bluuebunny/crossref_metadata_2025_split huggingface dataset:
Problems of Jet Physics
Authors: Scheuer P.A.G. 
Published: March 2016

What have I learned about jets since the Albuquerque meeting (IAL) Symposium No.97) last summer?There is where most of the gravitational energy is released, the funnels on the rotation axis of the accretion torus provide the right sort of geometry, and radiation is abundant. But the experts (Sikora & Wilson 1981 and references therein) never can make radiation-driven jets with Lorentz factor γ >> 1. Why? It is because, when an electron moves up through the funnel at a substantial fraction of the speed of light c, it sees as much radiation striking it trom the front as from behind; acceleration then stops. Large γ are attainable only if the source of radiation subtends an angle ≲ 1/γ behind the source.
New Possibilities in Time-Frequency Standards
Authors: Fliegel H.F. 
Published: March 2016

By invitation of Edoardo Proverbio, the President of Commission 31 (Time), this report attempts to summarize “new possibilities in time-frequency standards”. Especially because recent work in this highly specialized field is so diverse and so complex, I appreciate the support of our colleague, Dr. G. Busca, who will survey the extensive progress being made east of the Atlantic. I confine this paper to work that has appeared in the open literature from the United States and Canada.Since much of this work is proprietary, and since it is difficult to assign credit fairly, I will not attempt to give either a comprehensive or selected bibliography of references in this paper. There are three primary symposia where such work is published: (1) the annual IEEE International Frequency Control Symposia; (2) the more recently organized annual European Frequency and Time Forums; and (3) the (United States) Annual Precise Time and Time Interval (PTTI) Applications and Planning Meetings, organized jointly by the Department of Defense and NASA. The best general review of recent work on frequency standards, which appeared just before our last IAU General Assembly and is not yet outdated, is the special issue of the Proceedings of the IEEE, volume 79, no 7 (July 1991), and especially the papers “An Introduction to Frequency Standards” by Lindon L. Lewis, “Atomic Ion Frequency Standards” by Wayne M. Itano, and “Laser- Cooled Neutral Atom Frequency Standards” by Steven L. Rolston and William D. Phillips.
Seismic Solar Model
Authors: Goode P. 
Published: March 2016

The method of frequency inversion reveals that within the quoted observational errors, it is possible to achieve a precision of ∼ 10−3 in the sound speed determination through most of the Sun’s interior. Only for r < 0.05R⊙ is the precision ∼ 10−2. The accuracy of the density and pressure determinations is only slightly worse. Such restrictions impose significant constraints on the microscopic physical data, i.e. opacities, nuclear reaction cross-sections, and diffusion coefficients as well as on the solar age. The helioseismic age is consistent with that from meteorites.Recently released low-l solar oscillation data from the BISON network combined with BBSO data yield the most up-to-date solar seismic model of the Sun’s interior. For the core, the solar seismic model from the new data are consistent with the best, current standard solar models. An astrophysical solution to the solar neutrino problem fades away.
Concluding Remarks
Authors: Landstreet John D. 
Published: March 2016

I do not intend to try to summarize the various excellent papers that have been presented in this Joint Discussion on the accuracy of the HR diagram and related parameters. I will content myself with saying that I found them uniformly well-prepared and clear, and all very interesting. I want to thank the various speakers for doing their work so well!Instead, I propose - since we are in the Netherlands, after all - to assume the role of the good Dutch pastor for a few minutes and draw a few morals from what we have heard and seen. I think the papers presented point to several important recommendations to all of us, and it is these points that I would like to emphasize.The first moral that I find in this Joint Discussion is a very encouraging one for those of us who are interested in stellar astronomy and astrophysics. It is clear from this JD that our field is alive and well at the 22nd General Assembly of the IAU. This JD drew more poster papers than any of the other 19 JD’s held during this General Assembly, with 54 papers listed in the programme, compared to a mean of about 19 per JD. In addition, these poster papers come from five of the six inhabited continents, and if we allow a paper from New Zealand to stand in for Australia, then we have papers from all six. Not a bad representation, I think.
The Lithium Isotope Ratio In Old Stars
Authors: Nissen P.E. 
Published: March 2016

