bluuebunny/crossref_metadata_2025 huggingface dataset:
Relativistic Particle Acceleration in Plerions
Authors: Arons Jonathan , Tavani Marco
Published: July 2016

We discuss recent research on the structure and particle acceleration properties of relativistic shock waves in which the magnetic field is transverse to the flow direction in the upstream medium, and whose composition is either pure electrons and positrons or primarily electrons and positrons with an admixture of heavy ions. Particle-in-cell simulation techniques as well as analytic theory have been used to show that such shocks in pure pair plasmas are fully thermalized—the downstream particle spectra are relativistic Maxwellians at the temperature expected from the jump conditions. On the other hand, shocks containing heavy ions which are a minority constituent by number but which carry most of the energy density in the upstream medium do put ~20% of the flow energy into a nonthermal population of pairs downstream, whose distribution in energy space is N(E) ∝ E−2, where N(E)dE is the number of particles with energy between E and E + dE.

The mechanism of thermalization and particle acceleration is found to be synchrotron maser activity in the shock front, stimulated by the quasi-coherent gyration of the whole particle population as the plasma flowing into the shock reflects from the magnetic field in the shock front. The synchrotron maser modes radiated by the heavy ions are absorbed by the pairs at their (relativistic) cyclotron frequencies, allowing the maximum energy achievable by the pairs to be γ±m±c2 = mic2γ1/Zi, where γ1 is the Lorentz factor of the upstream flow and Zi, is the atomic number of the ions. The shock’s spatial structure is shown to contain a series of “overshoots” in the magnetic field, regions where the gyrating heavy ions compress the magnetic field to levels in excess of the eventual downstream value.

This shock model is applied to an interpretation of the structure of the inner regions of the Crab Nebula, in particular to the “wisps,” surface brightness enhancements near the pulsar. We argue that these surface brightness enhancements are the regions of magnetic overshoot, which appear brighter because the small Larmor radius pairs are compressed and radiate more efficiently in the regions of more intense magnetic field. This interpretation suggests that the structure of the shock terminating the pulsar’s wind in the Crab Nebula is spatially resolved, and allows one to measure γ1, and a number of other properties of the pulsar’s wind. We also discuss applications of the shock theory to the termination shocks of the winds from rotation-powered pulsars embedded in compact binaries. We show that this model adequately accounts for (and indeed predicted) the recently discovered X-ray flux from PSR 1957+20, and we discuss several other applications to other examples of these systems.

Subject headings: acceleration of particles — ISM: individual (Crab Nebula) — relativity — shock waves
Radio Emission From Snr 1987A
Authors: Kirk J. G. , Duffy P. , Ball Lewis
Published: July 2016

Diffusive acceleration of electrons by the blast wave of SNR 1987A is shown to provide a reasonable fit to observations of the radio emission since it reappeared in 1990 July. As well as determining the diffusion coefficient in this model, the observations indicate that the compression ratio of the gas subshock embedded in the blast wave is ~2.7, much lower than the value of 4 expected of a strong shock front. We propose that protons accelerated by the shock front itself cause the weakening of the subshock by both heating and accelerating the upstream plasma, producing a structure with a precursor to the gas subshock. Preliminary calculations using a fluid model for the energetic particle population indicate that these effects can significantly weaken the subshock just a few years after the explosion, if the same rate of particle injection is assumed as in other models of the acceleration of cosmic rays.

Subject headings: acceleration of particles — ISM: individual (SNR 1987 A) — radio continuum: ISM — shock waves
The Spectral Turnover of the “Filled-Center” Supernova Remnant 3C 58: Implications for When Acceleration Occurs
Authors: Green D. A.
Published: July 2016

IRASobservations the “filled-center” supernova remnant 3C 58 are used to derived upper limits of 0.4, 0.4, 0.8, and 1.5 Jy for its infrared flux density at 12, 25, 60, and 100 μm, respectively. These values imply a break in the spectrum of 3C58 above a few tens of gigahertz—two orders of magnitude below the break in the spectrum of the Crab Nebula—with a change of spectral index across the break for 3C 58 of more than 0.5. This is similar to previous results for another “filled-center” remnant, G74.9+1.2, from radio observations alone. This implies that their emission isnotdominated by a constant injection rate of particles with a power-law distribution. Either the break is due to synchrotron losses in the past, with little particle injection in recent times or it is intrinsic to the acceleration mechanism at work (presumably from a central neutron star).

