Showing papers in "Astronomy and Astrophysics in 1994"

The Cosmon model for an asymptotically vanishing time dependent cosmological 'constant'

Abstract: We investigate the coupled system of gravity and a scalar with exponential potential. The energy momentum tensor of the scalar field induces a time-dependent cosmological “constant”. This adjusts itself dynamically to become in the “late” universe (including today) proportional to the energy density of matter and radiation. Possible consequences for the present cosmology are shortly discussed. We also address the question of naturalness of the cosmon model. Whenever cosmology encounters potential difficulties in the description of the present universe cosmologists revive the discussion about the cosmological constant [1]. The discrepancy between the critical energy density expected from inflationary cosmology and lower dynamical estimates of this density has been attributed to the cosmological constant [2]. The discussion also pertains to the age of the universe [2] and the formation of structure [3]. In fact, a cosmological constant λ of the order of today’s critical energy density in the universe (λ ≈ (2 · 10eV )) strongly affects the present universe without altering the successful predictions of the hot big bang model at early stages of the evolution of the universe. Despite many attempts [4] we have at present no satisfactory understanding why λ should be much smaller than typical energy scales of the standard model or even the Planck mass Mp. For a time-independent cosmological constant it seems even harder to explain why it should be of the order of the present energy density. The latter depends on the age of the universe rather than on fundamental constants. It looks then not very natural that a constant λ should have a value which equals the energy density just at a time within the present cosmological epoch. In this work we consider a model where the cosmological “constant” varies with time such that the asymptotic solution for late times is characterized by a constant ratio λ(t)/ρ(t)[5], [6]. We discuss consequences for present cosmology and various alternatives how “early cosmology” could have made a transition to this type of “late cosmology”. We also briefly address the question of naturalness of an asymptotically vanishing cosmological “constant”. We start from the field equations for a scalar field φ coupled to gravity in a homogenous and isotropic universe (with k = 0 and H the Hubble parameter) φ+ 3Hφ+ ∂V ∂φ = q (1) ρ+ 3H(ρ+ p) + qφ = 0 (2) H = 1 6M2 (

Robust morphological measures for large-scale structure in the Universe

Abstract: We propose a novel method for the description of spatial patterns formed by a coverage of point sets representing galaxy samples. This method is based on a complete family of morphological measures known as Minkowski functionals, which includes the topological Euler characteristic and geometric descriptors to specify the content, shape and connectivity of spatial sets. The method is numerically robust even for small samples, independent of statistical assumptions, and yields global as well as local morphological information. We illustrate the method by applying it to a Poisson process, a `double-Poisson' process, and to the Abell catalogue of galaxy clusters.

Numerical expressions for precession formulae and mean elements for the Moon and the planets.

Abstract: We present, in this paper, a coherent set of formulae giving numerical expressions for precession quantities and mean elements of the Moon and the planets. First, using the notations of Lieske et al. (1977), we construct expressions for the precession quantities based upon the use of the secular variations of the ecliptic pole from the planetary theories built at the Bureau des Longitudes and taking into account recent determinations of the precession constant and of the obliquity in J2000. Also we give the derivatives of these expressions with respect to the masses of the planets, to the precession constant and to the obliquity. So, this set of formulae is applicable whenever the values of the planetary masses and of the constants are improved

Conditions for condensation and preservation of amorphous ice and crystallinity of astrophysical ices

Abstract: Conditions for formation and preservation of amor­ phous ice formed through condensation of water vapor on a substrate is investigated both theoretically and experimentally. The kinetic consideration of deposition of vapor leads to the condition for formation of amorphous ice on a cold substrate: the flux of water vapor onto the substrate should be larger than a critical flux Ds/a 4 == Fe, where Ds is the surface diffusion coefficient of water molecules on the substrate and a the lattice constant of the substrate. The validity of the derived condition has been confirmed by measuring the critical flux of water vapor onto a substrate of polycrystalline cubic ice as a function of tem­ perature. From the measured Fe, a surface diffusion coefficient of H20 molecules on polycrystalline cubic ice has been deter­ mined. With regard to the preservation condition, we derive a theoretical expression of the time scale te for crystallization of amorphous ice, which has the same form at low temperatures as the empirical formula used so far. A self-diffusion coeffi­ cient of amorphous ice is estimated from a comparison between the theoretical and empirical formulas of te. The results are ap­ plied to condensation of ice in molecular clouds, circum stellar envelopes of late-type stars, and the primordial solar nebula. Discussion is given on the crystallinity of ices in these sites.