The lithium isotope ratio in stars can be determined from high resolution observations of the profile of the Li I 6707 Å absorption line. Earlier studies of old F and G stars (Andersen et al. 1984, Maurice et al. 1984, Pilachowski et al. 1989) have led to upper limits of 6Li/7Li ranging from 0.05 to 0.10. Recently, Smith, Lambert & Nissen (1993) seem to have detected 6Li in HD 84937 - a metal-poor turnoff star with Teff ⋍ 6200 K and [Fe/H] ⋍ —2.4. An isotope ratio 6Li/7Li = 0.05 ± 0.02 was determined (see Fig. 1) The detection has been confirmed by Hobbs & Thorburn (1994), who derived 6Li/7Li = 0.07 ± 0.03. The main contribution to the quoted (1σ) errors comes from the noise in the spectrum (S/N = 400) and possible errors in the Doppler broadening of the Li line (Nissen 1994). This broadening is due to stellar rotation and macro-turbulent motions in the stellar atmosphere and can be determined from the profiles of unblended metallic absorption lines.As discussed in detail by Steigman et al. (1993) the presence of 6Li in the atmosphere of HD 84937 is consistent with the measured Be abundance (Boesgaard & King 1993) within the context of i) Standard Big Bang nucleosynthesis, ii) Pop. II cosmic ray nucleosynthesis and iii) standard (non-rotating) models for Li depletion. In particular, Steigman et al. derive D6 > 0.2, where D6 is the depletion factor for 6Li. As shown by Chaboyer (1994) standard stellar evolution models with new opacities predict D6≃ 0.4 for turnoff stars and subgiants with Teff > 5900 K. The same models predict D7 ≃ 1.0, i.e. no 7Li depletion for main sequence stars as well as subgiants with Teff ≥ 5800 K.
Dynamics of the M31 Planetary Nebula System
Authors: Hui Xiaohui 
Published: March 2016

AbstractWe made a spatially complete and kinematically unbiased survey of M 31 for planetary nebulae (PNs). The survey covers a projected area of 20 by 40 kpc centered on M 31. Subsequently, we measured the radial velocities of over 800 PNs using the Hydra multi-object spectrograph on the KPNO 4-m telescope. With this large dataset, a detailed dynamical study of the stellar disk, bulge and halo populations in M 31 is currently underway. We are focusing on the dynamical transition between the disk, bulge and halo, and a comparison between the kinematics of the PNs and globular clusters.
UV Bright Stars and Planetary Nebulae in Elliptical Galaxies
Authors: Ferguson Henry C. 
Published: March 2016

AbstractFar UV observations and optical studies of planetary nebula luminosity functions (PNLFs) offer complementary views of the late phases of stellar evolution in elliptical galaxies and spiral galaxy bulges. UV spectroscopy reveals that the hot stellar population is composite, with a mix of temperatures that varies from galaxy to galaxy. This changing mix is most likely due to changes in the relative numbers of stars that channel through the Post-Asymptotic Giant Branch (PAGB), Post-Early-AGB (PEAGB) and Extreme Horizontal Branch (EHB) phases of evolution. EHB stars appear to dominate the integrated λ < 2000 Å flux from galaxies with the strongest far-UV emission, but are too faint to resolve individually in even the nearest galaxies. Far UV images of M31 and M32 reveal a population of hot stars that are much brighter, but do not account for the majority of the far-UV flux. The sources detected are most likely low-mass PAGB stars (0.55 < M/M⊙ < 0.59). In contrast, the PNLF probes the PAGB star mass function at values greater than ∼ 0.6 M⊙. For a given galaxy the relative numbers of stars in these different branches of evolution are determined by the age and chemical evolution of the galaxy and by the physics of mass loss on the red giant branch. We review current constraints on the mass function of hot evolved stars in elliptical galaxies, highlight a few puzzles, and outline where future observations might contribute.
Active Galactic Nuclei in the Extreme Ultraviolet
Authors: Königl Arieh 
Published: March 2016