Subject headings: acceleration of particles — ISM: individual (3C 58) — radiation mechanisms: nonthermal — supernova remnants
Particle Acceleration in High-Energy Gamma-Ray Sources
Authors: Eichler David
Published: July 2016

Many proficient gamma-ray sources show energy spectra that are consistent with E−2 primary spectra. Such sources include recently identified gamma-ray quasars and some gamma-ray bursts. Assuming thick target conversion, this is consistent with shock acceleration, and the dominance of the gamma rays of the luminosity is also consistent with previous predictions of high production efficiency of fresh cosmic rays in shocks. The spectral cutoffs in the gamma rays may offer clues as to whether the high-energy particles are electrons or protons. Resolution of this matter might have implications for the nature of the sources and for theory of shock accelerated electrons.

Subject headings: acceleration of particles — gamma rays: bursts — shock waves
The Origins of High-Energy Cosmic Rays
Authors: Axford W. I.
Published: July 2016

Our current understanding of acceleration processes for Galactic and extragalactic cosmic rays is briefly reviewed. Shock acceleration in supernova remnants remains the most favored process for cosmic rays up to the “knee” of the all-particle total energy spectrum at 1014 - 1015 eV. The highest energy particles are almost certainly extragalactic, and the most favored sources are associated with active galactic nuclei in one way or another. The intermediate region between rigidities of 1014 and 1018 V is more difficult to understand, although a galactic origin is preferred at present. The problem of making a smooth join in the spectrum at the knee suggests that these particles should not be considered to be independent of those at lower energies.

Subject headings: acceleration of particles — cosmic rays — shock waves
Particle Acceleration and Nonthermal Energy Release as an Effect of Magnetoactive Disk Accretion Onto Gravitating Center
Authors: Ikhsanov N. R. , Pustil’nik L. A.
Published: July 2016

Observations of GRO and UHEGR show that a number of Galactic and extragalactic accreting systems release most of their energy in the ultrarelativistic energy range (more than GeV). This result contradicts one of the principal conclusions of the standard models of accretion about a predominantly thermal character of energy release. This contradiction is caused by ignoring in the standard approach the processes of generation and amplification of magnetic field in the case of accretion onto a magnetized gravitating center. A new approach taking into account the processes mentioned above is applied to disk accretion onto a nonmagnetized gravitating center, as well as onto a magnetosphere. It is shown that in both cases the accretion is strictly controlled by the magnetic field, which leads to new conditions of equilibrium and stability and turns on nonthermal processes of energy release. The resulting configuration of the magnetic field, in which the main energy release takes place in both cases, is called “Z-pinch,” and is formed in the polar region of an accreting object. Effective particle acceleration occurs in it owing to the chain of MHD and resistive plasma instabilities, resulting in current discontinuity with the formation of “double layers” and generation of electric fields close to the Dreicer limit in them. The maximum energies of the accelerated particles are limited by the value 10 EeV, that coincides with the results of UHEGR observations.

Subject headings: acceleration of particles — accretion, accretion disks — gamma rays: theory — MHD
Bohm Diffusion and Cosmic-Ray-Modified Shocks
Authors: Duffy Peter
Published: July 2016

A numerical solution to the problem of self-consistent diffusive shock acceleration is presented. The cosmic rays are scattered, accelerated and exert a back-reaction on the gas through their interaction with turbulence frozen into the local fluid frame. Using a grid with a hierarchical spacetime structure the physically interesting limit of Bohm diffusion (к ∝ pv), which introduces a wide range of diffusion lengthscales and acceleration timescales, can be studied. Some implications for modified shocks and particle acceleration are presented.

Subject headings: acceleration of particles — cosmic rays — diffusion — shock waves
Taiwan Oscillation Network and Small-Telescope Research at Tsing Hua University
Authors: Chou Dean-Yi
Published: July 2016

Two projects, the Taiwan Oscillation Network (TON) project and the earthshine project, at Tsing Hua University will be discussed. The TON is a ground-based network to measure solar intensity oscillations to study the solar interior. Four telescopes have been installed in Tenerife (Spain), Big Bear (USA), Huairou (PRC), and Tashkent (Uzbekistan). The recent scientific results from the TON data will be briefly discussed. The earthshine project is to measure the brightness of the dark portion of the lunar disk to obtain the Earth’s global albedo. The dark portion of the Moon is lit by the sunlight reflected from the Earth. The global albedo is linked to the global temperature of the Earth. The long-term measurement of earthshine will provide information on the long-term variation of the global temperature. An automated earthshine telescope is being developed at Tsing Hua University. It will be installed at Lulin Mountain in central Taiwan. The ultimate goal is to build a ground-based global network to measure the long-term variation of earthshine to learn about the long-term variation of the global temperature.
The Grid Giant Star Survey for the Space Interferometry Mission
Authors: Patterson Richard J. , Majewski Steven R. , Slesnick Catherine L. , Rhee Jaehyon , Crane Jeffrey D. , Polak Allyson A. , Kundu Arunav , Kunkel William E. , Geisler Douglas , Muñoz Ricardo , Arenas Jose , Seguel Juan , Gieren Wolfgang , Smith Verne V. , Benedict G. Fritz , Johnston Kathryn V.
Published: July 2016

