂuid with bulk viscosity. It turns out that the cosmological rerm Λ(t) is a decreasing function of time, which is consistent with recent observations of type Ia supernovae.
Rotating metrics admitting non-perfect fluids in General Relativity
(2011-07-06) Ibohal, Ng.
In this paper an application of Newman-Janis algorithm in spherical sym- metric metrics with the functions M(u, r) and e(u, r) has been discussed. After the transformation of the metric via this algorithm, these two functions M(u, r) and e(u, r) will be of the three variables u, r, θ. With these functions of three variables, all the Newman-Penrose (NP) spin coeﬃcients, the Ricci as well as the Weyl scalars have been calculated from the Cartan’s structure equations. From these NP quantities, a class of rotating solutions of Einstein’s ﬁeld equa- tions can be obtained. These solutions include (a) Kerr-Newman, (b) rotating Vaidya solution, (c) rotating Vaidya-Bonnor solution, (d) rotating Husain’s solution, (e) rotating Wang-Wu solutions. It is found that the technique de- veloped by Wang and Wu can be used to generate new embedded solutions, that the Kerr-Newman solution can be combined smoothly with the rotating Vaidya solution to generate Kerr-Newman-Vaidya solution, and similarly, Kerr- Newman-Vaidya-Bonnor solution of the ﬁeld equations. It has also shown that the embedded universes like Kerr-Newman de Sitter, rotating Vaidya-Bonnor- de Sitter, Kerr-Newman-Vaidya-de Sitter can be derived from the general so- lutions with Wang-Wu function. All rotating embedded solutions derived here can be written in Kerr-Schild forms, showing the extension of Xanthopoulos’s theorem. It is also found that all the rotating solutions admit non-perfect ﬂuids.
Probing the time-variation of the fine-structure constant: Results based on Si IV doublets from a UVES sample
(2011-07-06) Chand, Hum; Petitjean, Patrick; Srianand, R.; et al.
We report a new constraint on the variation of the ﬁne-structure constant based on the analysis of 15 Si iv doublets selected from a ESO-UVES sample. We ﬁnd ∆α/α = (+0.15 ± 0.43) × 10−5 over a redshift range of 1.59 ≤ z ≤ 2.92 which is consistent with no variation in α. This result represents a factor of three improvement on the constraint on ∆α/α based on Si iv doublets compared to the published results in the literature. The alkali doublet method used here avoids the implicit assumptions used in the many-multiplet method that chemical and ionization inhomogeneities are negligible and isotopic abundances are close to the terrestrial value.
Comprehensive CCd photometric study of the open clusterNGC 2421
(2011-07-06) Yadav, R. K. S.; Sagar, Ram
We present the UBV RI CCD photometry in the region of the open cluster NGC 2421. Radius of the cluster is determined as ∼ 3′ .0 using stellar density proﬁle. Our Study indicates that metallicity of the cluster is Z ∼ 0.004. The reddening E(B − V ) = 0.42±0.05 mag is determined using two colour (U − B) versus (B − V ) diagram. By combining the 2MASS JHK data with the optical data we determined E(J −K) = 0.20±0.20 mag and E(V −K) = 1.15±0.20 mag for this cluster. Colour- excess diagrams show normal interstellar extinction law in the direction of the cluster. We determined the distance of the cluster as 2.2±0.2 Kpc by comparing the ZAMS with the intrinsic CM diagram of the cluster. The age of the cluster has been esti- mated as 80±20 Myr using the stellar isochrones of metallicity Z = 0.004. The mass function slope x = 1.2±0.3 has been derived by applying the corrections of ﬁeld stars contamination and data incompleteness. Our analysis indicate that the cluster NGC 2421 is dynamically relaxed.
On temperature T01 for molecular hydrogen
(2011-07-06) Chandra, Suresh
Abstract: Temperature TOl for hydrogen molecule for the transition between the level J = 1 (ortho-Hj) and the level J = 0 (para-He) has been introduced by Spitzer et al. [1]. The work of Spitzer et al. [1]has been reinvestigated and we find that the concept of the temperature TOl is not valid.
Noise storm continua: power estimates for electron acceleration
(2011-07-06) Subramanian, Prasad; Becker, Peter A.