The extreme ultraviolet (EUV) wavelength interval (commonly defined to constitute the range 68 — 912 Å, or 13.6 — 182 eV) hes between the intensively studied FUV (912 - 3000 Å) and X-ray (1 - 68 Å) spectral regimes, but until recently has not been subjected to systematic investigations. This lack of information has been particularly acute in the case of active galactic nuclei (AGNs), where the very ability to carry out successful EUV observations has been seriously questioned. This has stemmed from the fact that, for a Galactic H I column density NG(H I) = 1 × 1020 cm−2, the interstellar transmission factor is 0.42 at 60 Å, but a factor of 10 smaller at 100 Å. In turn, there are no known regions of the interstellar medium (ISM) that have [NG(H I)/1020 cm−2] < 0.6 (resp., 0.8) for positive (resp., negative) latitudes (e.g., Heiles 1991).There are, however, several reasons why EUV observations of AGNs could provide important information about the nature of these objects. In the case of radio-quiet QSOs, the peak power is known to be emitted in the EUV range (several hundred A). Furthermore, in these sources there is evidence for a distinct soft X-ray component (representing a “soft excess” over an extrapolation of the ISM-absorbed hard X-ray component), whose properties are at present controversial. In particular, the first results of ROSAT PSPC observations of a complete sample of optically selected QSOs (Laor et al. 1994) have pointed to a spectral index αx = 1.50 ± 0.40 in the 0.2 − 2 keV range that flattens by Δαx ≈ 0.5 above ∽ 2 keV. This study has concluded that steep-αx sources are characterized by a week hard component. A similar distribution of αx has been inferred for soft X-ray-bright QSOs and Seyfert 1 galaxies observed in the ROSAT all-sky survey. On the other hand, previous observations (enumerated in Laor et al. 1994) by the Einstein and EXOSAT satellites yielded different soft X-ray spectral slopes and led to an alternative explanation of the steep-αχ objects. These questions could in principle be clarified by EUV measurements, which would then also help determine the soft X-ray emission mechanism.
Molecular Data for Stellar Opacities
Authors: Jørgensen Uffe Gråe 
Published: March 2016

In total, 40 neutral diatomic molecules, 2 molecular ions, and 7 polyatomic molecules are known from observed photospheric stellar spectra. Line data for opacity computations (i.e., lists of line frequencies, intensities, and excitation energies) exist for 17 of these molecules, although the data are complete only for a handful of them. A detailed description of stellar photospheric molecules can be found in Tsuji (1986), and the existing opacity data have been reviewed by Jorgensen (1995).Listed line frequencies in the data bases are either the measured values, or based on computed molecular constants obtained from fits to measured values. Attempts to compute ab initio line frequencies have so far resulted in lower accuracy than what is obtained by use of molecular constants. Published line strengths include measured values as well as ab initio values. For strong bands the ab initio intensities are as accurate as the laboratory values, whereas measured values for weak bands are generally more accurate than the ab initio values. The primary advantage of ab initio computations is therefore that the complete set of all transitions can be obtained. Exploratory studies have shown that completeness of the line data is crucial for the obtained stellar photospheric structure.As an alternative to the ab initio computations of the line intensities, fits to experimental data have been attempted. The most promising method seems to be to fit the dipole function by use of a Padé approximant. Combined with a potential fitted to experimental energy levels, such a dipole function can in principle be used to predict the complete list of band intensities and line intensities for all bands with energies up to the molecular dissociation energy. The part of the dipole function which corresponds to the largest stretching (or bending) of the molecule is the most uncertain in such fits as well as in ab initio computations. This part is responsible for most of the many weak transitions, and large uncertainties are therefore to be excepted in the computed intensities of the weak spectral bands. As these are of major importance for the stellar photospheric structure (due to their huge number and their pseudo continuous appearance in the spectrum), a particularly large effort is desirable in comparing computed intensities with laboratory data for a representative sample of weak bands. Unfortunately, only few measurements of weak bands exist.
Cepheids: Pulsation, Evolution, Opacity
Authors: Simon Norman R. 
Published: March 2016