NASA’s Space Interferometry Mission (SIM), scheduled for launch in 2009, will determine the positions of thousands of stars as faint as V = 20 to a precision better than 4 microarcseconds (µas). A key part of the mission is the Astrometric Grid, which is a reference frame of several thousand stars with V ≤ 13 against which all relative measurements will be calibrated. To serve as a reliable inertial reference frame, the Grid must be astrometrically stable against photocenter jitter (from planets, binary companions, flaring or spotting) at the ~ 4µas level. Sub–solar metallicity giant stars, by virtue of their intrinsic luminosity, can probe the Galaxy to greater distances than almost any other stellar type at the same apparent magnitude. Thus, distant (> 3 kpc) giants with V < 13 will have proportionately smaller astrometric jitter compared to other potential Astrometric Grid star candidates. The Grid Giant Star Survey is a patchwork all-sky survey to find sub–solar metallicity K giants for the Grid, and to provide a unique database for studies of Galactic stellar populations. We describe here the survey characteristics and give examples of results to date.
Development of the Far-ultraviolet Imaging Spectrograph on KAISTSAT-4
Authors: Han Wonyong , Min Kyoung Wook , Edelstein Jerry , Nam Uk-Won , Seon Jong-Ho , Seon Kwang-Il , Park Jang-Hyun , Yuk In-Soo , Jin Ho , Lee Woo-Baik , Kang Kyungin , Ryu Kwangsun , Rhee Jin-Geun , Lee Dae-Hee , Korpela Eric , Dixon W. Van Dyke
Published: July 2016

The Far-ultraviolet IMaging Spectrograph (FIMS) is a small spectrograph optimized for the observations of diffuse hot interstellar medium in far-ultraviolet wavebands (900–1150Å and 1335–1750Å). The instrument is expected to be sensitive to emission line fluxes an order of magnitude fainter than any previous missions. FIMS is currently under development and is scheduled for launch in 2002.
Search for δ Scuti Type Pulsating Components in Eclipsing Binary Systems
Authors: Kim S.-L. , Lee J.W. , Youn J.-H. , Moon H.-K. , Choo K.J.
Published: July 2016

We present observational results of four eclipsing binary stars, Y Cam, RZ Cas, AS Eri and RU UMi, which were reported to have δ Scuti type pulsating components.
Distinguishing KBO from Asteroid Occultations in TAOS
Authors: Lemme Claudia , Liang Chyng-Lan
Published: July 2016

The goal of the Taiwan-America Occultation Survey (TAOS) project is to estimate directly the number of Kuiper Belt Objects (KBOs) by measuring the rate of stellar occultations. Occultations of distant stars may also be caused by asteroids and this article describes how both types of occultations can be distinguished on the basis of their shadow ground speed on Earth and the observing angle.
A Simulation of Shell Structures of Comet Hale-Bopp in February 1997
Authors: Tao J. , Qian B.C.
Published: July 2016

We present a simulation based on the morphology of the inner coma of comet Hale-Bopp(C/1995 O1) in February 1997. The synthetic images can fit the observations well, suggesting a simple spin mode. The rotation parameters, obliquity of orbital plane to equator and argument of subsolar meridian at perihelion, are 71 and 81 degrees respectively.
Cometary Polarimetry
Authors: Sen Asoke K
Published: July 2016

Comets are known to have a high (2-10 percent) polarization, caused mainly due to dust scattering and resonance fluorescence emission. Since near earth comets are generally bright (integrated visual magnitudes of 10 or even brighter) and as they show high polarization near earth-sun location, one can try to perform imaging polarimetry of such objects with small (40 cm or even smaller) telescopes. By using a dichroic polaroid sheet either in front of the telescope tube or before the imaging detector at the Cassegrain plane, one can record cometary images with a good signal to noise ratio. By rotating the dichroic sheet in three discrete steps and then reducing the corresponding three comet images to a single image, one can determine the linear polarization value at each pixel location on the image. The error in polarization will typically be the inverse of the ‘signal to noise ratio’. Such polarization images of a comet help us to determine its dust properties and also to look for possible dust jet activities. This type of work is possible with small telescopes and minor instruments.
Fast Drift-Scan CCD Imaging and Photometry with Small Telescopes: Lunar Occultations and Speckle Interferometry
Authors: Núñez Jorge , Fors Octavi
Published: July 2016

