Showing papers in "Astrophysics and Space Science in 2002"

A Nonextensive Entropy Approach to Kappa-Distributions

Abstract: Most astrophysical plasmas are observed to have velocity distribution functions exhibiting non-Maxwellian suprathermal tails. The high energy particle populations are accurately represented by the family of kappa-distributions where the use of these distributions has been unjustly criticized because of a perceived lack of theoretical justification. We show that distributions very close to kappa-distributions are a consequence of the generalized entropy favored by non extensive statistics, which provides the missing link for power-law models of non-thermal features from fundamental physics. With regard to the physical basis supplied by the Tsallis nonextensive entropy formalism we propose that this slightly modified functional form, qualitatively similar to the traditional kappa-distribution, be used in fitting particle spectra in the future.

On fractional kinetic equations

Abstract: The subject of this paper is to derive the solution of generalized fractional kinetic equations. The results are obtained in a compact form containing the Mittag-Leffler function, which naturally occurs whenever one is dealing with fractional integral equations. The results derived in this paper provide an extension of a result given by Haubold and Mathai in a recent paper (Haubold and Mathai, 2000).

On the formation of massive primordial stars

Abstract: We investigate the formation by accretion of massive primordial protostars in the range 10 to 300 Mo. The high accretion rate used in the models (M = 4.4 × 10−3 M ⊙ yr 1) causes the structure and evolution to differ significantly from those of both present-day protostars and primordial zero-age main sequence stars. The stellar surface is not visible throughout most of the main accretion phase, since a photosphere is formed in the infalling envelope. Significant nuclear burning does not take place until a protostellar mass of about 80 M ⊙. As the interior luminosity approaches the Eddington luminosity, the protostellar radius rapidly expands owing to the radiation pressure. Eventually, a final swelling occurs when the stellar mass reaches about 300 Mo. This expansion is likely to signal the end of the main accretion phase, thus setting an upper limit to the protostellar mass formed in these conditions.

Bianchi Type-III String Cosmological Model with Bulk Viscous Fluid in General Relativity

Abstract: A Bianchi type-III string cosmological model with bulk viscous fluid for massive string is investigated. To get a determinate solution, a supplementary condition B=Cn, between metric potentials, is used whereB and C are function of time alone. The behaviour of the model in presence and absence of bulk viscosity, is discussed.

Formation of Intensive Magnetic Flux Tubes in a Converging Flow of Partially Ionized Solar Photospheric Plasma

Abstract: Theoretical model, explaining a phenomenon of formation of Intensive Magnetic Flux Tube (IMFT) in a converging flow of partially ionized solar photospheric plasma is considered. Special attention is paid to the fact of weak ionization (n/nn ∼ 10-4) of plasma in the photosphere. The cases of 2D magnetic slab and cylindric magnetic tube are considered. It was shown that in a converging flow of photospheric plasma thin magnetic tubes, or slabs with the characteristic scale L0 ∼ (1 ÷ 5) ċ 107 cm and magnetic field 1000 ÷ 2000 G can be generated. By this 2D magnetic slabs could be unstable with respect to an exchange instability and appear as an intermediate step during IMFT formation on the boundary of two supergranulation cells. Formation of compact strong magnetic field structures, and their energy balance are discussed. Stationary Joule energy dissipation taking place on the photospheric levels in the models of magnetic slab or IMFT under consideration increases towards the periphery of these objects and can exceed radiation looses. This can cause the occurrence of magnetic tubes with hot external envelopes, and modification of plasma temperature and density distribution, with respect to ones in a quiet atmosphere.

GAIA: An Astrometric and Photometric Survey of our Galaxy

Abstract: In October 2000, the GAIA astrometric mission was approved as one of the next two ‘cornerstones’ of ESA’s science programme, with a launch date target of 2010–12. GAIA will provide positional, radial velocity, and photometric measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 per cent of the Galactic stellar population. GAIA’s main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance questions such as when the stars in our Galaxy formed, when and how it was assembled, and its distribution of dark matter. The survey aims for completeness to V = 20 mag, with accuracies of 10 μas at 15 mag.

