bluuebunny/crossref_metadata_2025 huggingface dataset:
Seismology of δ Scuti stars in stellar clusters
Authors: Lebreton Y. , Michel E.
Published: March 2016

Several δ Scuti stars (A/F stars) have been observed during multi-site campaigns of observations and have been found to be multiperiodic pulsators. They are thus good candidates for asteroseismology from ground based observations. Observations provide the position of the star in the H-R diagram and some frequencies (about 5 for the STEPHI campaigns). Comparisons with models should then allow to identify the oscillation modes of the star and then give information on the unknown parameters of the star (mass, age, chemical composition). However the situation is complicated, since stellar models are based on a physical description of the stellar material, which is not well known in many respects. In particular A/F stars have convective cores, the extent of which depends on the amount of overshooting considered and on the description of this process.
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 Creation of Lithium in Giant Stars
Authors: Sackmann I.-Juliana , Boothroyd Arnold I.
Published: March 2016

A time-dependent “convective diffusion” algorithm for convective transport in the mixing-length framework has been coupled for the first time with a self-consistent full evolutionary computation, in order to investigate theoretically the creation of superrich lithium stars on the asymptotic giant branch. For intermediate mass stars in the mass range from 4 to 7 M⊙ with both Population I and II compositions, hot bottom burning in the convective envelope was found, with maximum temperatures Tce at the base of the convective envelope ranging from 20 to 100 million K, depending on stellar mass and mass loss rates. For Tce ≥ 40 million K, lithium-rich giants were produced (with log ε(7Li) ≳ 1, i.e., above the normal observed range in giants). For Tce ≥ 50 million K, superrich lithium giants were created, with log ε(7Li) ≳ 3 (i.e., larger than the present cosmic7Li abundance).
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.
Lithium Abundance, Diffusion and Turbulence
Authors: Michaud G. , Beaudet G.
Published: March 2016

Richer & Michaud (1993) calculated a series of envelopes fully coupled to non-rotating, constant mass, stellar evolution models of hydrogen burning stars with masses in the range of 1.2 to 2.2 M⊙, typical of A and F main sequence stars. They included He settling. The location of the theoretically predicted gap of the Hyades agrees quite well with the observed one, a result obtained without the introduction of any free parameter. At temperatures above the gap, while the observed lithium abundances are within a factor of 2-3 of normal values, the theoretical calculated curve drops to very low values. Diffusion velocities being fairly small, any other physical process with larger or similar velocities can reduce the effect of diffusion and produce the observed results. Mass loss is one such process. Another difficulty with the present theory is the width of the gap. Observations show that the observed gap is wider than the calculated one in the Hyades. This also suggests that other physical processes play an important role.
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 − 13.74

the 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.
Composition Gradients in Spiral Galaxies
Authors: Peimbert Manuel
Published: March 2016

Some topics on composition gradients are discussed. One of the most important processes responsible for the abundance gradients is the variation of the low-mass end of the IMF with O/H.
X-ray AGN; The View from ASCA
Authors: Kunieda Hideyo
Published: March 2016

ASCA capability of X-ray imaging spectroscopy up to 10 keV. allows us to examine the emission and absorption feature from AGN. Warm absorber, low energy lines and broad iron K lines are confirmed from Sy I’s. High sensitivity in broad energy band makes it possible to distinguish multiple components emerged by different processes. Detection of X-rays from faint sources tells us various galaxies may harbor AGN sometimes with obscuration tori. They might have considerable contribution to CXB.
Black Holes in Galactic Nuclei
Authors: van der Marel Roeland P.
Published: March 2016

The dynamical evidence for black holes (BHs) in galactic nuclei is reviewed, with emphasis on recent improvements in spatial resolution, methods for analyzing galaxy spectra and dynamical modeling. M31, M32 and M87 are discussed in some detail.
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

VLBI 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.
The Role of Opacities in Accretion Disks
Authors: Wehrse R.
Published: March 2016

