bluuebunny/crossref_metadata_2025_split huggingface dataset:
Solar Irradiance Variability and Luminosity Changes
Authors: Foukal Peter 
Published: March 2016

This review presents a discussion of radiometrie measurements of solar total irradiance variation, ΔS, on time scales of days, to the 15-year extent of present measurements. It does not cover variations in spectral irradiance, particularly the relatively large solar UV and EUV flux variations of great importance in studies of trends in stratospheric ozone, and of upper atmospheric heating variations.Analysis of the pyrheliometry of ΔS carried out from the Solar Maximum Mission, Nimbus-7 and ERB S satellites has shown that a) the sun brightens by 0.05 - 0.1% around activity maximum and; b) it dims by up to approximately 0.2% when large sunspot groups cross the disc. Empirical models of ΔS based on areas and photometric contrasts of sunspots, and on proxies of facular area, are remarkably successful in matching the observed record of ΔS, on time scales between days and the full 15- year record available since 1978. These models indicate that the opposite contributions of sunspots and faculae in active regions roughly balance over time scales of years, and the net solar brightening observed around peak activity is caused by increased emission from the photospheric magnetic network.
Magnetohydrodynamical Phenomena in the Solar Corona as revealed by Yohkoh
Authors: Uchida Yutaka 
Published: March 2016

In the talk, some of the new findings about the magnetodynamical phenomen a in the inner corona obtained by the satellite Yohkoh were reviewed. Here we give a very brief summary of it. (The long list of references has been omitted due to the limitation of space here.)One of the most remarkable findings by Yohkoh is that the corona is quite a dynamic entity. Such a dynamicity was recognized in its full form by the wide-dynamic range, high cadence imaging from Yohkoh-Soft X-ray Telescope (SXT) with the use of the video-movie presentation. It turned out that the background corona formed quite a dynamic system with the active regions (Uchida 1992, Tsuneta and Lemen 1992), and many examples of interesting dynamic interplay between them were found. It may well be said that the corona is not an “atmosphere” of the usual sense, but is an entity highly electromagnetic in character.
Atomic Data and Coronal Diagnostics
Authors: Mason Helen E. 
Published: March 2016

AbstractSpectroscopic diagnostics play an important role in determining the electron density and temperature structure, element abundances and dynamic nature of different features in the solar atmosphere. To ensure that these techniques are accurate it is necessary to interface solar analysis programs with the best available atomic data calculations. Recent progress in this work is presented and applications to planned space observations are discussed. In particular, the CDS and Sumer instruments, on the Solar Heliospheric Observatory, will make extensive use of spectroscopic diagnostics to study the heating processes for the solar corona and the acceleration mechanisms for the solar wind.
Link to the Physical Modelling with an Emphasis on Chemical Peculiarities
Authors: Mathys G. 
Published: March 2016

Frequently, the atmospheres of stars of spectral types B to F, on which the attention is particularly focussed in this Joint Discussion, are characterized by non-standard physical conditions: non-solar abundance patterns, surface in homogeneities, strong magnetic fields. The most extreme anomalies are found in the chemically peculiar (CP) stars but more moderate departures from standard atmospheres are observed in many stars in the considered temperature range. This can affect the derivation of fundamental stellar parameters in a number of ways, some of which will be presented in this contribution. The emphasis is set on those physical processes which are specific to CP stars. Due to lack of space, this review is necessarily incomplete: a selection of recent results of interest will be pointed out and some directions of investigation will be suggested.CP stars have an anomalous continuous energy distribution. Fitting this distribution by standard model atmospheres can only be achieved using models corresponding to different effective temperatures to represent different spectral ranges (e.g., Leone & Catalano 1991). This is obviously not physically meaningful: the effective temperature characterizes the total flux of a star, thus is unique. “Realistic” models must incorporate the anomalous elemental abundances. Such models are presently being developed by Kurucz (ATLAS12) and by the Vienna group (Weiss 1994, private communication). The latter is adapted from Muthsam’s (1979) code. One important result obtained by Muthsam is that the relation T(τ) appears steeper in magnetic Ap stars than in normal stars. This view recently received support from observations of rapid oscillations (Matthews et al. 1990). But even such realistic models may not represent satisfactorily CP star atmospheres, as some potentially important opacity sources may still be unknown. For instance, the broad continuum depressions observed in many magnetic CP stars have not been fully explained yet.
Tests of Evolution Models Using Eclipsing Binaries
Authors: Andersen J. 
Published: March 2016