The epochs of early-type galaxy formation

Abstract: By means of population synthesis models with variable α/Fe ratios we derive average ages, metallicities, and [α/Fe] element enhancements for a sample of 126 field and cluster early-type galaxies. We find a clear positive relation between [α/Fe] and velocity dispersion. Zero-point, slope, and scatter of this correlation are the same for cluster and field galaxies. In particular, the [α/Fe] ratios and mean ages of cluster ellipticals are positively correlated. This strongly reinforces the view that the [α/Fe] element enhancement in ellipticals is produced by star formation timescales rather than by variations of the initial mass function. These results indicate that the more massive the galaxy, the shorter is its star formation timescale, and the higher is the redshift of the bulk of star formation. This finding is not compatible with the predictions from models of hierarchical galaxy formation. The lenticular and field galaxies of the investigated sample do not follow the correlation between age and [α/Fe], but contain a non-negligible fraction of galaxies with young average ages and high [α/Fe] ratios.

The CO to H-2 conversion factor in normal late-type galaxies

Abstract: The molecular gas mass in nearby galaxies is generally estimated using 1 2 CO(1-0) line intensities and assuming the X conversion factor between I(CO) and N(H 2 ) measured in the solar neighborhood. It is however known that this X conversion factor is not universal since it changes with metallicity, cosmic ray density and UV radiation field. Far-IR data in the spectral range 100-1000 μm can be used to estimate the molecular gas content of late-type galaxies in an independent way of CO line measurements once a metallicity-dependent dust to gas ratio is assumed, allowing a direct estimate of X. This exercise is presented here for a large sample of galaxies with available multifrequency data. X spans from ∼ 10 2 0 mol cm - 2 (K km s - 1 ) - 1 in giant spirals to ∼ 10 2 1 mol cm - 2 (K km s - 1 ) - 1 in dwarf irregulars.

GAIA and the Extragalactic Distance Scale

Abstract: The local expansion field (υ 220 < 1200 kms−1) and the cosmic expansion field out to 30000 kms−1 are characterized by H 0 = 58 [kms−1 Mpc−1]. While the random error of this determination is small (±2 units), it may still be affected by systematic errors as large as ±10%. The local expansion is outlined by Cepheids and by Cepheid-calibrated TF distances of a complete sample of field galaxies and by nearby groups and clusters; the cosmic expansion is defined by Cepheid-calibrated SNe Ia. The main source of systematic errors are therefore the shape and the zero point of the P-L relation of Cepheids and its possible dependence on metallicity. GAIA will essentially eliminate these systematic error sources. Another source of systematic error is due to the homogenization of SNe Ia as to decline rate Δm 15 and color (B − V). GAIA will discover most of the 1100 SNe Ia within 10000 kms−1, which will occur during its four-year lifetime. If their photometric parameters can be determined from the ground, they will fix the dependence of the SNe Ia luminosity on Δm 15 and (B − V) with high accuracy. At the same time they will yield exquisite distances to an equal number of field galaxies. — GAIA will also revolutionize the very local distance scale by determining fundamental distances of the companion galaxies of the Milky Way and even of some spirals in- and possibly outside the Local Group from their rotation curves seen in radial velocities and proper motions. Moreover, GAIA will obtain trigonometric parallaxes of RR Lyrae stars, of red giants defining the TRGB, of stars on the ZAMS, of White Dwarf defining their cooling sequence, and of globular clusters, and determine the metallicity dependence of these distance indicators. It will thus establish a self-controlling network of distance indicators within the Local Group and beyond.