With the advent of new radiative opacities, the gross discrepancies in period ratio between observed stars and theoretical models have been eliminated in both the “beat” and “bump” Cepheid regimes (Moskalik, et al. 1992; Kanbur & Simon 1994). This paves the way for detailed modeling of the beat and bump stars.We begin here by summarizing one such effort, the full description of which is given by Simon & Kanbur (1994). In this study, the slope of the evolutionary luminosity-mass relation is taken to be dlogL/dlogM = 4.0 (Stothers and Chin 1991) and the intercept determined by forcing models which follow this slope to also conform with the bump Cepheid resonance condition P2/P0 = 0.50 at P0 = 10d (Simon & Schmidt 1976; Buchler =et al. 1990). We then model separately each of the dozen known galactic beat Cepheids, subject to the just mentioned evolutionary constraint imposed by the 10-day resonance. We find masses and luminosities, ranging from M = 4Mʘ, log L = 3.0 at P0 = 3d to M = 6Mʘ, log L = 3.5 at P0 = 1Od. Furthermore, a range of metallicity, 0.01<Z<0.02, is required among the beat stars, whose period ratios, P1/P0,are predicted to decrease as the metallicity rises. This trend has been observed (Andrievsky et al. 1993). However, “standard” evolutionary tracks for Z = 0.02 [e.g., the Geneva calculations (Schaller, et al. 1992) or the Padua calculations (Bressan, et al. 1993)] cannot produce Cepheids with parameters as given above - first because the models are somewhat underluminous at given mass, and second because the calculated blue loops do not reach the instability strip for M = 7Mʘ. On the other hand, the models described by El Eid (1994) do penetrate the strip at masses as low as 4 or 5 Mʘ, but display a luminosity-to-mass ratio considerably too small to satisfy the resonance condition for bump Cepheids. Possible resolutions to these problems include a deep Fe convection zone whose effect would be to lower the period ratios (Cox 1993), and/or an opacity increase near 1 × 106K, which could extend the blue loops (Stothers & Chin 1994). Each of these schemes require more research.
Water in Comets: Observations and Models
Authors: Crovisier Jacques 
Published: March 2016

AbstractCometary water, the main constituent of nucleus ices, is difficult to observe directly. Breakthroughs were made recently using new ground based infrared instruments and space techniques (ISO and SWAS) on comets C/1996 B2 (Hyakutake), C/1995 01 (Hale-Bopp), 103P/Hartley 2, and C/1999 H1 (Lee): observations of water hot bands, of water rotational lines, and measurements of the [D]/[H] ratio and ortho-to-para ratio of water. We discuss the laboratory data necessary to model the water spectrum and to interpret these observations.
Chemistry in the Envelopes around Massive Young Stars
Authors: van Dishoeck Ewine F. , van der Tak Floris F.S. 
Published: March 2016

AbstractRecent chemical studies of high-mass star-forming regions at submillimeter and infrared wavelengths reveal large variations in the abundances depending on evolutionary state. Such variations can be explained by freezing out of molecules onto grains in the cold collapse phase, followed by evaporation and high-temperature chemical reactions when the young star heats the envelope. Thus, the chemical composition can be a powerful diagnostic tool. A detailed study of a set of infrared-bright massive young stars reveals systematic increases in the gas/solid ratios and abundances of evaporated molecules with temperature. This ‘global heating’ plausibly results from the gradual dispersion of the envelopes. We argue that these objects form the earliest phase of massive star formation, before the ‘hot core’ and ultracompact H II region phase.
The New Generation of Ionization and Recombination Fronts
Authors: Dyson J.E. , Hartquist T.W. , Williams R.J.R. , Redman M.P. 
Published: March 2016

AbstractWe summarize previous work on hydrodynamic ionization fronts and new work where magnetic fields are incorporated into their structures. We describe recent work on recombination front structures and outline refinements which need to be made to both them and ionization fronts.
Spectrophotometry of Trans-Neptunian Objects
Authors: Romon Jennifer , Doressoundiram Alain , Barucci Maria Antonietta 
Published: March 2016

AbstractThe Trans-Neptunian Objects (TNOs) are very difficult to observe due to their faintness. Spectra can be obtained for only few objects. For this reason, broad band photometry is the only available technique to study their physical properties from a statistical point of view. To improve the knowledge about the physical nature of the Kuiper Belt, we have obtained several observing runs at different four-meter class telescopes (NTT - Chile; CFHT - Hawaii).The data presented here were obtained at NTT (ESO - Chile) during three observing runs from 1997 to 2000. We used the SUSI and SUSI 2 CCD cameras with the Bessel B, V, R, and I filters.
Particle Acceleration and Diffusion in Fossil Radio Plasma
Authors: Enβlin Torsten A. 
Published: March 2016

