bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling the Local Bubble
Authors: Cox Donald P.
Published: July 2016

Modeling the Local Bubble is one of those activities fraught with danger. It is very easy to be too naive, to fail to consider the dependence of the model on assumptions about the nearby ambient state, or the likelihood of such a structure. It is similarly easy to become so caught up in the details of the vicinity that it is unclear where to begin a necessarily idealized modeling effort. And finally, it is important to remember that the data we have may in some cases be lying to us, and that we have not yet learned to read their facial expressions quite carefully enough.

That said, I’ve tried in this paper to be helpful to those who may wish to take the risks. I surveyed the very most basic stories that the data seem to tell, and pointed out the standard coincidences that may be telling us a lot about what is happening, but may turn out once again to have been just coincidences. I’ve described 5 distinct conceptions that in one flavor or another pretty well survey the collection of mental images that have so far been carried by those who’ve attempted models. One may be right, or something entirely different may be more appropriate. It’s at least vital to realize that a conception comes first, followed by a simplified model of details. I’ve also included a long list of questions directed at observers. Some have partial answers, some one wouldn’t know today quite how to approach. But it is a list that students of the soft x-ray background, interstellar absorption lines, possible instrumentation, and the heliosphere may wish to review from time to time, just to see whether they can figure out how to be more helpful. There is another list for modelers, things the models must address, however-so-flimsily if necessary, because there are strong observational constraints (and stronger ones coming) on what can and cannot be present in the local ISM. To that I’ve added a few remarks concerning x-ray emission coming from beyond the Local Bubble, and another few on how x-ray emission from within the solar system might be contaminating what we see. That last bit is new, exciting, and possibly wrong, but it is an example of the ongoing wariness I believe one has to take toward the facts in the case. By the way, Dieter, it really was a great meeting.
The Evaporation of Nearby Clouds and the Soft X-ray Background
Authors: Slavin Jonathan D.
Published: July 2016

In recent years the nature of the low density clouds within the Local Bubble has been increasingly well characterized. These clouds, including the one which surrounds the solar system, are embedded in the hot gas and therefore should be evaporating via thermal conduction. If several evaporation fronts exist within the Local Bubble, the emissivity and spectrum of the hot gas is significantly different from a single temperature, equilibrium ionization plasma. We explore models in which the the temperature, density and ionization in the hot gas are influenced by cloud evaporation and compare the results with the observed Soft X-ray Background.
The Local Magnetic Field in the Milky Way
Authors: Uyaniker B. , Fürst E. , Reich W. , Reich P. , Wielebinski R.
Published: July 2016

A radio continuum survey at medium Galactic latitudes with the 100−m telescope of the Max-Planck-Institut für Radioastronomie is being carried out at a center frequency of 1.4 GHz in total power and linear polarization. We present polarization and depolarization structures detected at medium latititudes with no corresponding observable structure in total power emission. Existence of such kind of polarization structures imply that the emission, due to the Faraday modulation of the Galactic synchrotron foreground, occurs in thin sheetlike regions. This phenomenon is a sign for fluctuating local magnetic field.
Dust in Hot Environments: Giant Dusty Galactic Halos
Authors: Ferrara Andrea
Published: July 2016

I review some of the evidences for dust in the Local Bubble and in galactic halos and show that a general mechanism based on radiation pressure is capable of evacuating dust grains from regions dominated by massive star energy input and thus originate huge dusty halos. A Monte Carlo/particle model has been developed to study the dust dynamics above HII chimneys and the results, among other findings, show that dust can travel several kpc away from the plane of the parent galaxy. The cosmological implications of extragalactic dust are briefly outlined.
26Al in the Local Interstellar Medium
Authors: Knödlseder J. , Bennett K. , Bloemen H. , Diehl R. , Hermsen W. , Oberlack U. , Ryan J. , Schönfelder V. , von Ballmoos P.
Published: July 2016

We estimate the 1.8 MeV luminosity of the Sco-Cen association due to radioactive decay of 26A1 to (4 – 15) 10−5ph cm−2 s−1. We propose a low surface brightness, limb brightened bubble for the 1.8 MeV intensity distribution. The detectibility of this distribution with existing γ-ray telescopes is discussed.
Gravitational Fragmentation in Expanding Shells
Authors: Theis Ch. , Ehlerová S. , Palouš J. , Hensler G.
Published: July 2016

We investigate the gravitational fragmentation in expanding shells by applying an instability ’thermometer’ similar to the Toomre parameter for instabilities in self-gravitating disks. For Sedov–like evolving systems the onset of instability is mainly depending on the density of the ambient medium and the sound speed of the shell matter, whereas the energy injection rate is less important. Shells evolve towards gravitational instability, if the density gradient of the ambient medium is shallower than an isothermal profile, otherwise they become more stable. For density gradients flatter than ∝ r−1, the fragmentation becomes non-linear on the same time scale as the gravitational instability needs to start. In a homogeneous ambient medium the typical size of gravitationally unstable shells is 1 kpc for a gas density of n = 1 cm−3 and decreases to 10 pc for n = 104 cm−3.
Evidence for Azimuthal Asymmetry in Be Star Winds
Authors: Peters Geraldine J.
Published: July 2016