We use a generic stochastic acceleration formalism to examine the power Lin (erg s −1 ) input to nonthermal electrons that cause noise storm continuum emission. The analytical approach includes the derivation of the Green’s function for a general second-order Fermi process, and its application to obtain the particular solution for the nonthermal electron distribution resulting from the acceleration of a Maxwellian source in the corona. We compare Lin with the power Lout observed in noise storm radiation. Using typical values for the various parameters, we ﬁnd that Lin ∼ 1023−26 erg s −1 , yielding an eﬃciency estimate η ≡ Lout/Lin in the range 10−10 ∼ < η ∼ < 10−6 for this nonthermal acceleration/radiation process. These results reﬂect the eﬃciency of the overall process, starting from electron acceleration and culminating in the observed noise
Angular momentum transport in quasi-Keplerian accretion disks
(2011-07-06) Subramanian, Prasad; Pujari, B. S.; Becker, Peter A.
We reexamine arguments advanced by Hayashi &Matsuda (2001), who claim that several simple, physically motivated deriva- tions based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the na- ture of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expres- sion for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The deriva- tion is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.
Role of the space-time dimensions & the fluid equation of state in spherical gravitational collapse
(2011-07-06) Dadhich, Naresh; Ghosh, S. G.; Deshkar, D. W.
We study the spherically symmetric collapse of a ﬂuid with non-vanishing radial pressure in higher dimensional space-time. We obtain the general exact solution in the closed form for the equation of state (Pr = γρ) which leads to the explicit construction of the root equation governing the nature (black hole versus naked singularity) of the central singularity. A remarkable feature of the root equation is its invariance for the three cases: (D + 1, γ = −1), (D, γ = 0) and (D − 1, γ = 1) where D is the dimension of space-time. That is, for the ultimate end result of the collapse, D-dimensional dust, D + 1 - AdS (anti de Sitter)-like and D − 1 - dS-like are absolutely equivalent.
Suggesions for an interstellar C5H2 search
(2011-07-06) Chandra, Suresh; Shinde, S. A.
Laboratory detection of four isomers of C5H2 molecule have been reported by Travers et al. (1997), McCarthy et al. (1997), and Gottlieb et al. (1998). They suggested for detection of the ring-chain isomer of C5H2 (c-C5H2)incosmic objects, as it is the most stable one in comparison to the others. Two transitions 313−212 and 303−202 at 19.147 GHz and 19.606 GHz, respectively, of c-C5H2 have been detected in TMC-1. We suggest that the c-C5H2 may be identiﬁed in cool cosmic objects through its transition 313−404 at 4.3 GHz in absorption against the cosmic microwave background. Since in absence of availability of the collisional rates, we have used scaled values for them, we have checked the sensitivity of the lines on the rates by enhancing the rate for the transitions with ∆ka = 0 by a factor of 10. Though the transitions are not found sensitive, our results may be treated as qualitative in nature. This absorption line may play an important role for identiﬁcation of c-C5H2 in cosmic objects.
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
(2011-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We study the variation of the eﬀective ﬁne structure constant alpha for Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string theory naturally gives rise to a coupling between gauge ﬁelds and a scalar ﬁeld responsible for accelerated expansion of the universe. This leads to the change of alpha due to a dynamical evolution of the scalar ﬁeld, which can be compatible with the recently observed cosmological data around the redshift ˜ z < ∼ 3. We place constraints on several diﬀerent DBI models including exponential, inverse power-law and rolling massive scalar potentials. We ﬁnd that these models can satisfy the varying alpha constraint provided that mass scales of the potentials are ﬁne-tuned. When we adopt the mass scales which are motivated by string theory, both exponential and inverse power-law potentials give unacceptably large change of alpha, thus ruled out from observations. On the other hand the rolling massive scalar potential is compatible with the observationally allowed variation of alpha. Therefore the information of varying alpha provides a powerful way to distinguish between a number of string-inspired DBI dark energy models.
On the Variable-charged Black Holes Embedded into de Sitter Space: Hawking's Radiation
(2011-07-06) Ibohal, Ng.
In this paper we discuss the Hawking’s evaporation of the masses of variable-charged Reissner-Nordstromand Kerr-Newman, black holes embedded into the de Sitter cosmological universe by considering the charge to be function of radial coordinate. It has been shown that every electrical radiation of variable-charged rotating or non-rotating cosmological black holes will produce a change in the mass of the body without eﬀecting the Maxwell scalar and the cosmological constant. It is also shown that during the Hawking’s radiation process, after the complete evaporation of masses of both variable-charged Reissner- Nordstrom-de Sitter and Kerr-Newman-de Sitter black holes, the elec- trical radiation will continue creating negative mass naked singularities embedded into de Sitter cosmological spaces de Sitter black holes
Evalution of a CMDS imager as readout for imaging photon counters
(2011-07-06) Tandon, S. N.