In this paper we show how inherent features of typical CCD cameras can be used for fast (in the order of milliseconds) photometric observations (FPO), even with submeter class telescopes. This is based on a modified drift-scanning technique which we show to be advantageous for such kind of observations. In particular, we successfully carried out some lunar occultations and speckle interferometry observations using this technique. During the year 2000, we registered several occultation events. In particular, we present the occultation of SAO79031. The millisecond photometry of the event allows us subsequent data analysis for stellar diameter estimation and close binary detection. We also show an example of the application of the technique to speckle interferometry observations. Finally, we present the main characteristics of our project to build a new remotely controlled observatory near Barcelona.
Some Aspects of Astronomy at Maidanak Observatory
Authors: Hojaev Alisher S.
Published: July 2016

A brief overview for some aspects of astronomical investigations at Maidanak is given. An excursus is made into its history and past research. Future plans and their requirements are described.
Chemical Abundances of the Planetary Nebulae NGC 2392 and NGC 3242
Authors: Wu C.H. , Li J.Z. , Chang Z.W. , Lin C.Y. , Hu J.Y. , Ip W.H.
Published: July 2016

Planetary nebulae represent the end product of the evolution of low mass stars with M < 8M⊙. The central stars have masses between 0.55 and 0.85M⊙. This means a large amount of material must have been distributed in the interiors of the PNe. The observed variations of the chemical compositions and mass distributions therefore carry important information about the nature of the associated AGB superwinds and Red Giant outflows. (Kwok et al, 1978) A program for comprehensive multiwavelength study of PNe has been initiated at NCU. One recent study has to do with the spectrographic observations of the planetary nebulae NGC 2392 and NGC 3242 using the 2.16 m telescope of the Beijing Astronomical Observatory. Some preliminary results are presented here.
Russian/Former Soviet Union Experience in Professional Small Telescope Usage
Authors: Bochkarev N.G.
Published: July 2016

FSU astronomers traditionally use small telescopes (⊘ ≤1.5 m, hereafter STs) for both science and education. Russian/FSU experience here is among the largest world-wide. There are only 2 large and moderate-sized facilities in whole Russia: the 6 m telescope of SAO RAS and Russian-Ukrainian 2 m one on the 3100 m high peak Terskol in Central Caucasus.

Equipped with good light receivers and handled by skilled observers, STs can produce first class scientific data. Important results are typically yielded by long-time sequences of observations and/or new observational “know how”: good instrument/receiver design, appropriate selection of objects and moments, etc. Examples of what has been done with STs in FSU, within my memory, (in the last ≃ 1/3 century) are listed below, without a list of references, because of lack of space. The author plans to publish a larger article on this subject in Astr.&Aph.Trans.
APPENDIX: a List of Robotic Telescopes
Authors: Hessman Frederic V.
Published: July 2016

In order to help the reader keep somewhat abreast of the rapid development in the field of robotic telescopes, the current list on the web available at the Göttingen MONET website is included here as an appendix.
On Numerical Studies of Solar/Stellar Convection
Authors: Chan Kwing L. , Serizawa Katsuyo
Published: July 2016

It has become a cliché to say that convection lies at the foundation of the activities, magnetism, and dynamos of the Sun and cool stars – the title of this colloquium. However, it has not been possible, until recently, to study theoretically these phenomena and their connections with quantitative rigor. The new development owes much to the rapid growth in the power of computers and the spread of sophisticated numerical techniques.

Since the last decade numerical studies of solar and stellar convection problems thrive. The trend towards a greater use of computers in these studies will likely become stronger. Now is a good time to reflect on the numerical approach itself, so that we may understand it better and make better use of it in the future. Here we look at two aspects of this methodology. First (Section 2), we examine its relationships to the analytical approach and observation, for which we make a differentiation between numerical experimentation and simulation. It is obvious that the ultimate test of any theory, numerical or not, is to confront observations, but not generally realized is that the connection to analytical studies is an important requisite for the healthy development of numerical studies. This point is to be elaborated. Second (Section 3), we discuss the prevalence of pitfalls in the numerical approach; two examples of current interest in convection computations are analysed in detail. In Section 4, we go on to suggest that one of these pitfalls might have caused the discrepancy between the results of numerical modelling of the solar differential rotation and helioseismology.
Magnetoconvection Patterns in Rotating Convection Zones
Authors: Roberts Paul H.
Published: July 2016

