bluuebunny/crossref_metadata_2025_split huggingface dataset:
Observations of the Celestial Ephemeris Pole
Authors: Gross R.S. 
Published: March 2016

AbstractSpace-geodetic measurement systems are capable of determining: (1) a terrestrial, body-fixed reference frame defined in practice by the stated positions and secular motions of a set of observing stations, (2) a celestial, space-fixed reference frame defined in practice by the stated locations of celestial objects, and (3) the rotation parameters linking these two frames together. Five parameters are conventionally used to specify the orientation of the terrestrial frame with respect to the celestial frame: two nutation parameters, two polar motion parameters, and one spin parameter. The celestial ephemeris pole (CEP) is defined as the north pole of that axis about which the spin parameter (UT1) is measured. The two nutation parameters locate the CEP in the celestial frame, and the two polar motion parameters locate the CEP in the terrestrial frame. By examining the frame transformation matrices, an expression relating the location of the rotation pole to that of the CEP can be derived. In order to compare theoretical predictions with observations, results of models for the effect on the nutations of geophysical excitation processes such as diurnal oceanic current and sea level height variations should not only be given in terms of the location of the CEP (rather than of the rotation pole), but must also account for the resonance effects of the free core nutation.
Time Ephemeris
Authors: Fukushima Toshio 
Published: March 2016

The location-independent part of TCB–TCG, the difference between the two new time scales adopted by the IAU (1992), was integrated numerically for three JPL planetary/lunar ephemerides; DE102, DE200, and DE245. The differences among these three integrations were mostly explained by the difference in the adopted constants of the ephemerides. It was shown that the post-Newtonian correction and the perturbation by asteroids are negligible except for the mean rate, LC The comparison of these numerical integrations with the analytical formulas of Hirayama et al. (1987) and Fairhead and Bretagnon (1990) as well as their extended versions lead to the best estimate of LC asCombining this with the recent value of the geoid potential in Bursa et al. (1992), we estimated the value of LB, the scale difference between TCB and TT, as
Determining the solar structure from oscillation frequencies
Authors: Basu S. 
Published: March 2016

The inverse problem of finding the structure of the solar interior from the observed frequencies can be written aswhere, δωi is the difference in frequency of the ith mode between the solar data and the reference model, f1 and f2 are an appropriate pair of model parameters (e.g. sound speed squared c2, and density ρ), Ei is the mode kinetic energy, K(1) and K(2) are known functions of the reference model, and F(ω) is the unknown function added to account for uncertainties associated with the physics of the surface layers.
Spectroscopic Measurements of Stellar Rotation
Authors: Dravins Dainis 
Published: March 2016

This section title is identical to that of the first paper where the broadening of absorption lines in response to stellar rotation was discussed: Abney (1877).More accurate measurements now reveal also the details of stellar line shapes, making it possible to segregate the signatures of rotational and other broadening mechanisms: e.g. Gray (1992) and Smith & Gray (1976). To determine the rotation, fits can be made to line profiles (e.g. Anders et al., 1993), to their Fourier transforms (e.g. Dravins et al. 1990;Smith & Gray 1976), or to extended spectral regions (e.g. Kurucz et al. 1977).This review, however, concerns issues for [single-epoch] spectroscopic observations only, no temporal aspect will be discussed.What is observed is a rotationally broadened profile, the accuracy begins to get limited by the incomplete physical understanding of stellarline profiles and of the nature of stellar rotation. In order to disentangle the rotational broadening from other effects, one needs to know the ‘intrinsic’ (i.e. rotationally unbroadened) profile of the non-rotating star. How does this profile change with latitude and longitude across the stellar disk? What effects besides rotation are broadening the lines? What about mass loss, radial pulsation, non-radial oscillations, magnetic fields, spots, etc.? And the star might not even rotate as a rigid body, but perhaps differentially with respect to latitude and/or atmospheric height. All this has to be deduced from the often blended lines in complex spectra.
The Interstellar 7Li/6Li Ratio
Authors: Ferlet R. 
Published: March 2016