Stellar models are the means by which we describe and understand the distribution of stars in the HR diagram. A stellar model is, in principle, completely specified by the three fundamental parameters mass, chemical composition, and age. Comparing the properties of models and real stars with the same parameters will tell us if our recipe for constructing stellar models is realistic. Unfortunately, the only star for which all three are known independently of stellar models is the Sun. For stars of other masses and ages we must devise observational tests in which at least one fundamental parameter is unknown. Two such popular test objects are double-lined eclipsing binaries and star clusters.In suitable eclipsing binaries we can determine both masses and chemical composition; the absolute age is unknown, but the same for both stars. Since evolution depends most sensitively on the mass, eclipsing binaries provide a very direct test of the models, but only for two points on a single isochrone. In star clusters, neither ages nor individual masses are known, but the detailed shape and population of a well-observed cluster sequence in the HR diagram provide a number of additional probes into the models.
Tests of Evolution Models Using Star Clusters
Authors: Mermilliod J.-C. 
Published: March 2016

The fitting of isochrones over star-cluster colour-magnitude diagrams is a favourite test of stellar evolution models because it seems to be the easiest and most evident one: the star distribution in the colour-magnitude diagram represents a constant age locus, provided the clusters are old enough and the internal age dispersion is negligible. The ratios of evolved to main-sequence stars in open clusters, or of population along the red giant branch in globular clusters, could provide valuable tests, but it is difficult to estimate these ratios with small enough uncertainties. Furthermore the detailed surface chemical composition (element abundances and isotope ratios) is not known for many cluster stars and model predictions cannot be tested in various environments.Tests have been mostly devoted to the main sequence and the long debate between canonical precepts and core overshooting is still active. St others (1991) has argued that the use of the new opacities (Rogers & Iglesias 1992) reduces the necessity of adding overshooting to the models. Castellani et al (1992) computed extensive grids of models with canonical precepts. However, recent papers (Carraro et al. 1993; Meynet et al. 1993; Demarque et al. 1994) show that a moderate core overshooting (about 0.2 Hp) is still necessary. Basically, cluster observations call for more massive cores, whatever process is at work. An excellent review of the various ingredients included in models (equation of state, nuclear rates, opacities, convection) and the effects of their changes has been presented by Vandenberg (1991).
Lithium In Population II Stars
Authors: Thorburn J.A. 
Published: March 2016

AbstractSince the work of Spite & Spite (1982), halo main sequence turn-off stars have usually been considered a site where Li synthesized in the Big-Bang is wholly preserved and uncontaminated by other Li sources. However, the interpretation of halo dwarf Li abundances has become more complex as new samples of halo star Li I observations have become available. Linking the Li plateau abundance identically with the cosmological Li fraction is no longer warranted. At present, observational evidence favors a scenario where most—but not all—of the plateau abundance is cosmological in origin and a non-negligible amount is contributed subsequently by Galactic nucleosynthesis; severe, uniform Li depletion is constrained but not yet completely ruled out.
Lithium Abundance of Halo Stars With New Teff
Authors: Molaro P. 
Published: March 2016