Morphological Classification of Galaxies using Computer Vision and Artificial Neural Networks: A Computational Scheme

Abstract: The morphology of galaxies is an important issue in the large scale study of the Universe. The Hubble Deep Field project has already shown that the Universe contains billions and billions of galaxies. The Sloan Digital Sky Survey is expected to map the sky for one million galaxies. One of the major challenges facing astronomers today is how to automatically identify and classify large number of galaxies that will began to show up in the hundreds of thousands of digitized images from sky surveys. Today it is possible to address this problem with the help of advances occurring in computer vision and artificial neural networks technology. This paper describes a computational scheme to develop an automatic galaxy classifier. From the scheme it is possible to visualize several different types of automatic galaxy classifiers. Two types are presented here with prototype models. The first type uses the geometric shape features as the basis for classification. The second uses the direct pixel images of galaxies and artificial neural networks to do the classification. The results show that geometric shape features are very good indicators of different types of nearby galaxies. Three test cases were presented to the prototype geometric shape classifier and it was able to successfully classify all three of them. The direct image based neural network classifier was able to learn 97% of the 171 training patterns presented to it. However when the network was presented a test set of 37 independent patterns, it was only able to classify 57% percent of the test cases. This study demonstrates that a very robust and efficient automated galaxy classifier based on shape features and artificial neural network can be develop.

Gravity Anomaly During the Mohe Total Solar Eclipse and New Constraint on Gravitational Shielding Parameter

Abstract: By using a high-precision LaCoste-Romberg gravimeter, continuous and precise meas- urements were carried out during the March 9, 1997 total solar eclipse in Mohe region in Northeast China. The gravity variations were digitally recorded during the total solar eclipse so as to investigate the possible anomaly of the Sun and the Moon's gravitational fields on the Earth. After the careful processing and analysis of the observed data, no significant anomaly during the very solar eclipse was found. However, there are two 'gravity anomaly valleys' with near symmetrical decrease of about 6 ∼ 7 µgal at the first contact and the last contact. This is the anomaly phenomenon observed and reported for the first time in the literature. This paper is intended to explain the observed anomaly by conducting the tilt experiment due to the thermal stress and temperature change in the solar eclipse. A new constraint limit on gravitational shielding is thus obtained. Some analysis and discussions are presented although further studies and research are highly needed.

The light curve of V605 Aql – the `older twin' of Sakurai's Object

Abstract: `Nova Aquilae No. 4', discovered in 1919, later renamed V605 Aql, was recognized to be a final helium flash object only in the 1980s. Clayton and De Marco (1997) gave a detailed description of the available spectroscopic and photometric material. Here we try to re-analyze the photometric record. The photographic material is still available and has been used to construct a revised light curve. Only fragmentsof the visual observations were published; the remainder appears to be lost.

String theory, cosmology and varying constants

Abstract: In string theory the coupling ‘constants’ appearing in the low-energy effective Lagrangian are determined by the vacuum expectation values of some (a priori) massless scalar fields (dilaton, moduli) This naturally leads one to expect a correlated variation of all the coupling constants, and an associated violation of the equivalence principle We review some string-inspired theoretical models which incorporate such a spacetime variation of coupling constants while remaining naturally compatible both with phenomenological constraints coming from geochemical data (Oklo; Rhenium decay) and with present equivalence principle tests Barring a very unnatural fine-tuning of parameters, a variation of the fine-structure constant as large as that recently ‘observed’ by Webb et al in quasar absorption spectra appears to be incompatible with these phenomenological constraints Independently of any model, it is emphasized that the best experimental probe of varying constants are high-precision tests of the universality of free fall, such as MICROSCOPE and STEP

Nucleosynthesis and Stellar Evolution

Abstract: Two of the basic building blocks of galaxies are stars and the interstellar medium. The evolution of the abundance composition in the latter and especially the enrichment of heavy elements as a function of space and time reflects in turn the history of star formation and the lifetimes of the diverse contributing stellar objects. Therefore, the understanding of stellar evolution and its endpoints (mainly planetary nebulae, supernovae of type Ia and type II/Ib/Ic) is essential. Despite many efforts, a full and self-consistent understanding of supernovae (the main contributors to nucleosynthesis in galaxies) is not existing, yet. However, they leave fingerprints, seen either in spectra, lightcurves, radioactivities/decay gamma-rays or in galactic evolution. Here we want to address the composition of ejecta, their model uncertainties and relate them to constraints from abundance observations in galactic evolution.