Lithium-7 is now generally accepted to originate in the hot Big Bang nucleosynthesis (BBN), with a primordial abundance (7Li/H)⋍ 10-10 in excellent agreement with the observed uniformity of the Li abundance in very metal deficient Pop II stars.During the galactic evolution, both Li isotopes are created by spallation reactions of galactic cosmic rays (GCR) interacting with the ISM, that yield (7Li/H)⋍ 2 × 10-10 in 10 Gyrs, with a ratio (7Li/6Li)GCR=1.4. The major problem is then to explain the observed Pop I Li abundance, (7Li/H)PopI ∼ 10-9, of which only 30% is accounted for by BBN and GCR spallation, as well as the high 7Li/6Li ratio measured in meteorites, representative of the solar system formation epoch 4.6 Gyrs ago, (7Li/6Li)⊙=12.3, whereas the above mechanisms predict a ratio around 2.The existence of an extra stellar source of Li has been suggested. GCR spallation alone tends to decrease the 7Li/6Li ratio with time, and one should observe today an interstellar ratio ≃5-6 without production of Li in stars, or ≳6 with a steuar production. Measuring this ISM ratio thus provides a key test for the models of lithium evolution. If it is found to be ≲5, then another scenario would have to be considered.
The Galactic Chemical Evolution of Lithium
Authors: Matteucci F. 
Published: March 2016

Under the assumption that the abundance of 7Li in Population II stars represents the primordial Li abundance (with perhaps a small contribution from GCR spallation) and that GCR spallation/fusion processes cannot contribute to more than ≃ 10 − 20% of the Li abundance observed in Pop. I stars and in the solar system, one must conclude that most of Li in Pop. I stars has a stellar origin.Possible stellar Li producers are discussed: low mass AGB stars (2−5M⊙) (C-stars), high mass AGB stars (5 - 8M⊙), supernovae of type II (M > 10M⊙) and novae. The various problems connected with all of these sources are indicated: in particular, we discuss the Li production in AGB stars when evolutionary effects due to the metallicity are taken into account, and the fact that novae do not seem to be good candidate for Li production, as suggested by a recent nucleosynthesis study. We then calculate the yields from these stellar sources and predict the behavior of log N(Li) vs. [Fe/H] by means of a galactic chemical evolution model.We conclude that, although a unique model cannot be found, due to the uncertainties still existing in the stellar nucleosynthesis, the most likely scenario is that Li is partly produced in type II supernovae (v-induced nucleosynthesis) and partly in massive AGB stars.
Mass Loss and Lithium Abundances in Popii Stars: A New Approach of The Primordial Value
Authors: Vauclair Sylvie , Charbonnel Corinne 
Published: March 2016

The observations by Spite & Spite (1982), that the upper values of the lithium abundance in halo stars are one order of magnitude smaller than the ones observed in galactic stars, lead to a large debate about the primordial lithium abundance. The whole problem can be summarized by the following question: has lithium been depleted in the outer layers of halo stars, or has the original lithium been preserved at their surfaces since the beginning?It seems difficult to maintain the original lithium abundance in halo stars during all their lifetime. Either lithium is depleted due to element separation, or it is destroyed by nuclear reactions. Computations by Proffitt & Michaud (1989) showed that nowhere inside halo stars the lithium abundance could have remained at its original value.It was suggested by Vauclair (1988) that rotation-induced turbulence could lead to a nuclear destruction of lithium in halo stars large enough to explain their present abundances, with an original abundance equal to the present galactic one. It seemed possible that the “plateau shape” of the abundances be preserved if the turbulent diffusion coefficient decreased rapidly with radius, as in Zahn (1987) (see also Pinsonneault et al. 1992).
Chemical Abundances from Planetary Nebulae in Elliptical Galaxies
Authors: Walsh J.R. 
Published: March 2016

Currently over 2000 Planetary Nebulae (PN) have been detected in Local Group E and S0 galaxies by on/off band [O III]5007Å CCD photometry. PN can currently be detected out to Fornax and Virgo clusters.The magnitude of the brightest PN in the nearest elliptical galaxy (NGC 205) is m5007 20.7 (Ciardullo et al, 1989); for NGC 5128 (Cen-A) it is 23.5 (Hui et al, 1993a) and ≥ 25.6 for Virgo ellipticals (Jacoby et al, 1990). Given the relation between m5007 and line flux:m5007 = −2.5 log F5007 − 13.74the typical Hβ fluxes are ≤1.5×10-16 ergs cm−2 s−1 for NGC 5128 PN and ≤ 2.5×10−17 for Virgo PN. Obtaining spectra of these faint emisison lines is clearly a challenge. The only PN spectra studied in Local Group galaxies are 1 PN in M32 (NGC 221) (Log F(H/β) = −14.9; Jenner et al, 1979) and recently 15 in NGC 205 (Jacoby et al, 1995). None of the spectra are of high quality and the Te diagnostic line [O III]4363Å was not detected. There is evidence of possible high N abundance in the M32 PN.
A Supermassive Object in the Nucleus of NGC 4258?
Authors: Greenhill L.J. , Jiang D.R. , Moran J.M. , Reid M.J. , Lo K.-Y. , Claussen M.J. , Henkel C. , Wilson T.L. , Becker R. , Wouterloot J.G.A. 
Published: March 2016