The lithium observed in the atmospheres of unevolved halo stars is believed to be an essentially unproccessed element which reflects the primordial yields. The primordial nature of Li is inferred from the presence of a constant Li abundance for all the halo dwarfs where convection is not effective (Teff ≥ 5600 K). Such an uniformity is taken as evidence for the absence of any stellar depletion during the formation and the long life of the halo stars and also as evidence for the absence of any production mechanism acting either before or at the same time of the formation of the halo population. The existence of a real plateau has been recently questioned by Thorburn (1994), Norris et al (1994) and Deliyannis et al (1993). These results open the possibility of substantial depletion by rotational mixing where a certain degree of dispersion is foreseen for different initial angular momenta of the stars and/or to a significant Galactic Li enrichment within the first few Gyrs.Lithium abundances in a sample (39) of halo dwarfs have been redetermined by using the new Teff derived by Fuhrmann et al (1994) from modelling of the Balmer lines. These Teff are reddening independent, homogeneous and of higher quality than those based on broad band photometry. The sample represent a significant fraction of the presently available Li determinations, and form an unique sample with a good and homogeneous Teff Fuhrmann et al are also able to provide errors in the Teff for individual stars and in most cases they are as good as ± 50 K, which is a factor 2 smaller than the grossly estimated errors for photometric-based Teff. The Li equivalent widths have been taken from the literature and the theoretically derived random errors from Deliyannis et al (1993) or computed following their prescriptions. Abundances have been derived by generating new atmospheric models by using the ATLAS-9 code by Kurucz (1993) with enhanced α-elements and without the overshooting option. If we use the Kurucz 1993 grid which includes overshooting the Li abundance is increased by ≈ 0.08 dex.
The Rotational History of Solar-Type Stars: Does It Affect Lithium Abundances?
Authors: Bouvier J. 
Published: March 2016

Recent studies of lithium abundances in solar-type stars suggest that second order effects, such as rotation and/or mass-loss, might affect the rate at which lithium is depleted at the surface of cool stars. This conclusion mainly stems from the dispersion of lithium abundances observed in late-type stars with otherwise similar mass and age. The dispersion of lithium abundances is relatively small in pre-main sequence (PMS) stars [1], reaches a maximum of about 2 dex in ZAMS stars (Soderblom, this volume), and decreases later during main sequence evolution, although significant dispersion may still be present by the age of the Sun [2], and perhaps even during the giant stage (Balachandran, this volume).Such a pattern of lithium dispersion is difficult to understand in the framework of conventional models in which lithium burning occurs at the base of the convective zone of non-rotating, non-mass losing stars. That two stars of the same age and mass can exhibit different lithium content strongly suggests that other, so far neglected, mechanisms play a role in lithium depletion.In this context, it may be relevant to stress that the pattern of lithium dispersion varies with age in much a similar way than does the dispersion of stellar rotation, namely: the dispersion in both rotation rates and lithium abundances is relatively small in the PMS stage, then strongly increases from the PMS to the ZAMS, and subsequently decreases on the main sequence. The parallel evolution of the range of lithium abundances and of rotation rates observed in solar-type stars is strongly suggestive of a physical link between rotation and lithium depletion.
From AGB to PN in External Galaxies
Authors: Chiosi Cesare 
Published: March 2016

AbstractWe present new results from spectro-photometric models for elliptical galaxies in which detailed descriptions of the star formation history and chemical enrichment are taken into account. In particular, we focus on the important role played stars of different chemical composition evolving along the red giant branch (RGB), horizontal branch (HB), asymptotic giant branch (AGB), and finally planetary nebula (PN) stages in relation to the sources of UV flux in elliptical galaxies. A few important applications of these new galaxy models are briefly outlined.
The Planetary Nebula Luminosity Function (PNLF) of Our Galaxy Compared to those of other Galaxies
Authors: Méndez R. H. 
Published: March 2016

Nowadays it is possible to detect planetary nebulae, and measure their [O III] λ5007 fluxes, in many galaxies. Given this information it is relatively straightforward to build the [O III] λ5007 PNLF for any galaxy, because we can always assume all its PNs to be essentially at the same distance from us.Of course the situation is much more difficult for PNs in the solar neighborhood. We have two problems here: (1) the individual PN distances are poorly known, except for ∼ 25 objects; (2) in the literature there is no agreement on which are the 25 PNs with reliable distances. Clearly it was a wise decision to calibrate the PNLF method of extragalactic distance determinations using the distance to M 31 (Ciardullo et al. 1989, Jacoby et al. 1992). In these short notes I review if it may be possible to build a PNLF for our Galaxy (the answer is: yes, probably more than one) and discuss if it would be worth the trouble (the answer is: yes, it would) and what can be done to achieve that elusive goal.
Planetary Nebulae and the Cosmic Distance Scale
Authors: Jacoby George H. 
Published: March 2016