In addition to the well-known granulation and supergranulation of the solar convection zone (the “SCZ”), the presence of so-called “giant cells” has been postulated. These are supposed span the entire thickness of the SCZ and to stretch from pole to pole in a sequence of elongated cells like a “cartridge belt” or a bunch of “bananas” strung uniformly round the Sun. Conclusive evidence for the existence of such giant cells is still lacking, despite strenuous observational efforts to find them. After analyses of sunspot motion, Ribes and others believe that convective motions near the solar surface occurs in a pattern that is the antithesis of the cartridge belt: a system of “toroidal” or “doughnut” cells, girdling the Sun in a sequence that extends from one pole to the other. Galloway, Jones and Roberts have recently tried to meet the resulting theoretical challenge, with the mixed success reported in this paper.
Stellar Convection as a Low Prandtl Number Flow
Authors: Massaguer Josep M.
Published: July 2016

Thermal convection in the Sun and cool stars is often modeled with the assumption of an effective Prandtl number σ ≃ 1. Such a parameterization results in masking of the presence of internal shear layers which, for small σ, might control the large scale dynamics. In this paper we discuss the relevance of such layers in turbulent convection. Implications for heat transport – i.e. for the Nusselt number power law – are also discussed.
The Spatial Structure of Homogeneous Turbulence at High Reynolds Number
Authors: Vincent A. , Meneguzzi M.
Published: July 2016

A direct numerical simulation at resolution 2403 is used to obtain a statistically stationary three-dimensional homogeneous and isotropic turbulent field at a Reynolds number around 1000 (Rλ ≈ 150). The energy spectrum displays an inertial subrange extending over more than one decade. In the physical space, the strongest vorticity is organized in very elongated thin tubes. The typical length of a tube is around the integral scale of the flow and its thickness is a few dissipation scales.
Compressible MHD in Spherical Geometry
Authors: Valdettaro Lorenzo , Meneguzzi Maurice
Published: July 2016

The generation of magnetic field by a conducting, compressible fluid inside a spherical shell is studied by direct numerical simulations. A pseudo-spectral method is used in order to resolve accurately all the scales present in the problem. The range of parameters considered is the following: a unit Prandtl number, Rayleigh numbers up to 100 times critical, Taylor number 625, an aspect ratio of 2, a Mach number slightly less than 1, and pressure and temperature scale heights of the order of the thickness of the shell. A dynamo effect is observed for magnetic Prandtl numbers larger than 1. We present the properties of the turbulent flow, the role of the helicity and of the differential rotation in the enhancement of the magnetic field, and the spectral properties of the flow fields.
Rotational Effects on Reynolds Stresses in the Solar Convection Zone
Authors: Pulkkinen P. , Tuominen I. , Brandenburg A. , Nordlund Å. , Stein R.F.
Published: July 2016

Three-dimensional hydrodynamic simulations are carried out in a rectangular box. The angle between gravity and rotation axis is kept as an external parameter in order to study the latitude-dependence of convection. Special attention is given to the horizontal Reynolds stress and the∧-effect (Rüdiger, 1989). The results of the simulations are compared with observations and theory and a good agreement is found.
Fossil Magnetic Fields and Activity of Young Stars
Authors: Dudorov A.E. , Gorbenko E.E.
Published: July 2016

The strength of fossil magnetic fields in young stars is estimated. The interaction of a fossil magnetic field with convection in stellar envelopes is investigated. The influence of such a magnetic field, transformed to a small scale, on the activity of young stars is discussed.
The Sun’s Internal Differential Rotation From Helioseismology
Authors: Goode Philip R.
Published: July 2016

Well-confirmed helioseismic data from several groups using various observational techniques at different sites have allowed us to determine the differential rotation in the outer half of the Sun’s interior. The resulting rotation law is simple – the surface differential rotation persists through much of the convection zone with a transition toward solid body rotation beneath. To date there is no appealing evidence for a rapidly rotating core. There is however, weak evidence for a solar cycle dependence of the Sun’s internal rotation.
The Toroidal Magnetic Field inside the Sun
Authors: Krivodubskij V.N. , Dudorov A.E. , Ruzmaikin A.A. , Ruzmaikina T.V.
Published: July 2016

Analysis of the fine structure of the solar oscillations has enabled us to determine the internal rotation of the Sun and to estimate the magnitude of the large-scale magnetic field inside the Sun. According to the data of Duvall et al. (1984), the core of the Sun rotates about twice as fast as the solar surface. Recently Dziembowski et al. (1989) have showed that there is a sharp radial gradient in the Sun’s rotation at the base of the convection zone, near the boundary with the radiative interior. It seems to us that the sharp radial gradients of the angular velocity near the core of the Sun and at the base of the convection zone, acting on the relict poloidal magnetic field Br, must excite an intense toroidal field Bф, that can compensate for the loss of the magnetic field due to magnetic buoyancy.
Sun-as-a-star: Its Convective Signature and the Activity Cycle
Authors: Livingston W.
Published: July 2016