Extensive IUE observations from 1987-96 have provided compelling evidence for azimuthal asymmetry in the winds of Be stars. Two identified types of wind/FUV flux behavior are briefly discussed. Interpretations based upon both nonradial pulsations and rotation of one or more active regions appear plausible.
Optical Interferometry of Non-spherical Winds
Authors: Vakili Farrokh , Mourard Denis , Stee Philippe , Bonneau Daniel
Published: July 2016

Optical Long Baseline Interferometry (OLBI) combined with spectral resolution is capable to sounding the stratification of luminous star winds. In this paper we review some few, yet unique results obtained on P Cyg and Be stars from OLBI observations. We also examine the prospects for studying hot stars winds offered by optical synthesis arrays presently under construction.
The Wind Momentum – Luminosity Relationship of Blue Supergiants
Authors: Kudritzki Rolf-Peter
Published: July 2016

The prediction of the Wind Momentum - Luminosity Relationship (WLR) based on the theory of radiation driven winds is verified by quantitative spectroscopy of winds of A-, B- and O-supergiants. The relationship depends on spectral type. New stellar wind calculations are presented reproducing the observed spectral type dependence. The impact of spectral variability on the WLR is investigated by an analysis of some hundred spectra of the luminous A0Ia-supergiant HD92207 obtained within the Heidelberg Spectral Variability Survey and found to be small.

Finally, the WLR is discussed as a tool for the determination of extragalactic distances. Recent results obtained for the Galaxy, M31 and M33 are presented. The potential of the method is discussed with the conclusion that it may allow independent distance moduli to be obtained with an accuracy of ten percent out to the Virgo and Fornax clusters of galaxies.
Building Galaxy Models with Schwarzschild Method and Spectral Dynamics
Authors: Zhao Hongsheng
Published: July 2016

Tremendous progress has been made recently in modelling the morphology and kinematics of centers of galaxies. Increasingly realistic models are built for central bar, bulge, nucleus and black hole of galaxies, including our own. The newly revived Schwarzschild method has played a central role in these theoretical modellings. Here I will highlight some recent work at Leiden on extending the Schwarzschild method in a few directions. After a brief discussion of (i) an analytical approach to include stochastic orbits (Zhao 1996), and (ii) the “pendulum effect” of loop and boxlet orbits (Zhao, Carollo, de Zeeuw 1999), I will concentrate on the very promising (iii) spectral dynamics method, with which not only can one obtain semi-analytically the actions of individual orbits as previously known, but also many other physical quantities, such as the density in configuration space and the line-of-sight velocity distribution of a superposition of orbits (Copin, Zhao & de Zeeuw 1999). The latter method also represents a drastic reduction of storage space for the orbit library and an increase in accuracy over the grid-based Schwarzschild method.
Parallel/Vector Integration Methods for Dynamical Astronomy
Authors: Fukushima Toshio
Published: July 2016

This paper reviews three recent works on the numerical methods to integrate ordinary differential equations (ODE), which are specially designed for parallel, vector, and/or multi-processor-unit (PU) computers. The first is the Picard-Chebyshev method (Fukushima, 1997a). It obtains a global solution of ODE in the form of Chebyshev polynomial of large (> 1000) degree by applying the Picard iteration repeatedly. The iteration converges for smooth problems and/or perturbed dynamics. The method runs around 100-1000 times faster in the vector mode than in the scalar mode of a certain computer with vector processors (Fukushima, 1997b). The second is a parallelization of a symplectic integrator (Saha et al., 1997). It regards the implicit midpoint rules covering thousands of timesteps as large-scale nonlinear equations and solves them by the fixed-point iteration. The method is applicable to Hamiltonian systems and is expected to lead an acceleration factor of around 50 in parallel computers with more than 1000 PUs. The last is a parallelization of the extrapolation method (Ito and Fukushima, 1997). It performs trial integrations in parallel. Also the trial integrations are further accelerated by balancing computational load among PUs by the technique of folding. The method is all-purpose and achieves an acceleration factor of around 3.5 by using several PUs. Finally, we give a perspective on the parallelization of some implicit integrators which require multiple corrections in solving implicit formulas like the implicit Hermitian integrators (Makino and Aarseth, 1992), (Hut et al., 1995) or the implicit symmetric multistep methods (Fukushima, 1998), (Fukushima, 1999).
Motion near The Unit Circle in The Three-Body Problem
Authors: Broucke Roger A.
Published: July 2016

Many of the important applications of the circular planar restricted problem of three bodies involve motion in the vicinity of the unit circle, (as defined in canonical units). It is then of interest to develop simplified models which are valid in this region. These models preserve the gross characteristics of the original system but they possess simpler equations of motion.