Critical density solitary waves structures in a hot dusty plasma with vortex-like ion distribution in phase space

Abstract: The nonlinear properties of solitary waves structure in a hot dusty plasma consisting of isothermal hot electrons, non isothermal ions and high negatively charged massive dust grains, are reported A modified Korteweg-de Vries equation (modified KdV), which admits a solitary waves solution for small but finite amplitude, is derived using a reductive perturbation theory A nonisothermal ions distribution provides the possibility of coexistence of amplitude rarefactive as well as compressive solitary waves On the other hand, consideration of a critical ions density gives a stationary solution of solitary waves and the dynamics of small but finite amplitude of solitary waves is governed by Korteweg-de Vries equation (KdV) The properties of solitary waves in the two cases are discussed

A study of an inhomogeneous bianchi-I model in lyra geometry

Abstract: The inhomogeneous Bianchi-I model based on Lyra's geometry has been studied in the cosmological theory in presence of a massless inhomogeneous scalar field whose potential has a flat part. The field equations are solved using separation of variables and it is shown that one of the time part of the field equations are solvable for any arbitrary other cosmic scale function. Solutions for a particular form of cosmic scale (time part) is presented.

A 2.09-h Photometric Periodicity in GW Librae

Abstract: We have found a 2.09 h modulation in brightness of the dwarf nova GW Lib. This bears no special relationship to the 1.28 h spectroscopic period that is believed to be the orbital period. It was present in May 2001, but not in observations made in 1997 and 1998 and is of unknown origin. Similar unexplained periodicities are present in FS Aur and V2051 Oph.

Charged Static Fluid Spheres in General Relativity

Abstract: The work is concerned with the charged analogue of Bayin's paper (1978) related to Tolman's type astrophysically interesting aspects of stellar structure.

On Bianchi Type-I Mesonic Cosmological Model in Bimetric Theory

Abstract: Anisotropic spatially homogeneous Bianchi type-I cosmological model in bimetric theory of gravitation (Rosen, 1973) is considered. It is shown that the Bianchi type-I cosmological model does not exist in case of both meson field and mesonic perfect fluid (with or without mass parameter). Hence only vacuum models can be obtained (Reddy and Venkateswarlu, 1989).

Astronomical Engineering Revisited: Planetary Orbit Modification Using Solar Radiation Pressure

Abstract: As the Sun evolves along the main sequence its luminosity will grow, leading to a steadily increasing solar flux at the Earth with corresponding catastrophic consequences for the biosphere A novel means of avoiding this terminal route to human evolution has recently been proposed by Korycansky et al which utilises a series of grazing fly-pasts of the Earth with a small solar system body to increase the orbit radius of the Earth over a timescale of order 109 years This short paper will propose an alternative strategy which utilises a large reflective sail to generate a propulsive thrust due to solar radiation pressure It will be shown that if the sail is configured to be in static equilibrium relative to the Earth, the centre-of-mass of the Earth-sail system slowly accelerates This scheme offers some advantages in that the mass of the sail is four orders of magnitude less than the mass to be processed in the scheme of Korycanskyet al for trajectory correction manoeuvres alone In addition, the severe hazard posed by multiple grazing fly-pasts of the Earth by a small solar system body is avoided Although offering significant advantages, any thoughts of engineering on an astronomical scale clearly requires a leap of the imagination and a ready use of liberal assumptions