Perturbative Lagrangian approach to gravitational instability

Abstract: This paper deals with the time evolution in the matter era of perturbations in Friedman-Lemaitre models with arbitrary density parameter $\Omega$, with either a zero cosmological constant, $\Lambda = 0$, or with a non-zero cosmological constant in a spatially flat Universe. Unlike the classical Eulerian approach where the density contrast is expanded in a perturbative series, this analysis relies instead on a perturbative expansion of particles trajectories in Lagrangian coordinates. This brings a number of advantages over the classical analysis. In particular, it enables the description of stronger density contrasts. Indeed the linear term is the famous Zel'dovich approximate solution (1970). We present here a systematic and detailed account of this approach. We give analytical results (or fits to numerical results) up to the third order. We then proceed to explore the link between the lagrangian description and statistical measures. We show in particular that Lagrangian perturbation theory provides a natural framework to compute the effect of redshift distortions, using the skewness of the density distribution function as an example. Finally, we show how well the second order theory does as compared to other approximat- ions in the case of spherically symmetric perturbations. We also compare this second order approximation and Zel'dovich solution to N-body simulations in the description of large-scale structure formation starting from a power law (n=-2) power spectrum of Gaussian perturbation. We find that second order theory is both simple and powerful.

Interstellar oxygen, nitrogen and neon in the heliosphere

Abstract: Oxygen, nitrogen and neon pick-up ions of interstellar origin were detected for the first time with the Solar Wind Ion Spectrometer (SWICS) on board Ulysses The interstellar origin of these ions is established by the following criteria: (a) they are singly charged, (b) they have the broad velocity distributions characteristic of pick-up ions, with an upper limit of twice the solar wind speed, (c) their relative abundance as a function of distance from the sun corresponds to the theoretical expectation, and (d) there is no relation to a planetary or cometary source The interstellar abundance ratios He(+)/O(+), N(+)/O(+), Ne(+)/O(+) were investigated At approximately 525 AU in the outermost part of Ulysses' trajectory He(+)/O(+) = 175(sup +70 sub -50) N(+)/O(+) = 013(sup +005 sub -005) and Ne(+)/O(+) = 018(sup +010 sub -007) were determined For the interstellar gas passing through the termination region and entering the heliosphere (He/O)(sub 0) = 290(sup +190 sub -100), (N/O)(sub 0) = 013(sup +006 sub -006) and (Ne/O)(sub 0) = 020(sup +012 sub -009) were obtained from the pick-up ion measurements Upper limits for the relative abundances of C(+) and C were also determined

Experimental nuclear masses and the ground state of cold dense matter

Abstract: We study the consequences of recent progress in the experimental determination of masses of neutron rich nuclei for our knowledge of the ground state of cold dense matter. The most recent experimental data determine the ground state of cold dense matter up to ρ ≃ 10 11 g cm −3 . The composition and the equation of state of the ground state of matter, in this density interval, are calculated

Infrared spectroscopy of gas-phase PAH molecules. I. Role of the physical environment.

Abstract: In the PAH model, polycyclic aromatic hydrocarbons are responsible for the Unidentified IR (UIR) bands observed in emission in many astronomical objects. Although this model states that these molecules are isolated, and at high temperatures (T ∼ 1000 K), most laboratory spectra have been obtained for condensed molecules at low temperatures. In this paper, we present the IR absorption spectra (3-20 μm), measured for gas phase molecules at high temperatures. For the three studied species, pyrene (C 16 H 10 ), coronene (C 24 H 12 ), and ovalene (C 32 H 14 ), these spectra are compared to those obtained in condensed phases, solid state and rare gas matrices. The relative band intensities clearly depend on the molecular environment

Iterative blind deconvolution method using Lucy's algorithm

Abstract: Lucy's algorithm is iteratively applied in a blind deconvolution method This new approach permits the reconstruction of a greatly extended object from one speckle frame Speckle images of extended object may lie partly outside the active region of the detector Likewise, objects close to the boundary of the detector, but outside of the field of view, may contaminate the image The blind deconvolution method is adapted to deal with these situations Repeated use of the blind deconvolution method is also proposed Computer simulations are conducted and the effectiveness of the proposed method is demonstrated The method is applied to an image of the solar photosphere