AbstractVLBI images of the H2O megamaser in NGC 4258 and a time series of spectra taken over several years combine to make a compelling case that there is a compact molecular disk associated with a supermassive object in the nucleus of this galaxy. The images of the maser in the velocity rangenear the systemic velocity show a highly elongated structure with a major axis of about 0.009 pc, along which the gradient in line-of-sight velocity is essentially a constant of 7970 ± 40 km s-1pc-1. The observed acceleration of these spectral features by about 6-11 km s-1yr-1, the presence of high-velocity maser satellite emission, and the VLBI results suggest emission from a disk of diameter0.2 pc, rotating with a velocity of ~800 Km s-1, bound by a mass of 1.5×107M⊙and denisty of ≳3.6×109M⊙pc-3
New Atomic Data for Astronomy: An Introductory Review
Authors: Seaton M.J. 
Published: March 2016

Astronomers require the following basic atomic data: energy levels and wavelengths’, radiative transition probabilities; cross sections for photo-ionisation and for collisional processes; and line profile parameters. They also require processed data such as: level populations; opacities; radiation forces; line emissivities; and collisional rate-coefficients.Many of the data used by astronomers come from theoretical work. Experimental work is of importance in determining accurate wavelengths, in providing essential checks on theory for radiative probabilities and collision rates, and in the determination of line-profile parameters. Experimental studies are particularly important for processes of collisional ionisation.
Atmospheres and Winds of Hot Stars - Impacts of New Opacity Calculations and Continuing Needs
Authors: Butler Keith 
Published: March 2016

In this paper I review some recent advances in the use of large amounts of atomic data in the modelling of atmospheres and winds of hot stars. The review is highly selective but representative of current developments. A more general overview is to be found in Kudritzki and Hummer (1990) although the field is changing so rapidly that much has happened since then. The paper breaks down into three parts: work on line formation, in which the atmospheric structure is known and held fixed, is described first, then follows a description of the inclusion of line opacities in non-LTE in the atmosphere problem itself, and finally recent developments in the theory of radiatively driven stellar winds are summarized. Here special emphasis is given to a novel distance determination method based entirely on spectroscopie quantities. I close with a brief shopping list.In a series of papers, Becker and Butler (1992,1994a, b,c) have investigated iron and nickel spectra in sub-dwarfs using the complete linearization method of Auer and Heasley (1976). The method scales linearly with the number of frequency points so they were able to use well over 10000 frequencies to adequately describe the line opacities. Several thousand lines were treated explicitly and the resultant computed spectra gave execellent fits to observed Hubble spectra in the wavelength ranges dominated by the ions concerned.The different ionization stages gave consistent results for the iron and nickel abundances but only after line-blocking from millions of spectral lines in the far UV had been included. This was done using the Kurucz (1988) line lists coupled with line grouping as suggested by Anderson (1989) and described briefly in the next section.The line-blanketed atmospheres of Kurucz (1991) are the best available up to about 30000K, where non-LTE effects start to become important. Non-LTE line-blanketed atmospheres have become feasible because the computational requirements of the accelerated lambda iteration (ALI) method (Werner and Husfeld, 1985) also scale linearly with the number of frequency points. On the other hand, Anderson (1989) suggested grouping energetically adjacent atomic levels together to form pseudo-levels on the basis that although they might, as a group, be in non-LTE, they should be in LTE with respect to one another due to the large number of collisions between them. This greatly reduces the number of levels to be considered but instead gives rise to highly complicated pseudo line-profiles. Grigsby et al (1992), who did not use ALI, constructed the first grid of line-blanketed non-LTE models by using a variation on the Opacity Distribution Function concept to group line opacities into blocks thereby reducing the number of frequency points required. Dreizler and Werner (1993) on the other hand were able to sample the opacity as they used ALI in their models.
Astronomy in Nigeria
Authors: Onuora Lesley I. 
Published: March 2016