An accretion disk is formed when matter with angular momentum is flowing on a gravitating object (as e.g. a white dwarf, a neutron star, a young stellar object, or a black bole). It radiates because the transport of angular momentum (required for the matter to reach the central object) necessarily implies the conversion of potential energy into a form of energy that corresponds to higher entropy. Many aspects of the physics (as e.g. the mechanism for the heat generation) are not yet well understood but they are presently one of the centers of astronomical interest (see e.g. the books by Frank, King, and Raine, 1992, or by Wheeler, 1993).
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.
The Kottamia Observatory and Other Aspects of Astronomy in Egypt
Authors: Osman A.M.I.
Published: March 2016

The 74-inch (1.8m) telescope of Kottamia Observatory is the largest telescope in North Africa and the Middle East. It stands on a plateau 485m above sea-level and 70km to the east of Cairo. The latitude is 25° 55’.9 N and the longitude 31° 49’.5E. The telescope is of conventional type, having Newtonian (f/4.9), Cassegrain (f/18) and coudí (f/28.9) foci; it was built by Grubb-Parsons (U.K.). The main mirror is of Duran 50 glass, and has a clear aperture of 74 inches (1.8m) and a focal length, for its paraboloidal surface, of 360 inches (9.2m). The secondary mirrors are of fused quartz. The telescope is provided with a Newtonian camera, a single-channel photoelectric photometer, a Cassegrain spectrograph comprising two cameras with dispersions of 100 Amm−1 and 48 Amm−1 at 4800A and a coude spectrograph with two cameras allowing dispersions of 20Amm−1 and 6Amm−1. A four-component correcting lens has been supplied to be mounted at the Newtonian focus, in order to reduce the effect of coma on photographic observations.
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.
Grains in Astronomy - An Overview
Authors: Witt Adolf N.
Published: March 2016

Astrophysical processes in which grains play key roles are briefly reviewed.
Various Aspects of the Spectroscopy of PAHs (Polycyclic Aromatic Hydrocarbons)
Authors: Bréchignac Philippe
Published: March 2016

The 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

An 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.
Water in Sunspots and Stars
Authors: Bernath Peter F.
Published: March 2016

Water is found in a wide variety of cool stars and substellar objects such as brown dwarfs. The detection of water vapor in such objects is reviewed.
Infrared Spectroscopy of Jupiter and Saturn
Authors: Drossart Pierre
Published: March 2016

The 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

The 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

Massive 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.
Spectroscopy of the Ionizing Sources of UC HII Regions
Authors: Hanson Margaret M.
Published: March 2016

The utility of near-infrared, spectroscopic studies of central ionizing sources of UC HII regions is presented, in conjunction with a recently available, sophisticated atmospheric code, to constrain the physical conditions and environment of very massive stars at extremely early stages of evolution.
Signatures of the Youngest Starbursts: The Discovery of Ultradense HII Regions
Authors: Johnson Kelsey E.
Published: March 2016

Globular clusters are ubiquitous in the local universe, and their younger and bluer siblings, “super star clusters” (SSCs), have been observed in a large number of starburst galaxies. Recently a handful of ultra-young SSCs have been found still embedded in their birth material (e.g. Kobulnicky & Johnson 1999 (here after KJ99); Turner et al. 2000; Beck et al. 2000; Tarchi et al. 2000). Because of their similarity (although on a vastly larger scale) to ultracompact HII regions in the Galaxy, KJ99 dub these embedded massive star clusters “ultradense HII regions” (UDHIIs). I will review the discovery of UDHIIs, their modeled properties, and their connection to the more familiar Galactic ultracompact HII regions.
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

As 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.
Effects of Multiple Planetary Encounters on Kuiper Belt Objects
Authors: Berinde Ştefan
Published: March 2016

Nowadays many attempts are made to establish a qualitative and a quantitative connection between Kuiper Belt Population and Jupiter Family Comets. Basically, this can be thought as a diffusion process throughout the outer Solar System due to multiple close encounters with the giant planets. But, following the path of a body in such a process is not an easy task to be approached analytically nor numerically, because the motion is very chaotic and spread over a long time. A statistical approach seems to be a reasonable way and is the purpose of this paper.
Census of the Kuiper-Belt Objects - The Taiwan-America Occultation Survey (TAOS)
Authors: Chen W.P.
Published: March 2016

