2004 (IPP)
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Item What is needed of a tachyon if it is to be the dark energy?(2011-07-06) Copeland, E. J.; Garousi, M. R.; Sami, M.; et al.We study a dark energy scenario in the presence of a tachyon field φ with potential V (φ) and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity λ = −MpVφ/V 3/2 . If λ is a constant, which corresponds to an inverse square potential V (φ) ∝ φ−2 , there exists one stable critical point that gives an acceleration of the universe at late times. When λ → 0 asymptotically, we can have a viable dark energy scenario in which the system approaches an “instantaneous” critical point that dynamically changes with λ. If |λ| approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where |λ| is smaller than of order unity.Item Tracking pulsar dispersion measures using the GMRT(2011-07-06) Ahuja, A. L.; Gupta, Y.; Mitra, D.; et al.Item Time dependence of the proton-to-electron mass ratio(2011-07-06) Petitjean, Patrick; Ivanchik, A.; Srianand, R.; et al.We have measured with high precision the position of 79 molecular hydrogen absorption lines of Lyman and Werner bands from two absorption systems at zabs = 2.594733 and 3.024899, in the spectra of quasars Q 0405–443 and Q 0347–383, respectively, in order to constrain the cosmological variation of the proton-to-electron mass ratio, µ = mp/me.Dataareof the highest spectral resolution (R = 53 000) and signal to noise ratio (S/N = 30–70) for such quasars. The absorption lines are not saturated and their profiles can be modelled as simple Gaussian functions. We find a correlation between the observed redshift of the lines and the sensitivity of the line positions to a change in µ. This can be interpreted as a variation of µ with µ/µ = (2.97 ± 0.74) × 10−5 over the past ∼12 Gyrs. As this result is based on two systems one cannot rule out that unknown systematics could cause a false-alarm detection. Thus the result needs to be confirmed with additional data. It is also very important to improve the accuracy of the laboratory wavelengths as the significance of our result depends on the accuracy to which they are known.Item Limits on the time variation of the electromagnetic fine-structure constant in the low energy limit from absorption lines in the spectra of distant quasars(2011-07-06) Srianand, R.; Chand, Hum; Petitjean, Patrick; et al.Most of the successful physical theories rely on the constancy of few fundamental quantities (such as the speed of light, c, the fine-structure constant, α, the proton to electron mass ratio, µ, etc), and constraining the possible time variations of these fundamental quantities is an important step toward a complete physical theory. Time variation of α can be accurately probed using absorption lines seen in the spectra of distant quasars. Here, we present the results of a detailed many-multiplet analysis performed on a new sample of Mg ii systems observed in high quality quasar spectra obtained using the Very Large Telescope. The weighted mean value of the variation in α derived from our analysis over the redshift range 0.4 ≤ z ≤ 2.3 is ∆α/α = (−0.06 ± 0.06) × 10−5. The median redshift of our sample (z≃1.55) corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. This gives a 3σ limit, −2.5 × 10−16 yr −1 ≤ (∆α/α∆t) ≤ +1.2 × 10−16 yr −1, for the time variation of α, that forms the strongest constraint obtained based on high redshift quasar absorption line systems.Item 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.Item 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.Item Probing the cosmological variation of the fine-structure constant:Results on VLT-UVES sample(2011-07-06) Chand, Hum; Srianand, R.; Petitjean, Patrick; et al.Abstract. Development of fundamental physics relies on the constancy of various fundamental quantities such as the fine structure constant. Detecting or constraining the possible time variations of these fundamental physical quantities is an important step toward a complete understanding of basic physics. High quality absorption lines seen in the spectra of distant QSOs allow one to probe time variations of several of these quantities. Here we present the results from a detailed many-multiplet analysis, to detect the possible variation of fine-structure constant, performed using high signal-to-noise ratio, (∼70 per pixel), high spectral resolution (R ≥45000) observations of 23 Mg ii systems detected toward 18 QSOs in the redshift range 0.4 ≤ z ≤ 2.3 obtained