This work is motivated by a desire to check if CMOS detectors, which are somewhat inferior to CCDs in terms of noise and dark current, but are much simpler to use, could be used as readout in imaging photon counters. Tests have been made on the performance of a CMOS detector used as readout of an image intensifier. The main conclusions are that: a) the detector has adequate dynamic range as well as sensitivity for photon counting, b) with an optics for reduction by a factor about three between the intensifier and the readout, it is found that 10-20 micron wide slits are recorded with an rms width of - 0.14 pixels, i.e. - 11 microns on the slit-plane - this includes effects due to finite resolution of the intensifier, and c) the detector is capable of giving a rms precision of - 0.03 pixels ( - 0.7 microns) on centroids of images similar to those of intensified photon events.
Restrictions on the Physical Prescription for the Viscosity in Advection-Dominated Accretion Disks
(2011-07-06) Becker, Peter A.; Subramanian, Prasad
It has recently been demonstrated that the Shakura-Sunyaev prescription for the kinematic viscosity in an advection-dominated accretion disk yields physically reasonable solutions for the structure of the inﬂow close to the event horizon. In particular, no violations of relativistic causality occur at the horizon. This is somewhat surprising considering the diﬀusive nature of the angular momentum transport in the Shakura-Sunyaev scenario, and it is therefore natural to ask whether one can also obtain acceptable solutions for the disk structure based on the various alternative models for the viscosity that have been proposed, includ- ing the “deterministic” forms. In this paper we perform a rigorous asymptotic analysis of the structure of an advection-dominated accretion disk close to the event horizon of a nonrotating black hole based on three of the alternative pre- scriptions for the viscosity that have been suggested in the literature. We constrain the physical disk model by stipulating that the stress must van- ish at the horizon, which is the fundamental inner boundary condition imposed by general relativity. Surprisingly, we ﬁnd that none of the three alternative viscosity prescriptions yield physically acceptable disk structures close to the horizon when the zero-torque condition is applied, whether the ﬂow is in vertical hydrostatic equilibrium or free-fall. Hence we conclude that the original Shakura- Sunyaev prescription is the only one proposed so far that is physically consistent close to the event horizon. We argue that, somewhat ironically, it is in fact the diﬀusive nature of the Shakura-Sunyaev form that is the reason for its success in this application. Our focus here is on advection-dominated accretion disks, but we expect that our results will also apply to generalized disks provided that losses of matter and energy become negligible as the gas approaches the event horizon.
Some Bianchi Type I Viscous fluid cosmological models with a variable cosmological constant
(2011-07-06) Pradhan, Anirudh; Pandey, Purnima
Some Bianchi type I viscous ﬂuid cosmological models with a variable cosmological constant are investigated in which the expansion is considered only in two direction i.e. one of the Hubble parameter (H1 = A4 A ) is zero. The viscosity coeﬃcient of bulk viscous ﬂuid is assumed to be a power function of mass density whereas the coeﬃcient of shear viscosity is considered as constant in ﬁrst case whereas in other case it is taken as proportional to scale of expansion in the model. The cosmological constant Λ is found to be positive and is a decreasing function of time which is supported by results from recent supernovae Ia observations. Some physical and geometric properties of the models are also discussed.
Indo-U.S. Library of Coude Feed Stellar Spectra
(2011-07-06) Valdes, Francisco; Gupta, Ranjan; Rose, James A.; et al.
We have obtained spectra for 1273 stars using the 0.9m Coud´e Feed telescope at Kitt Peak National Observatory. This telescope feeds the coud´e spectrograph of the 2.1m telescope. The spectra have been obtained with the #5 camera of the coud´e spectrograph and a Loral 3K X 1K CCD. Two gratings have been used to provide spectral coverage from 3460 ˚ A to 9464 ˚ A, at a resolution of ∼1˚ A FWHM and at an original dispersion of 0.44 ˚ A/pixel. For 885 stars we have complete spectra over the entire 3460 ˚ A to 9464 ˚ A wavelength region (neglecting small gaps of < 50 ˚ A), and partial spectral coverage for the remaining stars. The 1273 stars have been selected to provide broad coverage of the atmospheric parameters Teff , log g, and [Fe/H], as well as spectral type. The goal of the project is to provide a comprehensive library of stellar spectra for use in the automated classiﬁcation of stellar and galaxy spectra and in galaxy population synthesis. In this paper we discuss the characteristics of the spectral library, viz., details of the observations, data reduction procedures, and selection of stars. We also present a few illustrations of the quality and information available in the spectra. The ﬁrst version of the complete spectral library is now publicly available from the National Optical Astronomy Observatory (NOAO) via FTP and HTTP.