We will also show that several simplified models can be seen as a perturbation of a very well known simple linear system: the Clohessy-Wiltshire equations used by NASA in all their rendezvous operations. These are actually very close to the well-known Hill problem. We will thus consider the Restricted problem as a perturbed Hill or Clohessy-Wiltshire problem. We also introduce the Clohessy-Wiltshire Lagrangian in polar coordinates.
On The Critical Phenomena in The Dynamics of Asteroids
Authors: Shevchenko Ivan I.
Published: July 2016

Two statistical effects in the long-term chaotic asteroidal dynamics are considered, namely the power-law character of the dependence of recurrence times on local Lyapunov times and the power-law decay in the tails of the recurrence distributions. The dependences in both cases are shaped by effects of anomalous transport, due to the presence of the chaos border in phase space, and by statistical selection effects.
Nonparametric Statistical Models of Globular Cluster Dynamics
Authors: Heacox William D.
Published: July 2016

Observations only partially constrain dynamical models of globular clusters and similar systems, in large part because distances and velocities are seen only in projection, and only at a single (unknown) orbital phase. As a result, some dynamical assumptions are necessary if the cluster dynamics and mass distribution are to be inferred from kinematic data — typically that mass follows light, or that the stars observed are the sole source of the gravitational field in which they move, or that orbital energies are completely thermalized, or that orbital angular momenta are exponentially distributed — most often some combination of these is invoked. Such assumptions, reasonable as they may seem, are almost never justified by the data per se for the cluster in question, and thus diminish the credibility of resulting estimates of cluster dynamics and, especially, of the presence and extent of dark matter.
Can Combination of ‘Kozai Effect’ and Tidal Friction Produce Close Stellar and Planetary Orbits?
Authors: Kiseleva L.G. , Eggleton P.P.
Published: July 2016

In binary stars, tidal friction dissipates a fraction of the orbital energy at constant angular momentum and will circularise binary orbits on a rather short rimescale compared with the nuclear rimescale, provided that at least one star of the binary has a radius comparable to the separation between binary components. This dissipation effectively ceases once the orbit is circularised. In a hierarchical triple system such dissipation cannot cease entirely, as neither inner nor outer orbit can become exactly circular because of the perturbation of the third distant body. Thus in such systems tidal friction can lead to a steady secular decrease of the inner semimajor axis, accompanied by transfer of angular momentum from the inner to the outer pair, persisting over the whole nuclear lifetime of the system. The situation can be even more dramatic if two orbits have high relative inclination i > 40° It can be shown analytically and numerically (see e.g., Kozai 1962, Marchal 1990, Kiseleva 1996 and references therein) that for triple systems with high relative inclination there is a quasi-periodic change of the inner eccentricity (on a timescale ) during which it reaches a maximum value . This value only depends on the inclination i between the two orbital planes; other parameters affect only the timescale. For example, if we approximate a triple stellar system like β Per (Algol) (m1 = 0.8M⊙, m2 = 3.7M⊙, Pin = 2.87 days; m3 = 1.7M⊙, Pout = 1.86 yr, eout = 0.23; i = 100°) as three point masses, then the inner eccentricity ein cycles rather smoothly between 0 and 0.985, while i fluctuates between 100° and 140°. We call these fluctuations ‘Kozai cycles’. Such ‘Kozai cycles’ do not actually occur in this semi-detached system: they can be damped to a small value by tidal friction, but in fact they are also strongly reduced by the non-dissipative effect of the quadrupole moments of the two stars in the inner pair. This effect produces apsidal motion which is much more rapid than the apsidal motion due to the third star, and so prevents the Kozai cycles from operating.
Secular Perturbations near Mean Motion Resonances
Authors: Christou A.A.
Published: July 2016

In this work we attempt to make progress into assessing the importance of secular interactions between planetary satellites. In recent years, discrepancies have been observed in the expected positions of small planetary satellites (Bosh & Rivkin, 1996; Roddier et al., 1998). The existing ephemerides-producing algorithms for these objects assume fixed, elliptical and inclined orbits whose rate of precession is determined by oblateness alone. Even though the masses of these satellites are quite small relative to the planet (∼ 10−9 – 10−10) their small mutual separations and the existence of much larger satellites further out leaves open the possibility that in some cases at least the fixed-orbit assumption is only a crude approximation to reality. Two important dynamical mechanisms through which these orbits may evolve are resonant or secular interactions. In order to explore the possibility of the latter we have set up a simple planar system where an satellite in a circular orbit around a spherical planet is perturbing a massless particle which moves in proximity to various mean motion resonances. We aim to examine the effect of the resonance on the particle’s reference orbit by measuring the secular frequency. The effects of oblateness have not been taken into account as they are adequately modeled by orbit-fitting theories and can thus be readily subtracted.
Analytical Approximation of the Dissipative Standard Map
Authors: Celletti Alessandra , Penna Gabriella Della , Froeschlé Claude
Published: July 2016

We investigate the dynamics of a dissipative standard mapping defined by the equations

where y ∈ R, x ∈ T and ε is a real parameter, we refer to 0 < α < 1 as the “dissipative parameter” and to ψ as the “dissipative coefficient” (ε = α = 0 provides an integrable mapping). Notice that the dynamics is contractive, since the jacobian of the above mapping equals to 1 − α. In particular, we want to compare (see Celletti et al., 1997) the solutions associated to the conservative map (i.e., α = 0) with that related to (1) (α ≠ 0). For simplicity, we consider the case when α = ε2 and construct explicit approximate solutions to the conservative and dissipative systems, using a suitable parametrization like in (Celletti and Chierchia, 1988).
Molecular Thermal Emission and Its Relationship to Circumstellar Absorption, Stellar Absorption, and Stellar Emission in Red Variables
Authors: Wallerstein George
Published: July 2016