One major problem in Africa in general is to convince governments and agencies that astronomy is relevant to Africa. Attention has been focussed on technology transfer, neglecting science and research. This attitude encourages the continued dependence on industrialized countries.In Nigeria there has been some success in projecting the idea that Space Science does not just mean remote sensing, but that basic space science, i.e. astronomy and astrophysics, cosmology, planetary science etc. is important and necessary. Evidence of this is that Nigeria’s expert committee on space policy recommended that one of three proposed National Centres should be for basic space science, laying emphasis on fundamental physics, astronomy and astrophysics, solarterrestrial interactions and their influence on climate, planetary and atmospheric studies. In addition, the Government of Nigeria hosted the Third UN/ESA Workshop on Basic Space Science in October, 1993.
Astronomy in South Africa
Authors: Warner Brian 
Published: March 2016

This note reviews those aspects of astronomy in South Africa that may be useful for the support of astronomical development on the rest of the African continent.Optical and infrared astronomy is largely consolidated at the Sutherland site of the South African Astronomical Observatory (SAAO), about 300 km from Cape Town. The administrative and technical headquarters of this institution occupy the buildings of the old Royal Observatory (founded in 1820) in Cape Town. In Cape Town there is a twin 18/24 inch (0.45/0.6m) refractor and an 18-inch (0.45m) photometric reflector still operational. At Sutherland the 74-inch (1.8m Radcliffe) reflector is equipped for spectroscopy, CCD imaging and IR photometry. Newtonian and coudé foci are available but rarely used. Also at Sutherland there are 40-inch, 30-inch and 20-inch telescopes (approx. 1m, 0.75m and 0.5m) used primarily for CCD imaging (40-inch), optical and infrared photometry (30-inch) and UVBRI standard photometry (20-inch). An automatic photoelectric telescope (30-inch aperture) is nearing completion. Observing conditions at Sutherland provide about 50% photometric time.
Various Aspects of the Spectroscopy of PAHs (Polycyclic Aromatic Hydrocarbons)
Authors: Bréchignac Philippe 
Published: March 2016

AbstractThe exploitation of the recent observations (in particular by ISO) of the near infrared emission bands (UIBs) requires a good knowledge of the spectroscopic properties of the main proposed carriers, i.e. the Polycyclic Aromatic Hydrocarbons, which are expected to act as efficient UV/visible to infrared free-flying light converters.The charge state of the interstellar PAHs strongly affects their spectroscopy. The laboratory study of the electronic transitions of PAH cations in the gas phase enables to address important questions, like their possible contribution to the Diffuse Interstellar Bands (visible and near-IR absorption), and the mechanism responsible for electronic to vibrational intramolecular energy conversion.Microcanonical simulations based on hybrid quantum/classical theoretical methods reveal how strongly affected are the infrared intensities by both the charge state and the vibrational temperature.
Crystalline Silicates in Space
Authors: Waters L.B.F.M. , Molster F.J. 
Published: March 2016

AbstractAn overview is given of the occurrence and properties of crystalline silicates in space. Crystalline silicates are found in circumstellar shells of both young and evolved stars, and are present in Solar System comets. These silicates are Mg-rich and Fe-poor, in contrast to amorphous silicates. We discuss the formation mechanism of crystalline silicates and the apparent lack of these grains in the interstellar medium.
Infrared Spectroscopy of Jupiter and Saturn
Authors: Drossart Pierre 
Published: March 2016

AbstractThe spectroscopy of giant planets in the infrared range gives access to a remote sensing of many physical parameters. The composition, pressure/temperature structure, and the cloud structure all contribute to the spectrum, in solar reflected light below 3 micrometer as well as thermal emission above, from atmospheric levels ranging from the mesosphere down to the troposphere. Imaging spectroscopy revealing the variability of the atmosphere gives access to spatial and temporal evolution of these parameters, constraining the meteorological evolution of the planets.
Fundamental Geodetic Parameters 1999
Authors: Groten Erwin 
Published: March 2016