AbstractPlanetary nebula luminosity functions provide very accurate distances to galaxies as judged by comparisons with other techniques, including Cepheids; systematic effects cannot be significant. The method is empirical and is a consequence of the shallow initial-final mass relationship predicted by stellar evolution and directly measured. From distances to ∼ 30 galaxies, the PNLF yields H0 = 79, in excellent agreement with recent Cepheid discoveries in Virgo.
Chromospheric and Coronal Spectra
Authors: Jordan Carole 
Published: March 2016

The interpretation of chromospheric and coronal spectra requires accurate ionization and recombination rates, collision strengths and transition probabilities. Recent projects to improve calculations of opacities in stellar interiors have led to a large amount of new atomic data. Some current and potential applications of atomic data to chromospheric and coronal spectra are mentioned below.Strong chromospheric lines are optically thick, and the solution of the radiative transfer equations can depend on atomic data for other species contributing to the background opacity. Many lines in the spectra of stars with hot coronae are excited by electron collisions, but in the cooler non-coronal giants radiative processes involving the H Lyman α and β lines become more important (see Jordan 1988a). Photo-ionization rates from ground configuration excited terms and oscillator strengths to high levels are still needed.Fe II is an important ion producing emission lines in stellar chromospheres. Several excitation mechanisms contribute to the observed spectra (Jordan 1988b). Permitted lines to the ground term and low lying metastable terms have high optical depths and transfer photons to spin forbidden lines sharing a common upper level (e.g. mults. uv 1 and uv 3 transfer photons to mults. uv 32 and 61). Line intensity ratios yield the optical depth in the optically thick lines. The strong H Ly α line in cool giants and supergiants excites high levels in Fe II, resulting in strong decays in multiplets such as uv 391 and 399. A large number of f-values are required to interpret the lines formed by these radiative processes. Nahar & Pradhan (1994) have published some results from the Opacity Project, calculated by using the close coupling method and observed energy levels (which introduce some allowance for relativistic effects). In most cases these f-values agree with experimental results and the calculations by Kurucz (1988) to within 10%. The latter are still needed for the interpretation of stellar uv spectra because of the treatment of spin-forbidden lines.
Interpreting the Spectra of Chemically Peculiar Stars
Authors: Lanz Thierry 
Published: March 2016

Similarly to many astrophysical studies, a spectroscopic analysis of chemically peculiar (CP) stars requires extensive, accurate sets of atomic data. Our purpose is to show here that the CP stars can also serve as a laboratory for checking the accuracy of the atomic data. Despite additional challenges toward modeling the atmospheres of the CP stars, they have a definite advantage as laboratories for atomic physics: their strong chemical anomalies make some spectral features especially prominent in their spectra, which can be better checked against theoretical predictions. This statement may be applied to both to stars exhibiting large over- or under- abundances. We discuss several examples how CP stars can be used along this idea.We will focus here on the chemically peculiar stars of the upper main-sequence, which encompass the classical Am and Ap stars. Chemical anomalies are widespread on the main sequence for spectral types B. to F. These stars mostly show weak helium lines, and abnormally strong lines from iron-peak and heavier (e.g. the lanthanides) elements. A classical characteristics of Am stars in their weak Ca lines, while Ap stars may exhibit either strong Si or Hg lines. CP stars can be grouped according to the most obvious line strength anomalies, which are related to the effective temperature and to the presence or the absence of a magnetic field. In magnetic CP stars, the chemical elements are not uniformly distributed over the stellar surface, but they seem to be concentrated (or depleted) in rings or polar caps following the magnetic geometry. Magnetic CP stars have also broad absorption features in their ultraviolet and visible spectra that can used photometric peculiarity criteria. Using the new extensive photoionization data from the Opacity Project (OP), the UV absorption features can be identified and the UV spectrum can be well reproduced.
Astronomy in Saudi Arabia
Authors: Al-Malki M.B. 
Published: March 2016