AbstractThe strong activity of radio galaxies should have led to a nearly ubiquitous presence of fossil radio plasma in the denser regions of the inter-galactic medium as clusters, groups and filaments of galaxies. This fossil radio plasma can contain large quantities of relativistic particles (electrons and possibly protons) by magnetic confinement. These particles might be released and/or re-energized under environmental influences as turbulence and shock waves. Possible connections of such processes to the formation of the observed sources of diffuse radio emission in clusters of galaxies (the cluster radio halos and the cluster radio relics) are discussed.
Jupiter’s Aurora: Solar Wind and Rotational Influences
Authors: Waite J. Hunter , Clarke John T. , Walker R.J. , Connerney John E.P. , McComas D. , Riley P. , Lewis William S. 
Published: March 2016

AbstractJovian auroral emissions are observed at infrared, visible, ultraviolet, and x-ray wavelengths. As at Earth, pitch-angle scattering of energetic particles into the atmospheric loss cone and the acceleration of current-carrying electrons in field-aligned currents both play a role in exciting the auroral emissions. The x-ray aurora is believed to result principally from heavy ion precipitation, while the ultraviolet aurora is produced predominantly by precipitating energetic electrons. The magnetospheric processes responsible for the aurora are driven primarily by planetary rotation. Acceleration of Iogenic plasma by rotationally-induced electric fields results in both the formation of the energetic ions that are scattered and the formation of strong, field-aligned currents that communicate the torques from the ionosphere. In addition to rotation-driven processes, solar-wind-modulated processes in the outer magnetosphere may lead to highly, time-dependent acceleration and thus also contribute to jovian auroral activity. Observational evidence for both sources will be presented. See Waite et al. (2001, Nat., 410, 787).
MGS TES Results: Atmospheric Structure, Aerosols, and Dynamics
Authors: Conrath Barney J. , Pearl John C. , Smith Michael D. , Christensen Philip R. 
Published: March 2016

AbstractThermal emission spectra from the MGS TES instrument permit retrieval of atmospheric properties that are of interest in the study of Martian meteorology, including temperature, dust, and water ice. Examples of analyses of zonal mean meridional thermal sructure for equinox and solstice conditions are presented.
Variable and Constant Features on Titan from HST
Authors: Lorenz Ralph D. , Lemmon Mark T. , Smith Peter H. 
Published: March 2016

AbstractObservations with HST WFPC-2 from 1994 to 1997 show a number of features which can be attributed to the surface (notably the large as-yet-unnamed bright region, several smaller bright areas and a number of dark regions). The delineation of these features in several datasets is presented. Other features are variable and are due to atmospheric phenomena: these include the total brightness of Titan (drop by 5% 1994-1997 at 350 nm; increase by 7% at 889 nm), the hemispheric north-south asymmetry, and the hint of a south polar hood.Additionally, small variable features, apparently due to clouds, are documented. As well as variability from one year to the next, these show anomalous center-to-limb brightness behaviour, and a 673/940 nm color much bluer than the large surface feature, consistent with clouds in the troposphere.
Hipparcos Luminosities and Asteroseismology
Authors: Bedding Timothy R. 
Published: March 2016

AbstractAsteroseismology involves using the resonant frequencies of a star to infer details about its internal structure and evolutionary state. Large efforts have been made and continue to be made to measure oscillation frequencies with both ground- and space-based telescopes, with typical precisions of one part in 103–104. However, oscillation frequencies are most useful when accompanied by accurate measurements of the more traditional stellar parameters such as luminosity and effective temperature. The Hipparcos catalogue provides luminosities with precisions of a few percent or better for many oscillating stars. I briefly discuss the importance of Hipparcos measurements for interpreting asteroseismic data on three types of oscillating stars: δ Scuti variables, rapidly oscillating Ap stars and solar-like stars.
MeV Neutrino Sources: The Sun and the Supernovae
Authors: Turck-Chièze Sylvaine 
Published: March 2016

AbstractThe two sources of MeV neutrinos, the Sun and the Supernovae are very interesting to study nowadays. Seismology aboard SoHO has properly constrained the solar emitted neutrino fluxes and consequently contributes to an unambiguous evidence of the solar neutrino oscillations. The main role of seismology stays nevertheless the description of the magneto-hydrodynamic processes and this progress provokes a renewal of the stellar discipline. Important results are noticed these three years, for the Sun. Future will be richer implying a large number of stars with an evident impact on supernovae and solar neutrino properties.