The discovery of thermal SiO emission from cool stars (Buhlet al. 1975, Reid and Dickinson 1976) and the subsequent discovery of additional stars with SiO and CO thermal emission (Lo and Bechis 1977, Lambert and van den Bout 1978, Dickinsonet al. 1978) solved the problem of the velocity of the center of mass of those stars and permitted improved models of their atmospheric kinematics. Morris and Alcock (1977) have shown that the broad emission features are indeed thermal, and that the emission must come from a region much larger than the volume of the star. Once the velocity of the center of mass of the star has been determined we can establish the velocities of the stellar absorption lines, stellar emission lines and circumstellar absorption features. The purpose of this paper is to assemble the available data for such stars and to note the relationships among the various velocities.
Recent Work on the RCB Stars
Authors: Feast M.W.
Published: July 2016

A general review of the R Coronae Borealis variables was given at the Moscow meeting (Feast 1975). What I wish to do now is to go over the more recent work and to see where we stand with the interpretation of these stars. It seems particularly appropriate to talk about RCB stars here in New Zealand where Frank Bateson and his collaborators are doing such a magnificent job in monitoring these variables. Their work is essential if, for a representative sample of these objects, we are to find out such matters as the frequency and depths of minima, the rates of decline, possible long term brightness variations, changes in periods for pulsating RCB stars and so on.
Cepheids with Beat Periods
Authors: Faulkner D. J.
Published: July 2016

The double-mode or beat Cepheids form a class of about a dozen objects near the short period end of the classical Cepheid instability domain, which exhibit two (or three) simultaneous modes of pulsation (see Table 1). Their importance lies in the fact that the two periodicities (P0and P1, thought to be the fundamental and first overtone modes of radial oscillation) may be used, together with the results of linear pulsation theory, to yield a mass and radius estimate for each star (MbeatRbeat) based on period measurements alone. This is so because the numerous periods that have now been calculated for stellar models of different characteristics may be represented by fitting formulae for each mode, depending largely on stellar mass and radius. Since mass and radius determinations are rather scarce in astronomy, the beat Cepheids should provide an excellent check on our ideas of stellar evolution, particularly during the stage of core helium-burning. Unfortunately, this check has led to some severe problems in interpreting events at the lower end of the classical Cepheid domain.
Results from an analysis of 800 Palomar Schmidt Plates
Authors: Luyten W. J.
Published: July 2016

When, in 1962, I began repeating the early Palomar Survey plates and hand-blinked them, I realized immediately that if 1 wanted to do more than just a representative sample of the sky, the blinking process would have to be automated. Fortunately I knew some of the engineers of Control Data Corporation, and soon we got together and evolved a plan for the construction of such an automated-computerized plate scanner and measuring machine. There were the usual delays with funding, but on 1 July 1965 I received a contract from the National Aeronautics and Space Administration=originally this was for $276,000 and three years. It took a little longer, and cost more - which is not unusual for this kind of work - and in early April 1970 the machine was finished at a cost of $762,000. But - the machine was a great deal better than we could have hoped. Originally I had specified that the measuring accuracy - i.e. the repeatability accuracy of the machine - should be three microns, but I also said that if this proved too difficult to attain I could live with a five micron accuracy. When completed, the machine proved to be fantastically and superlatively successful.
Fundamental position astronomy and transportable Zenith Cameras
Authors: Pilowski K.
Published: July 2016

In continuation of previous works of the author (Astronomische Station of the Technical University of Hannover: Monographie 1973; Supplement I, no. 9, 1974; Suppl. II, no. 10, 1976; Suppl. Ill in preparation), a completely systematic solution of the “Fundamental Positional Astronomy” and “Transportable Zenith Cameras” is given as a new form of a purely stellar Spherical Astronomy, in theoretical foundation, instrumentation and observation in a very simple way, as with traditional means, based on geographical position determination.

There are two possibilities:

A purely stellar absolute fundamental system (fundamental catalogue and absolute inertial systems) and a stellar-geographical fundamental system (fundamental catalogue, absolute inertial systems, equator position in time), serving additionally for the determination of the geographical position and the earth’s rotation (absolute earth time). The first theory is based on two selected zenith fields, the second on one selected zenith field and the earth’s rotation.
Status of SRS Catalogue
Authors: Polozhentsev D.
Published: July 2016

Since 1962 the observatories of the southern and northern hemisphere have been working jointxyon meridian observations of the programs of the “Southern Reference Stars (SRS)”. The SRS catalogue will contain about 20000 stars, which will give a chance to use it as a reference catalogue for photographic astrometry of the southern sky. It was suggested that the SRS observations be reduced in the fundamental catalogue system. However, shortly after the commencement of the work it came out that the RA of FK4 stars southwards of -10° in decl. have large systematic and accidental errors. In 1973 a suggestion was made that a preliminary improvement of the RA system of FK4 stars should be made before using it for the SRS.
Views of the future development of Fundamental Astrometry
Authors: Tucker R. H. , Teleki G. , Counselman C. C. , Hog E. , Marcus E. , Débarbat S. , Fogh Olsen H. J. , Gliese W. , Klock B. L. , Murray C. A. , Nikoloff I. , Potter H. I. , Tavastsherna K. N. , Wood H.
Published: July 2016

The paper presents various views of the future of fundamental astrometry, arranged under the following headings:

1 General and Introductory Remarks

2 The New Techniques

2.1 Modernised Classical Astrometry

2.2 Laser Ranging

2.3 Radio Astrometry

2.4 Space Astrometry

2.5 Other Techniques

3 The Impacts and Implications

3.1 Prospects for Classical Astrometry

3.2 Criteria for cessation of observations

3.3 Need for adequate connection of observational series

3.4 Planning of Observational Programmes

4 Points Emerging

4.1 Fundamental and Relative Astrometry

4.2 Fundamental and Inertial Systems

4.3 Data Processing

4.4 The Role of IAU Commission 8

4.5 Proposed Observational Programmes

5 Bibliography
The Two Dimensional Representation of Three Dimensional Interferometer Measurements
Authors: Frater R.H.
Published: July 2016

A convolution technique for the reduction of three dimensional interferometer measurements to two dimensions is described. With the addition of relatively simple hardware to a general purpose computer the technique allows fast, efficient processing of three dimensional data.
Problems with Maximum Entropy
Authors: Ponsonby J.E.B.
Published: July 2016

Two rival expressions for entropy have been used in maximum entropy image formation. Attention is drawn to the fact that both have been shown to be limiting cases of a general expression for the entropy of the photon distribution swept up during an observation. All measures of entropy are based on some physical model of the radiation mechanism. It is argued that knowledge of its character should be fully included in the image formation process.
The Introduction of A Priori Knowledge in Certain Processing Algorithms
Authors: Högbom J.A.
Published: July 2016

A common problem in radio synthesis work is that of determining the brightness at N grid points in the map domain when there are only n<N independent interferometer measurements available. The missing (N-n) equations can in principle be replaced by an equivalent amount of information in the form of a priori knowledge about the brightness distribution. One way of doing this is to add an equation of the form

i.e. some function of the brightness distribution is maximized subject to the condition that the map be compatible with the n measurements. This automatically gives (N-n) new equations, leaving us in the pleasant situation of having as many equations as there are unknowns.
Instabilities in a Polytropic Atmosphere
Authors: Antia H.M. , Chitre S.M.
Published: July 2016

The density in the outer layers of stars varies by several orders of magnitude and it is desirable to include the full effects of compressibility in any study of instabilities arising in stellar convection zones. In an unstable compressible fluid-layer that is thermally conducting both the oscillatory and non-oscillatory motions can simultaneously arise. The conditions under which the oscillatory acoustic modes can be overstabilized in a polytropic atmosphere are examined. It is argued that the linearized perturbation theory breaks down when applied to an inviscid complete polytrope which has vanishing density and temperature at the top for both optically-thin and optically-thick approximations. However, the linearized theory is demonstrated to be self-consistent when viscosity and thermal conductivity are included in the study of complete polytropes.
Thermal and Continuum Driven Convection in B-Stars
Authors: Nelson George Driver
Published: July 2016

Two regions of convective instability are present in the photosphere of a typical B-star (Teff = 30,000 K Log g = 4.0). One is the usual thermal instability caused by the helium ionization. The other is driven by the continuum radiation pressure in a thermally stable layer. Mixing length and anelastic modal representations of these unstable regions show that the rapid radiative cooling of temperature fluctuations limits the velocities to an amplitude of a few meters per second, much too small to account for the observed line broadening and asymmetries.
Mesoturbulence
Authors: Traving G.
Published: July 2016

The influence of a stochastic velocity field with a finite scale length 1 on the transfer of line radiation is described by means of a generalization of the transfer equation. Micro- and macroturbulence are contained in this mesoturbulence approach as limiting cases 1 → O and 1 → ∞ respectively.
Differential Rotation and Magnetic Activity of the Lower Main Sequence Stars
Authors: Belvedere G. , Paterno L. , Stix M.
Published: July 2016

We extend to the lower main sequence stars the analysis of convection interacting with rotation in a compressible spherical shell, already applied to the solar case (Belvedere and Paterno, 1977; Belvedere et al. 1979a). We assume that the coupling constant ε between convection and rotation, does not depend on the spectral type. Therefore we take ε determined from the observed differential rotation of the Sun, and compute differential rotation and magnetic cycles for stars ranging from F5 to MO, namely for those stars which are supposed to possess surface convection zones (Belvedere et al. 1979b, c, d). The results show that the strength of differential rotation decreases from a maximum at F5 down to a minimum at G5 and then increases towards later spectral types. The computations of the magnetic cycles based on the αω-dynamo theory show that dynamo instability decreases from F5 to G5, and then increases towards the later spectral types reaching a maximum at MO. The period of the magnetic cycles increases from a few years at F5 to about 100 years at MO. Also the extension of the surface magnetic activity increases substantially towards the later spectral types. The results are discussed in the framework of Wilson’s (1978) observations.
“Carbon-Rich” White Dwarfs
Authors: Bues I.
Published: July 2016