In the last twenty-five years, more than 100 astronomers have graduated from two astronomical departments in the Kingdom: one at King Saud University (KSU) in Riyadh and the other in King Abdulaziz university (KAU) in Jeddah. About 15% to 20% of those graduated are now working in fields related to astronomy, among them five with Ph.D. degrees and three with M.Sc. degrees. Most of the other astronomy graduates are teaching mathematics, physics or general science in schools.The total number of academic staff in the two universities is ten Ph.Ds (five Saudis and five from abroad) and five research assistants, with no post-graduate students. Each department has a Celestron 14-inch (approx. 0.35m) with photo-counters, a 15-cm coudé refractor, solar laboratory, small planetarium and astronomical measuring instruments. In addition, KAU has a CCD camera, while KSU has a double telescope with 45cm Ritchey-Chrétien optics and a 24cm Schmidt telescope. All these observational facilities are located near the universities, where light pollution is very high and limits the observations. There is a plan to move the KSU double telescope away from Riyadh. Because of a lack of students and Saudi teaching staff, and cuts in the budget for universities, it is planned to turn the two astronomy departments into groups in the respective physics departments.
Laboratory Analogues of Cosmic Dust
Authors: Mutschke H. , Clément D. , Dorschner J. , Fabian D. , Jäger C. , Henning Th. 
Published: March 2016

Abstract
Understanding the properties and the evolution of cosmic dust requires laboratory experiments for measuring basic physical and chemical data and simulating the interaction of dust grains with their cosmic environments. We give a short review of such laboratory experiments using analogue materials of cosmic dust.
H2 in Space
Authors: Flower D.R. 
Published: March 2016

AbstractMolecular hydrogen may be excited by collisions with other particles (as in shock waves), by ultraviolet pumping (as in photon-dominated regions), through its formation on the surfaces of dust grains (in interstellar clouds), or by absorption of the cosmic background radiation (in the primordial gas). We consider the adequacy of our knowledge of H2, from the viewpoint of being able to reliably predict the spectrum arising from these excitation mechanisms.
Atomic Data Needs for X-ray Astronomy
Authors: Brickhouse Nancy S. 
Published: March 2016

AbstractWith the launches of the Chandra X-ray Observatory and XMM-Newton, high resolution X-ray spectra of cosmic sources are broadening our understanding of the physical conditions, such as temperature, density, ionization state, and elemental abundances. X-ray emitting astrophysical plasmas can be generally classified by their dominant ionization mechanism, either collisional ionization or X-ray photoionization. The atomic data needs are significantly different for these two cases; however, for both cases it is important that we identify robust and accurate diagnostics and that we verify completeness of the broadband models. We discuss the status of the atomic data currently used in atomic databases for X-ray astronomy, in view of theoretical and experimental atomic physics considerations.
Report on Astronomical Constants
Authors: Fukushima Toshio 
Published: March 2016

AbstractGiven was additional information to a previous report on the recent progress in the determinations of astronomical constants (Fukushima 2000), including the revision of Lg, Δp and the offsets of CEP. As a summary, presented was the (revised) IAU 2000 File of Current Best Estimates of astronomical constants, which is to replace the former 1994 version (Standish 1995).
Dynamical Evolution of Trans-Neptunian Objects in High-Eccentricity Orbits
Authors: Emel’yanenko V.V. 
Published: March 2016

AbstractThe evolution of near-parabolic orbits with perihelia in the trans-neptunian region has been studied, considering the action of planetary, Galactic and stellar perturbations for the age of the Solar System. This investigation has led to the conclusion that the observed trans-neptunian objects in high-eccentricity orbits might originate from the inner core of the Oort cloud.