From May 1980 to April 1990 we have observed several times a year the solar irradiance spectrum from 5000Å to 6300Å using the 1-m Fourier Transform Spectrometer. During the ascending phase of the solar cycle, spectrum line asymmetry diminishes about 15% as the total magnetic field encreases. At some epoch near the time of total magnetic field maximum, line asymmetry ceases to track surface magnetism and abruptly assumes a value appropriate to solar minimum.
Towards Magnetic Images of Rapidly Rotating Late-type Stars
Authors: Donati J.-F. , Semel M.
Published: July 2016

We present the first magnetic detections using Zeeman-Doppler Imaging (ZDI) in the two bright rapidly rotating RS CVn systems HR 1099 and σ2CrB, and discuss their compatibility with various recent results on magnetic activity in cool stars.
Doppler Mapping of CP-star Surfaces with Weak and Strong Magnetic Fields
Authors: Khokhlova V.L.
Published: July 2016

A code for Doppler mapping of the distribution of chemical elements on stellar surfaces (Goncharskij et al., 1977, 1982) has been used during the past decade to make maps for several CP-stars. The code is based on Tikhonov’s method of solution of so-called ill-posed inverse problems and on an analytical expression of local line profiles in the form of the Milne-Eddington solution of the radiative transfer equation. This code takes into account the atmospheric model of the star through an assumed central depth of very strong and saturated lines, and through limb-darkening coefficients. As a solution one gets local line profiles and hence local equivalent widths.
Surface Inhomogeneities on the W UMa star AC Bootis
Authors: Linnell Albert P.
Published: July 2016

New photometric data and two older data sets for AC Boo can be represented with a common set of Roche model parameters and two starspot pairs which drift only in longitude and are located on the cooler component.
Long-term Starspot Behavior of BY Dra and EV Lac
Authors: Pettersen B.R. , Oláh K. , Sandmann W.H.
Published: July 2016

New starspot photometry of BY Dra and EV Lac, extending over a decade is analyzed in combination with data from the literature to study the long-term behaviour of spottedness on these stars. Both stars show brightness suppression as well as brightness modulation so spots exist both near the poles and at lower latitudes. Polar spots have been continuously present on BY Dra for the last 14 years. (O–C) diagrams for lightcurve minima reveal systematic, sinusoidal variations in BY Dra with a period of 13–14 years. In EV Lac the spots have been located on the same hemisphere for the last ten years.
The Spot Activity of FK Comae Berenices
Authors: Jetsu Lauri , Pelt Jaan , Tuominen Ilkka , Nations Harold
Published: July 2016

The active regions of FK Comae Berenices show a flip-flop behaviour, i.e. the concentrated part of spot-activity shifts exactly to the other side of stellar surface, and then remains on the same longitude for a time interval from a few years to a decade. The activity shows excellent phase coherence with respect to these two active longitudes separated 180 degrees from each other. FK Comae may provide a physical example of a non-linear dynamo, which shows surprisingly simple observational changes in the pattern of the magnetically induced spot configurations.
Recent Advances in the Observation and Analysis of Stellar Magnetic Fields
Authors: Saar Steven
Published: July 2016

There has been considerable progress recently in the study of magnetic fields on late-type stars. Advances in the theory include investigation of systematic effects, new and improved methods of analysis for unpolarized and circularly polarized lines, new models of stellar dynamos and of broadband linear polarization, and the first studies of the thermodynamic nature of stellar magnetic regions and their vertical structure. Observationally, there have been new measurements, particularly of young and active stars (including the first detection of a field on a pre-main sequence object), a new monitoring campaign, the first indications of the relative temperatures in stellar plages, and the first measurements of fields in a single stellar active region. I discuss the results in the framework of stellar activity and surface structure.
Spectroscopy of Southern Active Stars
Authors: Vilhu O. , Gustafsson B. , Walter F.M.
Published: July 2016

Hα and HeI D3 lines were observed in five active cool stars over their relatively short rotation periods. Three of the stars (AB Dor, Rstl37B, HD82558) are in the pre main sequence phase (as deduced from their lithium abundances), while two are post main sequence FK Comae type stars (HD32918 and BD-223467) (Vilhu et al., 1991). We present the results of these observations and point out the physical similarity of the Hα and He D3 transitions (Fig.1).
A Dynamo Model for Magnetic Stars with Long Periods
Authors: Schüssler M.
Published: July 2016