8 white dwarfs of spectral type λ4670, LP 93-21, EG182, BPM 27606, EG82, G257-38, EG148, G99-37 and L97-3, with more or less well observed carbon bands in their spectra, have been analyzed by detailed model atmosphere techniques. For the very strong band stars the blanketing effect of the molecular band systems of the Swan band and the Deslandres-d’Azambuja band had to be included in the computation of the models. The best fit of the fluxes with scanner observations and spectra is obtained for abundance ratios of 103 ≤ He/H ≤ 105, 800 ≤ He/C ≤ 5000, with LP 93-21 being the most carbon-rich object. The results are summarized in Table 1.
Non-Radial Oscillations in Rotating White Dwarfs
Authors: Papaloizou J. , Pringle J.E.
Published: July 2016

In previous work (Papaloizou and Pringle, 1978), we investigated analytically the low frequency spectrum of a rotating pulsating star, taking the effects of rotation fully into account. In that case we found that not only are the g-modes substantially modified by the rotation, but also that a new set of modes appear (the r modes) which are similar to Rossby waves in the Earth’s atmosphere and oceans.
Time-Dependent Accretion Disks in Dwarf Novae
Authors: Bath G.T. , Pringle J.E.
Published: July 2016

Over the past decade it has become increasingly clear that the once commonly held belief that the explosions of dwarf novae are caused by runaway thermonuclear reactions of accreted hydrogen on the surface of a white dwarf is not easily reconcilable with the growing observational data now extending from infra-red to x-ray wavelengths. It cannot fit the disk location of the outburst, the outburst energetics and repetition period (Bath et al. 1974) and the spectrum changes at outburst. The discovery that dwarf novae are semi-detached binaries opened the possibility that the explosions are caused by bursting mass accretion on to the white dwarf (Smak (1971), Bath (1973), Osaki (1974)). This model has now become a powerful tool for investigating the physical and spectroscopic properties of these systems.
Major Observatories Versus Economy-Class Observatories in Space
Authors: Dyson Freeman J.
Published: July 2016

Big space-instruments are essential for observing fine details of selected objects. Small space-instruments are essential for scanning large areas of sky, for discovering and cataloguing new objects, and for monitoring variable or transient sources of radiation. If astronomy is to flourish, both types of instrument must be provided. The problem is not to choose between big and small. The problem is to find the optimum mixture of big and small. Various examples from past history and from future plans illustrate the importance of maintaining a variety of different types of instrument.
Occultation Astronomy
Authors: Dyson Freeman J.
Published: July 2016

The era of occultation astronomy, studying dark objects by observing occultations of bright ones, has begun. The paper of Duncan, Quinn and Tremarne, “The Origin of Short-Period Comets”, Astrophys. J. Letters, 328, 69-73 (1988), greatly improved the prospects for occultation astronomy by demonstrating the existence of a second comet reservoir, the Kuiper Belt, much closer to us than the Oort Cloud and concentrated toward the ecliptic plane. Charles Alcock at Livermore (private communication) has begun work on a practical system of small telescopes to observe occultations of stars by comets. I here propose a system similar in concept of Alcock’s but using different hardware. We need to try various systems on a small scale to find out which are most cost-effective. My proposal is based on the Multiple Telescope Robotic Observatory (MTRO) developed by Boyd and Genet at Fairborn Observatory in Arizona. See Russell M. Genet, “Multiple-Telescope Robotic Observatories in Space”, submitted to P.A.S.P. (1990).
The UV Imager for the Israeli Scientific Satellite
Authors: Brosch N.
Published: July 2016

Israel will orbit a satellite dedicated to scientific research. One of the two experiments studied for deployment on this platform is a three-channel imager in the ultraviolet proposed by Tel Aviv University and designed jointly by staff of the Wise Observatory and of El-Op (Electro-Optical Industries, Ltd.). The design provides very significant scientific returns in a small payload and for a moderate cost.
A Theoretical Review of Diffusive Shock Acceleration
Authors: Jones Frank C.
Published: July 2016

We discuss the fundamental ideas of particle acceleration in plasma shocks with emphasis on those features that are required to produce the “universal” power-law spectrum. We compare shock acceleration with the more familiar second-order or stochastic acceleration and see that they are not too different in many respects. We discuss the features of shock acceleration that make it appealing and some of its problems as well.

Subject headings: acceleration of particles — MHD — plasmas — shock waves
Acceleration of Electrons at a Curved Shock
Authors: Vandas M.
Published: July 2016

The paper deals with the acceleration of electrons at a nearly perpendicular curved shock wave and results are compared with the case of a plane shock wave. We formulate the equations of motion of an electron in adiabatic approximation and solve them in a special case. The results show that the energy gain of reflected electrons at a curved shock depends on the ratio of the radius of curvature to the shock thickness; for a low ratio the energy gain is significantly lower than at a plane shock, while the energy gain of transmitted electrons is approximately the same as at a plane shock. The analytical solution provides a tool for direct calculations of fluxes of accelerated electrons near the curved shock of given parameters.

Subject headings: acceleration of particles — shock waves
Gamma-Ray Precursors of Solar Flares
Authors: Rieger E.
Published: July 2016

Bursts have been observed by the gamma-ray spectrometer on SMM at medium- and high-energy gamma-rays that precede the flare maximum. The negligible contribution of nuclear lines in the spectra of these events and their impulsive appearance suggests that they are hard-electron-dominated events superposed on the flares. Spatial resolution at gamma-ray energies will be necessary to decide whether this kind of bursts is cospatial with the flares or whether they occur in the flares’ vicinity.