An updated list of radio flux density calibrators

Abstract: We present new flux densities in the range 0.7 cm ≤ λ ≤ 21 cm for the radio sources in the calibrator list of Baars et al. (1977). Most of these sources show significant variability on the time scale of a decade, with the notable exception of 3C286 and 3C295. We use these sources to link our relative measurements to the absolute scale of Baars et al.; for the other sources we present updated spectra. We assess the suitability of the individual sources for use as flux density calibrators

Is dark matter in spiral galaxies cold gas? I: Observational constraints and dynamical clues about galaxy evolution

Abstract: Based on dynamical constraints about the Hubble sequence evolution, observational data and a number of conspiracies, we propose that the dark matter around spiral galaxies is in the form of cold gas, essentially in molecular form and rotationally supported. The existence of a much larger amount of cold gas in the outer discs of spirals is in accordance with dynamical studies taking into account the bar phenomenon and the secular growth of bulges, leading to the general conclusion that spiral galaxies have to evolve along the Hubble sequence from Sd to Sa over a couple of Gyr's. If so, the varying M/L ratio along the Hubble sequence suggests that dark matter is transformed into stars, i.e. dark matter should be in a sufficiently diluted form of gas

The SOLA method for helioseismic inversion

Abstract: The Subtractive Optimally Localized Averages (SOLA) method is a versatile and efficient technique for inverting helioseismic data The SOLA method is based on explicit construction of Backus-Gilbert averaging kernels, but whereas the more usual formulations of the optimally localized averages (OLA) method use a multiplicative penalty function to localize the kernels, the distinctive idea of SOLA is that one specifies a desired target form for the kernels and then minimizes the integrated squared difference between the kernels and the target form This allows great versatility in the choice of target form, and furthermore SOLA has the significant advantage of being computationally more efficient than the usual OLA formulations A Gaussian target function is a useful choice, and we use the example of determining the Sun's internal rotation to explore how the parameter values (such as the Gaussian's width) should best be chosen

Detection of a polycyclic aromatic molecule in comet P/Halley

Abstract: New spectroscopic data obtained with the three-channel spectrometer of Vega 2 show, after subtracting the dust-scattered solar continuum, that a broad band emissive feature between 342 and 375 nm progressively arises. Four bands at 347, 356, 364 and 374 nm can be identified in this feature. The 1/p type intensity increase (p: projected distance between nucleus and line of sight) shows that the molecules responsible for the emission are most probably of the parent-type. A search for a molecule emitting fluorescence bands between 342 and 375 nm showed that phenanthrene was a possible candidate. A laser-induced fluorescence experiment was conducted in the laboratory to record the dispersed emission spectrum of phenanthrene under jet-cooled conditions

The contribution of AGNs to the X-ray background.

Abstract: We report the results of a detailed analysis of the contribution of various classes of AGNs to the extragalactic X-ray background (XRB). The model is based on the unification schemes of AGNs and on their related X-ray spectral properties in the light of recent observational results. The integrated emission from AGNs, when folded with an appropriate cosmological evolution law, can provide a good fit to the XRB over a wide energy range, from several to $\sim$ 100 keV, while it contributes only about 74\% of the ROSAT soft XRB. The model predictions have been checked against all available observational constraints from both hard and soft X-ray surveys (counts, redshift distributions and average X-ray source spectral properties).

Determination of the extragalactic-planetary frame tie from joint analysis of radio interferometric and lunar laser ranging measurements

Abstract: Very Long Baseline Interferometry (VLBI) observations of extragalactic radio sources provide the basis for defining an accurate non-rotating reference frame in terms of angular positions of the sources Measurements of the distance from the Earth to the Moon and to the inner planets provide the basis for defining an inertial planetary ephemeris reference frame The relative orientation, or frame tie, between these two reference frames is of interest for combining Earth orientation measurements, for comparing Earth orientation results with theories referred to the mean equator and equinox, and for determining the positions of the planets with respect to the extragalactic reference frame This work presents an indirect determination of the extragalactic-planetary frame tie from a combined reduction of VLBI and Lunar Laser Ranging (LLR) observations For this determination, data acquired by LLR tracking stations since 1969 have been analyzed and combined with 14 years of VLBI data acquired by NASA's Deep Space Network since 1978 The frame tie derived from this joint analysis, with an accuracy of 0003 sec, is the most accurate determination obtained so far This result, combined with a determination of the mean ecliptic (defined in the rotating sense), shows that the mean equinox of epoch J2000 is offset from the x-axis of the extragalactic frame adopted by the International Earth Rotation Service for astrometric and geodetic applications by 0078 sec +/- 0010 sec along the y-direction and y 0019 sec +/- 0001 sec along the z-direction

Tidal disruption and the appearance of periodic comet Shoemaker-Levy 9.