More than two hundred transneptunian Kuiper-Belt Objects (KBOs) have so far been identified. Because of their large distances from the Sun, and their intrinsically small sizes, only the largest members (diameters more than 100 km) can be observed by direct imaging with large telescopes. Even smaller KBOs, though expected to be more numerous on theoretical grounds, cannot be seen directly unless they happen to block out the light from background stars. The Taiwan-America Occultation Survey (TAOS) project is a collaboration among the Lawrence Livermore Laboratory (USA), Academia Sinica, National Central University (both of Taiwan), and other institutes, that aims to measure the frequency of such chance stellar occultations, and thereby conduct the census of the Kuiper-Belt population down to a few kilometer sizes. An array of robotic telescopes are being set up in central Taiwan to monitor the brightness variation of several thousand stars at a rate of a few Hz. Observations will be operated in a synchronized and coincidence mode, so the sequence and timing of any candidate occultation event can be recorded and distinguished against a false detection. The full survey is expected to start in the fall of 2000.
Rotational Braking, Angular Momentum Transport and the Light Element Abundances
Authors: Balachandran Suchitra C.
Published: March 2016

Three critical tests are applied to the models that hypothesize the transport of angular momentum and material via turbulence. The models fail on all three counts.
Main Sequence Evolution, Abundance Anomalies and Particle Transport
Authors: Michaud Georges , Richard Olivier , Richer Jacques
Published: March 2016

The availability of large atomic data bases has made it possible to calculate stellar evolution models taking into detailed account the abundance variations of all important contributors to opacity. In a first step, in addition to nuclear reactions, the atomic diffusion, radiative accelerations and opacity are continuously calculated during evolution taking the abundance changes of 28 species into account. This leads to the first self consistent main sequence stellar evolution models. In A and F stars (M ≥ 1.5Mʘ) an iron peak convection zone is shown to appear at a temperature of 200000 K. The calculated surface abundance anomalies, that follow without any arbitrary parameter, are very similar to those observed in AmFm stars in open clusters except that they are larger by a factor of about 3. The second step, is then to introduce a competing hydrodynamical process. To reduce the calculated anomalies to the observed ones, turbulence has been introduced. It is found that the mixed zone must be about 5 times deeper than the iron convection zone. Detailed comparisons to a few AmFm stars have been carried out. The determination of the abundance anomalies of a large number of atomic species (20 to 30 are probably accessible) makes it possible to constrain stellar hydrodynamics. In clusters, the original abundances and age may be known and the accurate determination of surface abundances may constrain turbulence, mass loss and differential rotation when the required atomic data bases are available and used for the modeling of particle transport in stellar evolution.
Hydrodynamical Models of Mixing beyond a Convection Zone
Authors: Freytag Bernd
Published: March 2016

Numerical radiation hydrodynamics simulations of stellar surface convection have gained a high level of reliability and perform very well in the confrontation with observations. Similar simulations of convection zones in stellar interiors are much more difficult to set up because of the huge range of time-scales to cover. Therefore, simplified models have to be used, where the results have to be carefully extrapolated to apply them to stellar interiors. Simulations of shallow surface convection zones show that the overshooting velocity fields extend far beyond the region with significant convective energy flux.
Some Comments on the Problem of Rotation and Mixing in Stars
Authors: Mestel Leon
Published: March 2016

In the 1950’s, Sweet (1950) and Öpik (1951) rediscussed the resolution of the von Zeipel paradox in a uniformly rotating stellar radiative zone, showing that the time-scale of the Eddington (1929)-Vogt (1925) thermally-driven circulation would be of the order of the Kelvin-Helmholtz time τK H, divided by an appropriate value for the ratio q of centrifugal to gravitational acceleration. In a Cowling-type star, the streamlines emerge near the poles of the convective core, traverse the radiative envelope, and return to the core at the equator. In a rapid rotator, the time τc of circulation is still short compared with the time-scale τn for transmutation in the core of H into He, and it seemed that the mean molecular weight μ would remain nearly uniform, with the star evolving up and slightly to the left of the Main Sequence in the H-R Diagram; while in a moderate rotator, there could still be enough mixing to affect noticeably the expected evolution towards the Giant Domain. Prima facie, significant mixing would occur provided the value q(rc) of q at the convective core surface exceeded τK H/τn ≈ 10−3.
Early Observations and Modern Ephemerides
Authors: Standish E.M.
Published: March 2016