Subject headings: Sun: flares — Sun: X-rays, gamma rays
Acceleration from Field-Aligned Potential Drops
Authors: Haerendel Gerhard
Published: July 2016

Unstable field-aligned currents are seen as the origin of field-aligned potential drops. They convert energy stored in magnetic shear stresses into kinetic energy. A good fraction of this energy is carried by runaway electrons and ions out of the acceleration region. The paper emphasizes the analogy with mechanical fractures. Simple expressions for the energy conversion rate and the parallel potential drop are derived, the two being linked by the critical current density needed for instability. The origin of the currents (generator) lies mostly in a region remote from that of energy conversion (fracture zone). The transmission of shear stresses and energy from the generator plasma, where the primary forces are applied to the fracture zone is also considered. A closed set of relations allows quantitative evaluation of the energetic particle production efficiency. The decoupling of the plasma on either side of the fracture zone which allows fast stress relief is described in detail, as well as a stationary model for the Alfvén wave interaction between fracture zone and generator plasma. A simple concept of the nature of the anomalous resistivity generated by the unstable current leads to an expression for the magnetic diffusivity inside the fracture zone and an estimate of the latter’s extent parallel to the magnetic field, whereas its width and length transverse to B follow from the macroscopic relations. Finally and as an example, the theory is applied to the problem of fast electron (and ion) acceleration well above 1 MeV seen to occur in many solar flares. It is obvious that this process belongs to the most powerful production processes of high-energy particles in stellar magnetic fields.

Subject headings: acceleration of particles — MHD — plasmas
Gamma Rays from the Geminga Pulsar: Variations with time and Phase
Authors: Grenier I. A. , Bennett K. , Buccheri R. , Gros M. , Henriksen R. N. , Hermsen W. , Kanbach G. , Sacco B.
Published: July 2016

Pulsed γ radiation from the Geminga pulsar was recorded by the COS B satellite from 50 MeV to 5 GeV between 1975 and 1982. It has been analysed to derive the source spectral properties as a function of time and phase. The two main peaks are separated by 0.50 ± 0.01 in phase. Significant pulsed emission has also been detected in both phase regions between the two main peaks. Significant spectral differences with phase have been found; the emission from the first peak is slightly softer than from the second one and the Interpeak 2 radiation is extremely soft. The first peak emission remained stable within the statistics over 7 years while the flux from the other phase intervals changed significantly with time. The phase dependence of the variability implies that all the observed source emission should be pulsed and that it consists of four discrete beams with different apertures and spectra. The beams characteristics are strikingly similar to those of four γ-ray beams generated by the Vela pulsar.

Subject headings: gamma rays: observations — pulsars: individual (Geminga, Vela)
Gamma-Ray Burst Spectral Breaks and Source Beaming
Authors: Baring Matthew G.
Published: July 2016

The principal discovery of the ComptonGamma-Ray Observatory(GRO) concerning gamma-ray bursts is that these sources are isotropic but with a comparative deficiency of fainter sources, suggesting that they are probably cosmological in origin. If they are at such large distances from Earth then they are extremely luminous and compact. A consequence of this is that two-photon pair production attenuation of the gamma-ray continuum cannot be avoided unless the source radiation is substantially beamed. Most sources do not display gamma-ray turnovers although a few GRB detected byGROexhibit distinct spectral breaks in the MeV range. A derivation of the relationship defining of the degree of beaming in burst sources with spectral breaks due toγ-γattenuation, as a function of source spectral index and break energy, is presented. It is found that sources at distances of ~1 Gpc must typically be beamed with bulk Lorentz factors of around 103-104, indicating powerful bulk acceleration in bursts, although these Lorentz factors are reduced markedly for steep source spectra. Since the source spectra are not strongly Comptonized, such beaming will blueshift the γ-γ attenuation breaks to energies much higher than 1 MeV; an absolute lower bound to the source bulk Lorentz factor is determined from this additional constraint. This blueshifting suggests that those sources with MeV breaks may not be cosmological, or that their breaks are produced by a mechanism that dominates γ-γ attenuation at these energies.

Subject headings:gamma-rays: bursts — radiation mechanisms: nonthermal
On The Location of the Acceleration and Emission Sites in Gamma-Ray Blazars
Authors: Dermer Charles D. , Schlickeiser Reinhard
Published: July 2016

Compton scattering of external radiation by nonthermal particles in outflowing blazar jets is dominated by accretion-disk photons rather than scattered radiation to distances ~ 0.01-0.1 pc from the central engine for standard parameters, thus clarifying the limits of validity of the model by the present authors and the model of Sikora, Begelman, & Rees. On the basis of contemporaneous Ginga X-ray and EGRET gamma-ray observations, we estimate the radius of 3C 279’s gamma-ray photosphere to be smaller than estimated by Blandford. There is thus no need to require that the acceleration and emission sites of gamma-ray blazars to be located farther than ~ 102 - 103 gravitational radii from the central engine. We argue that lineless BL Lac objects, rather than quasars, are more likely to be detected in the TeV energy range.