using UVES at the VLT. We validate our procedure and define the selection criteria that will avoid possible systematics using detail analysis of simulated data set. The spectra of Mg ii doublets and Fe ii multiplets are generated considering variations in α and specifications identical to that of our UVES spectra. We show our Voigt profile fitting code recovers the variation in α very accurately when we use single component systems and multiple component systems that are not heavily blended. Spurious detections are frequently seen when we use heavily blended systems or the systems with very weak lines. Thus we avoided heavily blended systems and the systems with Fe ii column density < 2×1012 cm−2 while analysing the UVES data. To make the analysis transparent and accessible to the community for critical scrutiny all the steps involved in the analysis are presented in detail. The weighted mean value of the variation in α obtained from our analysis over the redshift range 0.4 ≤ z ≤ 2.3 is ∆α/α = (−0.06 ± 0.06) × 10−5 . The median redshift of our sample is 1.55 and corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. The 3σ upper limit on the time variation of α is −2.5 × 10−16 yr −1 ≤ (∆α/α∆t) ≤ +1.2 × 10−16 yr −1 . To our knowledge this is the strongest constraint from quasar absorption line studies till date.Item Multicolor surface photometry of lenticulars galaxies I. The Data(2011-07-06) Barway, Sudhanshu; Mayya, Y. D.; Kembhavi, A.K.; et al.We present in this paper multicolor surface and aperture photometry in the B, V,R and K′ bands for a sample of 34 lenticular galaxies from the UGC catalogue. From surface photometric analysis, we obtain radial profiles of surface brightness, colors, ellipticity, position angle and the Fourier coefficients which describe the departure of isophotal shapes from purely elliptical form and find the presence of dust lanes, patches and ring like structure in several galaxies in the sample. We obtain total integrated magnitudes and colors and find that these are in good agreement with the values from the RC3 catalogue. Isophotal colors are correlated with each other, following the sequence expected for early- type galaxies. The color gradients in lenticulars are more negative than the corresponding gradients in ellipticals. There is a good correlation between B − V and B − R color gradients, and the mean gradient in the B−V,B−R and V −K′ colors are −0.13±0.06, −0.18±0.06, −0.25±0.11 magnitude per dex in radius respectively.Item General Relativistic Effects of Strong Magnetic Fields on the gravitational force: A driving engine for Gamma-$ SGRM?(2012-07-06) Malheiro, Manuel; Ray, Subharthi; Mosquera Cuesta, Herman J.; et al.Abstract. In general relativity all forms of energy contribute to gravity and not only just ordinary matter as in Newtonian Physics. This fact can be seen in the modified hydrostatic equilibrium equation for relativistic stars pervaded by magnetic (B) fields. It has an additional term coupled to the matter part as well as an anisotropic term which is purely of magnetic origin. That additional term coming from the pressure changed by the radial component of the diagonal electromagnetic field tensor, weakens the gravitational force when B is strong enough and can even produce an unexpected change in the attractive nature of the force by reversing its sign. In an extreme case, this new general relativistic (GR) effect can even trigger an instability in the star as a consequence of the sudden reversal of the hydrostatic pressure gradient. We suggest here that this GR effect may be the possible central engine driving the transient giant outbursts observed in Soft Gamma-ray Repeaters (SGRs). In small regions of the neutron star (NS), strong magnetic condensation can take place. Beyond a critical limit, these highly magnetised bubbles may explode releasing the trapped energy as a burst of γ-rays of ∼ 1036−40 erg.Item Automated Classification of 2000 Bright IRAS Sources(2011-07-06) Gupta, Ranjan; Singh, Harinder P.; Volk, K.; et al.An 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 IRAS (Infrared Astronomical Satellite) spectra in the region 8µm to 23µm. The database has been classified into 17 predefined classes based on the spectral morphology. We have been able to classify over 80 percent of the sources correctly in the first instance. The speed and robustness of the scheme will allow us to classify the whole of the LRS database, containing more that 50,000 sources, in the near future.