A α - effect dynamo model is presented which can be relevant for the group of magnetic stars.with observed periods between 1 y and 72 ys. The model is based on an axisymmetric α2- dynamo including non-linear effects due to the “cut off α- effect”; no differential rotation is taken into account. There are oscilliations of the magnetic field with periods in the right order of magnitude under the assumption of an outer convection zone between R ≥ r ≥.5 R ….7R. In the sense of this model therefore these stars should be young objects passing from their Hayashi track down to the main sequence.
Magnetic Field And Chemical Anomalies of Ap Stars
Authors: Dolginov A.Z.
Published: July 2016

It is shown that owing to the “battery” effect the spatial nonuniform helium distribution in Ap stars causes a toroidal magnetic field of about 103- 104gauss. Possible circulation of stellar matter in this field produce a poloidal magnetic field of the same order as the toroidal one even if the velocities of circulation are about 10-4-10-5cm/s. The chemical anomaly in turn is supported and amplified by the magnetic field. It is pointed out that the “battery” effect is likely to take place in the liquid core of the earth and in components of close binaries if they are nonuniformly heated by their companions.
Origin of the Anomalous Abundances of Ap Stars
Authors: Catalano F.A.
Published: July 2016

The existence of early type stars with peculiar spectra has been known since the beginning of the century (Maury 1897; Cannon 1901; Lockyer and Baxandall 1906). Also the spectrum and light variability of some of these stars has been known for a long time (Lundendorff 19065 Belopolsky 1913; Guthnik and Prager 1914). Later the membership of peculiar stars to a well defined group of stars having similar properties was evidenced by Morgan (1933), who suggested that the spectral anomalies were correlated with the surface temperature. Since then a great amount of observational and theoretical efforts have been made with the aim of understanding the peculiar stars phenomenon.
Abundance Anomalies, Diffusion and Constraints on F, A and B Stellar Envelope Models
Authors: Michaud G.
Published: July 2016

A review of diffusion in F, A and B stars is presented. The basic equations are introduced in a simple and approximate way. Meridional circulation, magnetic fields, surface gravity and age are all factors that contribute to variations in anomalies for stars of a given effective temperature. Detailed calculations for each of these effects, as well as of radiation forces in the atmosphere, are needed before the required detailed comparison between theory and observations can be made for individual stars. When it is made this comparison should put strong constraints on stellar atmosphere or envelope models. At the present level of uncertainty no difficulty between theory and observations appears definitely established.
Diagnostics of Strontium Abundance In Ap Stellar Atmospheres
Authors: Praderie Francoise
Published: July 2016

Analysis of strontium abundance has been performed from Sr II resonance lines in radiative equilibrium model atmospheres characterized by Teff= 9650 K and Teff= 12500°K. One first compares LTE and nLTE determination of the abundance in the case where the abundance is uniform with depth. Second, one derives the effect of concentrating all strontium in a “cloud”, which is the situation produced if diffusion acts to sort the various elements.
Rotation and Binarity
Authors: Jaschek Carlos , Jaschek Mercedes
Published: July 2016

The observational data bearing upon rotation and binarity are reviewed. Special emphasis is placed upon some anomalies in the behaviour of close binaries.

The purpose of this paper is to summarize the progress on these subjects since the last Ap star meeting.
The Peculiar A Stars: Atmospheric Parameters and Abundance Peculiarities
Authors: Hack M.
Published: July 2016

We have known for more than 40 years (Morgan 1933) that a group of stars having abnormal spectra exists among the main sequence stars included between B5 and F2. They represent at least 15% of the normal stars with the same color and luminosity.

They can be divided in two large groups and their properties are summarized in Tables 1 and 2. They are the Ap stars (including the various subgroups, Mn stars, Si stars, Sr-Cr-Euor cool Ap stars) and the Am, Fm stars.
Variation of Heavy Element Lines in the Ap Stars
Authors: Aslanov I.A. , Rustamov Yu. S. , Kowalski M.
Published: July 2016

Lines of U II were found in the spectrograms of the Ap stars HR 465, 17 Com A and HD 224801. Pm II lines were found in HR 465. These lines vary in intensity in HR 465 with a period of 6h41m, in 17 Com A of 71m, and in HD 224801 of 6h.
Some Peculiar Early-Type Stars in the Southern Milky Way (1 = 237° - 332°)
Authors: Rickman Hans
Published: July 2016