There is a great variety of planetary and lunar observations recorded throughout history. Even today, some of these are useful for the improvement of ephemerides, though one must use a lot of judgment and caution - for surprising reasons. Five different sets of observations are presented, each with a story and a lesson: Galileo’s observations of Neptune, Lalande’s observations of Neptune, Williams’ measurements of the 1780 solar eclipse, Robertson’s 1811 transit timings, and solar eclipses in general.
Oxygen Abundances: New Results from [O I] Lines
Authors: Sneden Christopher , Primas Francesca
Published: March 2016

From new high resolution spectra of metal-poor giants we use equivalent widths of [O I], Sc II, and Fe II lines to derive new values of [O/Sc] and [O/Fe]. These results are generally consistent with previous determinations of oxygen abundances from [O I] lines, but the star-to-star scatter is greatly reduced.
Spectroscopic Influence of Temperature Inhomogeneities
Authors: Cayrel Roger , Steffen Matthias
Published: March 2016

The effect of temperature inhomogeneities on the formation of two oxygen lines and one Fe II line has been investigated based on 2-D radiation hydrodynamics simulations of convection in the atmospheres of metal-poor stars. It was expected that the IR O I triplet lines would be enhanced by the temperature inhomogeneities, more than the forbidden [O I] line or Fe II lines. The actual computations done on two snapshots under the assumption of LTE have not confirmed this expectation, instead suggesting that the main difference between ‘classical’ and ‘dynamical’ atmospheres of metal-poor dwarfs is the significantly cooler mean temperature of the dynamical upper photosphere (τRoss ≾ 0.01).
An Independent View?
Authors: Lambert David L.
Published: March 2016

The current dispute over the oxygen abundance in metal-poor stars has been viewed from a distance by this observer who would claim a measure of independence despite an intermittent interest in ox-gen abundance determinations over 3 decades. This summary attempts to persuade all participants whether they advocate a ‘high’ or ‘low’ oxygen abundance or are simply bemused by the dispute that in resolving the present disagreements we shall learn not only about the oxygen abundance of these oldest Galactic stars but shall achieve a deeper understanding of the atmospheres of the stars.
Atomic Hydrogen at High Redshift
Authors: Briggs F.H.
Published: March 2016

After the production of the ionizing background by the first generation of
stars, neutral gas must be confined to sufficiently high density to be self-shielding and remain neutral. Neutral gas is an identifier of the presence of confining gravitational potentials and a tracer of the kinematics of the potential. Kinematical studies are being extended to neutral atomic gas at high redshift.
Hidden Star Formation: The Ultraviolet Perspective
Authors: Meurer G.R. , Heckman T.M. , Seibert M. , Goldader J. , Calzetti D. , Sanders D. , Steidel C.C.
Published: March 2016

Many recent estimates of the star formation rate density at high redshift rely on rest-frame ultraviolet (UV) data. These are highly sensitive to dust absorption. Applying a correlation between the far-infrared (FIR) to UV flux ratio and UV color found in local starbursts to galaxy samples out to z ∼ 3, one can account for most of the FIR background. However, the correlation is based on a sample that does not include the most extreme starbursts, Ultra Luminous Infrared Galaxies (ULIGs). Our new UV images of ULIGs show that their FIR fluxes are underpredicted by this correlation by factors ranging from 7 to 70. We discuss how ULIGs compare to the various types of high-z galaxies: sub-mm sources, Lyman Break Galaxies, and Extremely Red Objects.
Prospects for Future Far-Infrared/Submillimeter Studies of the High-Redshift Universe
Authors: Blain Andrew W.
Published: March 2016