Delay times between geoeffective solar disturbances and geomagnetic indices

Abstract: We have examined delay times between solar disturbances (X-ray flares and DSFs) and storm sudden commencements(SSC) as well as between SSC and major geomagnetic storms. To carry out cross-correlation analysis of these point series data, we have introduced a new correlation measure which is defined by the ratio of the median value of the absolute residual differences between two sets of time series data to the one determined from hypothetical target series. We have confirmed from the correlation analyses that (1) the most probable traveling time of a solar disturbance from the Sun to the Earth is estimated to be about 2 days for a disturbance associated with major (X and M class) solar flares, and about 3 days for a disturbance associated with DSFs, (2) long-duration flares are better correlated with SSCs than short-duration flares, (3) travelling times of solar disturbances strongly depend on the heliolongitude where they originate, and (4) solar disturbances associated with flares and DSFs at the western limb can hardly reach the Earth.

A new self creation cosmology

Abstract: A theory is described which produces continuous creation by adapting that of Brans-Dicke. The universe is seen to be created out of the zero point energy field by self-contained gravitational, scalar, and matter fields. The theory is conformally equivalent to General Relativity in vacuo.Both the Jordan and the Einstein frames are physical and they conserve energy and four-momentum respectively. The conformal equivalence has the consequence that predictions of the theory in solar system experiments are identical with General Relativity, but definitive experiments exist which distinguish between the two theories. The cosmological solution yields a linear expansion with a dynamical density parameter Omega of $$\tfrac{1}{3}$$ anda cold matter density parameter of $$\tfrac{2}{9}$$ , but the universe is closed. The theory is free of the horizon, smoothness and density problems of GR and therefore does not need Inflation. It does however require an exotic equation of state with negative pressure and it is suggested that this is provided by a false vacuum or zero point energy determined, and there forelimited by, its field equations thereby overcoming the ‘lambda problem’.

Two-Fluid Bianchi Type II Cosmological Models

Abstract: In this paper we present a class of solutions of Einstein's field equations describing two-fluid models of the universe in a locally rotationally symmetric Bianchi type II space-time. In these models one fluid is the radiation distribution which represents the cosmic microwave background and the other fluid is the perfect fluid representing the matter content of the universe. It is found that both the fluids are comoving in the locally rotationally symmetric Bianchi type II space-time. The behaviour of the radiation density, matter density, the ratio of the matter density to the radiation density and the pressure has been discussed. A subclass of solutions is found to describe models of a spatially homogeneous and partially isotropic universe evolving from a radiation dominated era to a pressure free matter dominated era.

Transient Modulation of Cosmic Ray Intensity: Role of Magnetic Clouds and Turbulent Interaction Regions

Abstract: Two distinct regions of shock-associated magnetic clouds, (i) magnetically turbulent regions formed due to interaction between magnetic cloud and ambient magnetic field i.e. turbulent interaction region (TIR), and magnetically quiet region called magnetic cloud have been considered separately and correlation of interplanetary plasma and field parameters, magnetic field strength (B) and solar wind speed (V), with cosmic ray intensity (I) have been studied during the passage of these two regions. A good correlation between B and I and between V and I has been obtained during the passage of sheath when the magnetic field is high and turbulent, while these correlation have been found to be poor during the passage of magnetic clouds when the field is strong and smooth. Further, there is a positive correlation between enhancement in field strength and its variance in the sheath region. These results strongly support the hypothesis that most Forbush decreases are due to scattering of particles by region of enhanced magnetic turbulence. These results also suggest that it will provide a better insight if not the magnetic field enhancement alone but in addition, the nature of magnetic field enhancement is also considered while correlating the field enhancements with depressions in cosmic rays.

CDM, feedback and the hubble sequence

Abstract: We have performed TreeSPH simulations of galaxy formation in a standard ΛCDM cosmology, including effects of star formation, energetic stellar feedback processes and a meta-galactic UV field, and obtain a mix of disk, lenticular and elliptical galaxies. The disk galaxies are deficient in angular momentum by only about a factor of two compared to observed disk galaxies. The stellar disks have approximately exponential surface density profiles, and those of the bulges range from exponential to r 1/4, as observed. The bulge-to-disk ratios of the disk galaxies are consistent with observations and likewise are their integrated B — V colours, which have been calculated using stellar population synthesis techniques. Furthermore, we can match the observed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is (M/L I ) ≃ 0.7. The ellipticals and lenticulars have approximately r 1/4 stellar surface density profiles, are dominated by non-disklike kinematics and flattened due to non-isotropic stellar velocity distributions, again consistent with observations.