Subject headings: acceleration of particles — BL Lacertae objects: general — galaxies: jets — gamma rays: theory — radiation mechanisms: nonthermal
Oblique Magnetohydrodynamic Cosmic-Ray-Modified Shocks: Two-Fluid Numerical Simulations
Authors: Frank Adam , Jones T. W. , Ryu Dongsu
Published: July 2016

We present the first results of time-dependent two-fluid cosmic-ray (CR) modified MHD shock simulations. The calculations were carried out with a new numerical code for one-dimensional ideal MHD. By coupling this code with the CR energy transport equation we can simulate the time-dependent evolution of MHD shocks, including the acceleration of the CR and their feedback on the shock structures. We report tests of the combined numerical method including comparisons with analytical steady state results published earlier by Webb, as well as internal consistency checks for more general MHD CR shock structures after they appear to have converged to dynamical steady states. We also present results from an initial time-dependent simulation which extends the parameter space domain of previous analytical models. These new results support Webb’s suggestion that equilibrium oblique shocks are less effective than parallel shocks in the acceleration of CR. However, for realistic models of anisotropic CR diffusion, oblique shocks may achieve dynamical equilibrium on shorter timescales than parallel shocks.

Subject headings: cosmic rays — methods: numerical — MHD — shock waves
Cataclysmic Variables: A ’SWOT’ Analysis
Authors: Warner Brian
Published: July 2016

A brief review of the nature and subtypes of cataclysmic variables (CVs) is given. The catalogue of CVs is still very incomplete. All-sky surveys should add large numbers of CVs, which will improve knowledge of the space density of these systems. It is pointed out that the nova-like variables, which are the most difficult to discover, are the subtype having the highest space density. Their discovery is therefore the highest priority – they fix the frequency of CVs, which is important in population syntheses of binary stars.
The Rotation and Variability of T Tauri Stars: Results of Two Decades of Monitoring at Van Vleck Observatory
Authors: Herbst W.
Published: July 2016

A brief history of photometric monitoring of T Tauri stars with a 0.6 m telescope at Wesleyan University is given. This is followed by discussion of three recent results: 1) the mass dependence of stellar rotation rates in the Orion Nebula Cluster, 2) a check on stellar radii estimates of pre-main sequence stars, and 3) the nature of a very peculiar object in NGC 2264.
Variable Stars in the Globular Cluster M92
Authors: Kopacki Grzegorz
Published: July 2016

Results of a search for variable stars in the central region of the Oosterhoff II type globular cluster M92 are presented.
Project MONICA for the Study of Time-Variable Phenomena of the Jovian Sodium Cloud and the Io Plasma Torus
Authors: Chang Chien-Pang , Ip Wing
Published: July 2016

Because of active volcanism, large amounts of gas and dust particles are being injected from the Galilean satellite, Io, into the Jovian system. The neutral cloud of sodium atoms and the plasma torus of sulfur ions provide very useful information on Io’s interaction with the Jovian magnetosphere. A program called MONICA (Monitoring of Neutral and Ionized Atoms Clouds) was established at NCU with a view to participate in an international campaign during the flyby of the Jovian system by the Cassini spacecraft in December, 2000. Spectrographic observations were carried out using the 2.16m spectrograph of the Beijing Astronomical Observatory in Xing-Long. A progress report is presented here.
Long-Term Coudé Radial-Velocity Studies With a 1.2-m Telescope
Authors: Scarfe C.D.
Published: July 2016

I have used the 1.2-m telescope and coudé spectrograph of the Dominion Astrophysical Observatory for more than 30 years in a program of radial-velocity observations of binary stars. The program was begun with photographic plates as detectors, but for 20 years the primary detector has been the radial-velocity scanner, which cross-correlates stellar spectra with an artificial mask.

Since some of the binaries under observation have periods of several years, the instrument’s stability is an important consideration. I have therefore been obliged to observe standard stars and asteroids to check its performance. These observations are of relevance to efforts to improve the IAU standard star system.

I will describe the telescope, spectrograph and scanner, and will briefly discuss some of the results obtained for a selection of binary and multiple stars.
Turbulent Convection: A New Model
Authors: Canuto V. M.
Published: July 2016

We use the latest models of turbulence to compute a new expression for the turbulent convective flux, Fc. The new values of Fc are up to ten times larger than those given by the mixing length theory, MLT. Astrophysical considerations indicate that the new model fares better with observational data than the MLT.
Large Scale Convection in Stars : Towards a Model for the Action of Coherent Structures
Authors: Rieutord Michel , Zahn Jean-Paul
Published: July 2016

We show that, representing the descending fluid in a convection zone by a porous medium, the differential rotation of the (rising) fluid is very close to that in an axisymmetric model of the convection zone with anisotropic viscosity.
Convective Cores in Stellar Models
Authors: Roxburgh I.W. , Monteiro M.
Published: July 2016

The condition for homogeneous radiative stellar models to be marginally stable to convection at the centre is investigated for the family of models where the opacity к and energy generation ε are given by power laws in temperature and density κ = κo ρα T−β, ε = εo ρ Tη. The Naur-Osterbrock (1953) condition 6η > 6 + 10β − 15α is a necessary but not sufficient condition. A better estimate is obtained by taking the effective polytropic index n = dlogP/dlogT - 1 to be a linear function of temperature T throughout the model. This gives the condition

The predictions of this condition agree well with results for a set of stellar models 0 ≤ α ≤ 1, 0≤ β ≤ 5.