Observations made using COBE, SCUBA, ISO and MAMBO have provided a reasonable working knowledge of both the intensity of the submm and far-infrared background radiation and the source counts of luminous high-redshift dusty galaxies. However, because there are uncertainties in the background intensity determinations, the samples of detected galaxies are small, and most importantly, their redshift distributions are very incomplete, details of the evolution of dusty galaxies remain unresolved. The next steps forward in the field will be the launches of SIRTF and ASTRO-F, the commissioning of SOFIA and new, more capable ground-based mm/submm-wave cameras - BOLOCAM, SHARC-II and SCUBA-II - the use of ultra-long duration balloon experiments, such as BLAST, the construction of ALMA and the arrival of FIRST, and ultimately the advent of space-borne far-infrared interferometers, such as SPECS. There are also exciting prospects for direct mm/submm-wave CO-line redshift surveys using wide-band spectrographs. Using these new facilities, the number of high-redshift dusty galaxies known will be increased dramatically. Spectroscopy using SIRTF, SOFIA and FIRST will probe the astrophysical processes within these sources in detail, hopefully addressing the open question of the fraction of the counts and background radiation that is generated by the formation of high-mass stars and by active galactic nuclei (AGNs). The spatial and spectral structure of distant dusty galaxies will finally be resolved in detail using ALMA and SPECS.
Optical Analysis of Cluster Mergers
Authors: Girardi Marisa
Published: March 2016

An increasing amount of data has revealed that many clusters are very complex systems. Optical analyses show that several clusters contain subsystems of galaxies suggesting that they are still in the phase of relaxation, possibly after a phase of cluster merging. I briefly review the main results about substructure, and the connection between cluster dynamical status and galaxy properties. Useful comparisons with the results derived from X-ray data are also discussed.
First Results from the FUSE Mission
Authors: Sonneborn George , Moos H. Warren
Published: March 2016

The Far Ultraviolet Spectroscopic Explorer (FUSE) provides high-resolution spectroscopy at far ultraviolet (FUV) wavelengths (905–1187Å) of many astronomical objects in order to address fundamental questions related to the origin of the universe. FUSE, a cooperative project of the United States, Canada, and France, was launched in June 1999 for a three-year primary mission. The instrument sensitivity permits studies of many extragalactic sources at high spectral resolution. The key science drivers for the mission design are absorption-line spectroscopy of the ISM and IGM to investigate D/H, O VI, and molecular hydrogen abundances and distribution in the Galaxy and beyond to z ~ 0.3. Most of the observing time is available to Guest Investigators. This Joint Discussion summarizes some of the scientific results determined primarily from the first several months of FUSE observations.
Composition and Cloud Structure of Jupiter’s Deep Atmosphere
Authors: Atreya Sushil K. , Mahaffy Paul R. , Niemann Hasso B. , Owen Tobias C.
Published: March 2016

Abundances of ammonia, hydrogen sulfide, water, methane and other hydrocarbons, noble gases and their isotopes, etc. were measured for the first time to the 22 bar level. The ratios of the heavy elements to hydrogen were found to be enriched by a factor of 2-3 relative to solar, implying a large influx of cold planetesimals into Jupiter.
Spectroscopy of Planetary Atmospheres and Surfaces with ISO
Authors: Bézard Bruno
Published: March 2016

Spectroscopic observations of Solar-System objects with ISO provided a wealth of new and exciting results. Major findings relevant to planetary atmospheres and surfaces are presented here. Rotational lines of HD were observed for the first time in the four giant planets, yielding reliable determinations of the D/H ratio. The 14N/15N ratio could also be measured in the upper troposphere of Jupiter. Many new hydrocarbons were detected in the giant planets, improving our understanding of the methane photochemistry. In addition, oxygen compounds (H2O and CO2) were detected in their upper atmospheres and in Titan, pointing to exogenic sources of oxygen. Many bands of SO2 ice were observed on Io, providing information on the physical state of this volcanic compound on the surface. Finally, spectra of Mars revealed the presence of a few so-far undetected mineralogical features, tentatively attributed to carbonates.
The Surface of Titan from Adaptive Optics Observations
Authors: Gibbard Seran G. , Macintosh Bruce A. , Max Claire E. , Roe Henry , de Pater Imke , Young E.F. , McKay Christopher P.
Published: March 2016