Constraints on the Solar Internal Magnetic Field from a Buoyancy Driven Solar Dynamo

Abstract: We report here results from a dynamo model developed on the lines of the Babcock-Leighton idea that the poloidal field is generated at the surface of the Sun from the decay of active regions. In this model magnetic buoyancy is handled with a realistic recipe – wherein toroidal flux is made to erupt from the overshoot layer wherever it exceeds a specified critical field B c (105 G). The erupted toroidal field is then acted upon by the α-effect near the surface to give rise to the poloidal field. In this paper we study the effect of buoyancy on the dynamo generated magnetic fields. Specifically, we show that the mechanism of buoyant eruption and the subsequent depletion of the toroidal field inside the overshoot layer, is capable of constraining the magnitude and distribution of the magnetic field there. We also believe that a critical study of this mechanism may give us new information regarding the solar interior and end with an example, where we propose a method for estimating an upper limit of the difusivity within the overshoot layer.

Hydrogen-poor planetary nebulae

Abstract: Five planetary nebulae are known to show hydrogen-poor material nearthe central star. In the case of A58, this gas was ejected following alate thermal pulse similar to Sakurai's Object. In this paper I will reviewthese five objects. One of them, IRAS 18333 –2357, may not be a truePN. I will show that there is a strong case for a relation to the [WC]stars and their relatives, the weak emission-line stars. The surfaceabundances of the [WC] stars are explained via diffuse overshoot intothe helium layer. The hydrogen-poor PNe do not support this: theirabundances indicate a change of abundance with depth in the heliumlayer. A short-lived phase of very high mass loss, the r-AGB, isindicated. Sakurai's Object may be at the start of such a phase, and mayevolve to very low stellar temperatures.

Effect of Flow-Tube Geometry on Solar Wind Properties

Abstract: A mathematical description of the solar wind flow-tube geometry is proposed. The expansion factor of the flow tube f(r) (=a/r2, r is the heliocentric distance and a is the flow-tube cross-section area)increases monotonically from 1 at the coronal base to m at rc, and approaches its asymptotic value f∞ nearly in a width of2σc. The flow tube with fm=finfin is demonstrated to be approximately equivalent to that given by Kopp and Holzer (1976) for the fast solar wind, and it presumably represents slow wind tubes as fm is substantially larger than f∞. In terms of an Alfven wave-driven solar wind model, the effect of the flow-tube geometry on solar wind properties is examined. It is found that with the same flow conditions at the coronal base an expansion factor which increases monotonically with the radial distance results in a fast solar wind solution, whereas a flow tube which undergoes an expansion-contraction-reexpansion process creates a slow solar wind solution. Among the four flow-tube parameters the maximum expansion factor fm has the strongest effect, and the associated Laval-nozzle formed by the contraction and reexpansion of theflow tube plays a crucial role in determining solar wind properties. It is suggested that one must take the effect of the flow-tube geometry into account while constructing reasonable flow-tube models for the slow solar wind.

Viscous cosmological models with particle creation in Brans-Dicke theory

Abstract: The effect of bulk viscosity on the evolution of the spatially flat Friedmann-Robertson-Walker models in the context of open thermodynamical systems, which allow particle creation, is analyzed within the framework of Brans-Dicke theory. Particle creation and bulk viscosity have been considered as separated irreversible processes. To accommodate the viscous pressure and creation pressure, which is associated with creation of matter out of gravitational field, the energy-momentum tensor is modified. Dynamical behaviour of the models have been discussed.