AbstractThe present (2000) set of primary “constants” and “current best estimates” is being reviewed. The emphasis within IUGG is on the consideration and numerical evaluation of temporal changes of relevant parameters. The accuracy of many parameters has been substantially improved, but many numerical values did not change significantly. Therefore, to avoid confusion IUGG/IAG are reluctant in introducing new official parameter sets but keep “current best estimates” up-to-date.
Report of IAU Working Group on ‘Non-Rigid Earth Nutation Theory’
Authors: Dehant V. 
Published: March 2016

With respect to the publication in the Proceedings of the IAU Colloquium 180 (Dehant 2000), I have a few additional remarks that I would like to make. There were three competing models:1. MHB2000 of Mathews et al. (2000), (see also Mathews 2000), constructed from a rigid Earth nutation series with about 1400 terms and a transfer function obeying the sum rules, based on the fit of a semianalytical nutation theory to VLBI observations, with resonances determined by geophysical parameters including those estimated from the fit, incorporating the atmospheric annual effect fitted to observations, including ocean tide effects based on admittances with frequency dependence due to the FCN (Free Core Nutation) resonance and to ocean dynamics (fitted to ocean tide data), including electromagnetic couplings of the fluid core, and with mutual consistency maintained in the treatments of nutations, solid Earth tides and ocean tides which influence one another;2. GF2000 of Getino and Ferrándiz (2000), using a global Hamiltonian approach for 106 waves fitted to VLBI data, incorporating a resonance with global parameters fitted to observations such as the FCN free mode frequency, compliances, and dissipation coefficients, incorporating ocean corrections from Huang et al. (2000) with a frequency dependent resonance and fitted atmospheric corrections, and necessitating empirical corrections;3. SF2000 of Shirai and Fukushima (2000a and 2000b) or Herring (2000, not published), empirical models, based on a simple resonance formula fitted to the VLBI observation.
PN Jeffreys-Vicente Equation
Authors: Xu Chongming , Wu Xuejun , Soffel Michael 
Published: March 2016

In our work (Xu et al. 2000) for the first time a complete and closed set of post-Newtonian dynamical equations for elastically deformable, rotating astronomical bodies has been deduced. These bodies are assumed to differ only slightly from corresponding stationary and axisymmetric fluid reference bodies and show no internal dissipation, i.e., they are composed of a perfectly elastic medium. For such bodies a set of perturbation equations is derived by means of the Carter and Quintana formalism in a suitably defined rotating coordinate system. The post-Newtonian dynamical equation for the displacement field is the central result of this paper together with the various equations that define the quantities appearing therein. It is the post-Newtonian version of the well-known Jeffreys-Vicente (J-V) equation (Jeffreys & Vicente 1957) that forms the basis of any local theory of global geodynamics. PN J-V equation reads:where sa is the covariant displacement field, (ρ is the density of tolal mass-energy, p is the isotropic pressure), WG a relativistic geopotential in rotating coordinate, Θ the volume dilation, δWG Euler variation of WG, K the compression modulus, is the shear tensor, W and Wα are the scalar and vector potential in rotating coordinate.
Structure and Conditions in Massive Star Forming Giant Molecular Clouds
Authors: Williams Jonathan 
Published: March 2016

AbstractMassive stars form in clusters within self-gravitating molecular clouds. The size scale of these clusters is sufficiently large that non-thermal, or turbulent, motions of the gas must be taken into account when considering their formation. Millimeter wavelength radio observations of the gas and dust in these clouds reveal a complex, self-similar structure that reflects the turbulent nature of the gas. Differences are seen, however, towards dense bound cores in proto-clusters. Examination of the kinematics of gas around such cores suggests that dissipation of turbulence may be the first step in the star formation process. Newly formed stars, on the other hand, replenish turbulence through their winds and outflows. In this way, star formation may be self-regulated. Observations and simulations are beginning to demonstrate the key role that cloud turbulence plays in the formation and evolution of stellar groups.
Infrared Spectroscopy of the Centaur Asbolus with the ESO-Very Large Telescope
Authors: de Bergh Catherine , Barucci Maria-Antonietta , Le Bras Aurélie , Romon Jennifer , Schmitt Bernard , Cuby Jean-Gabriel 
Published: March 2016

AbstractAs part of an observing programme with the ISAAC infrared spectrometer at the 8-m Antu telescope of the ESO-Very Large Telescope (Chile) devoted to the study of the surface composition of Kuiper Belt Objects and Centaurs, we obtained, in May 1999, spectra of the red Centaur Asbolus in the J, H and K bands. The spectra appear featureless.