Saturn’s largest moon Titan is the only satellite in the Solar System with a substantial atmosphere. Photolysis of methane creates a hydrocarbon haze in Titan’s atmosphere that is opaque to visible light. The new adaptive optics system on the 10–meter W. M. Keck Telescope enables us to observe Titan with a resolution of 0.04 arcseconds, or 20 resolution elements across the disk. By observing at near-infrared wavelengths that are methane band windows we can see through Titan’s hydrocarbon haze to the surface beneath. Recent adaptive optics images of Titan both in broadband (J, H, and K) filters and in narrowband filters that selectively probe Titan’s surface and atmosphere allow us to determine surface albedo and properties of the hydrocarbon haze layer. Future observations will include high-resolution spectroscopy coupled with adaptive optics to obtain spectra of individual surface features.
Advances in Understanding of the Martian Climate
Authors: Richardson Mark I.
Published: March 2016

Data collected by the Viking mission to Mars resulted in a picture of the Martian climate which stood largely unmodified for over a decade. When a challenge did come in the mid-1990’s it resulted from ground-based and HST observations which suggested lower global-average temperatures and dust opacities, and more atmospheric water ice than inferred from Viking. These observations prompted suggestions of climate change orders of magnitude larger and faster than anything contemplated for the Earth. A combination of new data from the Mars Global Surveyor Thermal Emission Spectrometer (TES) and reanalysis of Viking-era data have resulted in a new picture of the Martian climate. It is now clear that no significant climate change has occurred and that the “cooler and cloudier” conditions observed in the 1990’s for northern summer applied equally well to the Viking era. TES observations have provided detailed information on the cycle of air temperature and water ice clouds which support and extend the ground-based and HST observations. The disagreement with Viking observations has been found to result from faults in the Viking Orbiter Infrared Thermal Mapper (IRTM) 15 µm channel, the lack of analysis of IRTM data applicable to water ice, and the misinterpretation of Viking Lander opacity measurements. The TES observations provide a rich data base which is now allowing a new picture of the Martian climate to be constructed in which water vapour and water ice clouds may play a significant role in modulating the annual cycles of dust and air temperature.
Hipparcos Data Validation
Authors: van Leeuwen Floor
Published: March 2016

An overview is presented of tests performed to check the statistical properties of the formal errors on the Hipparcos parallaxes. It is shown that there is no evidence for systematic or correlated errors beyond a correlation level of 0.12 and an angular scale of 1.2 degrees.
Statistical Effects from Hipparcos Astrometry
Authors: Arenou Frédéric , Luri Xavier
Published: March 2016

The Hipparcos astrometry is used mainly for the derivation of stellar physical quantities such as luminosity, masses and velocity. However, sample selections on data with observational errors or an intrinsic dispersion may lead to biased estimates, especially when the error distributions are non-Gaussian. We review the classical biases and the ways to avoid them through the use of statistical methods.
Generation and Amplification of Magnetic Fields in the Early Universe
Authors: Stone James M.
Published: March 2016

A very brief review is given of processes that may be responsible for the generation of initial seed magnetic fields in the early Universe, and that can amplify those fields to the levels observed in galaxies in the current epoch.
Magnetic Fields in Irregular Galaxies
Authors: Chyży Krzysztof T.
Published: March 2016

Radio data of large irregular galaxies reveal some extended synchrotron emission with a substantial degree of polarization. In the case of NGC 4449 strong galaxy-scale regular magnetic fields were found, in spite of the lack of ordered rotation required for the conventional dynamo action. The rigidly rotating large irregular NGC 55 shows vertical polarized spurs connected with a network of ionized gas filaments. Small dwarf irregulars show only isolated polarized spots.