Abstract: A unified model is formulated to interpret quantitatively the observed characteristics of the nuclear train, the two dust trails, and the tail region of P/Shoemaker-Levy 9 in terms of a collisionally modified rotation velocity distribution of the comet's debris. Since there is no evidence for the comet's out-gassing, the model does not assume activity. The discruption of the parent comet was due primarily to tidal stesses during its extremely close approach to Juptier in July 1992. The original nucleus is found to have been most probably approximately 5 km in radius or, equivalently, approximately 10(exp 17) g in mass. The dynamical separation of the debris occurred apparently approximately 2.2 hr after the perijove passage, even though the actual fragmentation of the original mass is likely to have begun before closest approach. Physical breakp was accompanied by ubiquitous low-velocity collisions among the particulates, resulting in a rearrangement of the initial rotaional velocities into a rapidly 'thermalized' distribution, characterized by a long tail of relatively high velocities for the debris that populates the dust trails far from the nuclear train. Compelling evidence is presented for secondary fragmentation events, indicative of the comet's continuing disintegration. Secondary fragmentation may be caused by rotational bursting of the massive fragments that had been cracked but unbroken during the tidal disruption. Impact conditions and possible interactions of the comet's debris with the jovian system during the encounter in July 1994 are described.

Isothermal elliptical gravitational lens models.

Abstract: Gravitational lens models for observed lensing systems are often based on quasi-elliptical lenses. The use of elliptical mass distributions is motivated by observations of galaxies and by the assumption that mass follows light. Elliptical mass distributions are also expected on theoretical grounds. On the other hand, since elliptical matter distributions are in general more difficult to handle, quasi-elliptical lens models, in which the isopotential curves are ellipses or in which an external shear component is added onto a spherical deflector, are often used for model fitting or for statistical lens studies. However, elliptical potentials correspond to unphysical matter distributions if the ellipticity is large

Are magnetic null points important in solar flares

Abstract: When they are present, null points (points where the 3 components of the magnetic field vanish) determine the topology of the magnetic field configuration. It has been suggested that the presence of nulls is related to flares, and that they settle the region where the energy is released by magnetic reconnection. Since solar flares are initially coronal events, nulls should be located above the photosphere. Therefore, we investigate the location of nulls in configurations formed by a parasitic and a main bipole (4 magnetic sources). For both potential and linear-force free field approaches, we have found that a null was present in the corona only when the two bipoles were nearly antiparallel. The flaring regions analysed here are those where the magnetic topology has been related to flare kernels in previous papers

COMPTEL observations of Ti-44 gamma-ray line emission from Cas A

Abstract: The Compton Telescope (COMPTEL) telescope aboard the Compton Gamma-Ray Observatory (CGRO) is capable of imaging gamma-ray line sources in the MeV region with a sensitivity of the order 10(exp -5) photons/(sq cm s). During two observations periods in July 1992 and February 1993 the Galactic plane in the region of the young supernova remnant Cas A was observed, showing evidence for line emission at 1.16 MeV from the decay of Ti-44 at a significance level of approximately 4 sigma. This is the first time a supernova remnant has been detected in the gamma-ray line from Ti-44 decay. Adopting a distance of 2.8 kpc to the Cas A remnant, the measured line flux (7.0 +/- 1.7) x 10(exp -5) photons/(sq cm s), can be translated into a Ti-44 mass ejected during the Cas A supernova explosion, between (1.4 +/- 0.4) x 10(exp -4) solar mass and (3.2 +/- 0.8) x 10(exp -4) solar mass, depending on the precise value of the Ti-44 mean life time and on the precise date of the event. Implications of this result for supernova nucleosynthesis models are discussed.

GeV/TeV gamma-ray emission from dense molecular clouds overtaken by supernova shells

Abstract: The γ-radiation produced in regions of supernova remnants (SNRs), being in their Sedov phase, and overtaking dense molecular clouds, is expected to be sufficiently high for detection by current satellite and ground based instruments The combined GeV/TeV observations of only a few of this type of objects, in particular from SNR G782+21, could provide the first direct evidence of the acceleration of the nuclear component of galactic cosmic rays in supernova shells