In the course of the Stockholm Observatory Spectral Survey of the Southern Milky Way apparent peculiarities have been found in a number of stars of early spectral types. Some of these stars have been subject to further observations at the European Southern Observatory in Chile. A list of the stars at present considered as peculiar is presented together with finding charts in view of forthcoming observations. The list contains all pieces of information collected up to the present time for each one of the stars.
On the Inhomogeneity of Chemical Composition over the Surface of 21 Per
Authors: Glagolevsky Yu.V. , Kozlova K.I. , Lebedev V.S. , Polosukhina N.S.
Published: July 2016

The magnetic variable star 21 Per has been studied from 4 and 8 Å/mm spectra obtained with the 2.6 - meter reflector of the Crimean Astrophysical Observatory. Spectral line intensities (Wλ) and radial velocities (Vr) have been measured.
Photoelectric Measurements of Strong Visual Absorption Features in Ap Stars
Authors: Vogt N. , Maitzen H. M.
Published: July 2016

As we have heard from Dr. Maitzen’s paper there are two pronounced flux depressions in the visual spectra of Ap stars, the nature of which is still unknown. The λ4100 depression is very broad (several hundred Å) and can be measured photoelectrically in the Strömgren system using the m1-index. The λ5200 feature can be observed by measurements in 3 filters: g1(5020 Å), g2(5240 Å) and y (Strömgren). The corresponding index d (Maitzen, 1976b) is defined by the difference between g2, and the mean of g1and y, all expressed in magnitudes.
Radio Observations of Magnetic Stars
Authors: Altenhoff W. , Pfleiderer J. , Weiss W.
Published: July 2016

Thirteen Ap stars have been observed with the Effelsberg 100-m-telescope, at a wavelength of 2 cm. No star was detected. The detection limits ranged from 3 to 8 mJy.
The Spectroscopic Binary Characteristics of the Mercury-Manganese Stars
Authors: Aikman G.C.L.
Published: July 2016

The 80 confirmed and suspected mercury manganese stars identified prior to 1974 and north of declination -20° have been studied from 654 high dispersion spectrograms. (2.4 - 15 Å mm-1) for velocity and degree of peculiarity. The binary frequency based on velocity variability criteria alone appears to be a normal main sequence value of about 50%, and is probably independent of the degree of enhancement of the mercury and manganese lines. New spectroscopic orbital elements are given for six systems: α And, 2 Per, κ Cnc, φ Her, HR 6620 and 46 Dra. The Hg Mn spectrosopic binaries appear to be fairly normal in their orbital characteristics, except that periods less than 3 days are conspicuously absent; there is also weak evidence for more nearly circular orbits amongst the short period Hg Mn binaries than in comparable normal binaries.
Ultraviolet Observations of the Bp Star 3 Centauri A
Authors: Hack M. , Stalio R.
Published: July 2016

3 Cen A (B5 III p) is well known for its remarkable peculiarities consisting in an excess of Phosphorous by a factor of ~100, of Krypton by a factor of ~103, of Gallium by a factor of ~104and deficiencies of Sulphur by a factor of ~10 and of Helium by a factor of 6. Moreover Helium consists by 84% of He3(Bidelman, 1960;Sargent and Jukagu, 1961; Jukagu, Sargent and Greenstein, 1961; Jukagu and Sargent, 1963; Hardorp, Bidelman and Prölss, 1968). The last two papers give atmospheric abundances computed by the method of atmospheric models. The best parameters derived by these authors are Te= 18150°K, log g = 3.7, ξ ≤ 2 km/s. The Ca II lines observable in the spectrum are believed to be of stellar origin, the interstellar components being negligible or absent. The main abundance peculiarities found by Hardorp et al. (1966, 1968) are the following: He, C: deficient by a factor of 10; 0: deficient by a factor of 3, Mg: by a factor of 2, S: by more than a factor of 10; Si: in excess by a factor of 2, P: by a factor of 100, Mn and Fe: by factors of 10 and 3 respectively, Ga, Kr: by factors of 104and 3x103respectively.
Rapid Variations in the Spectrum of the Magnetic Variable Star HD 215441
Authors: Polosukhina N.S. , Chuvaev K.K.
Published: July 2016

The star HD 215441 has the largest magnetic field ever recorded, about 34 Kgauss (1), varying without change of polarity. There are some indications of smooth brightness variations of this star with amplitude ∆m = 0.2 mag. as well as of small smooth variations of linear polarisation correlated with the brightness variations (2-4).
Discussion of the Oblique Rotator Model Cases Of 108 Aqr and x Serp.
Authors: Mégessier C. , Khokhlova V. , Ryabchikova T.
Published: July 2016

My talk will be on the oblique rotator model which was first proposed by Stibbs (1950), and since received success and further developments. I shall present two different attempts at describing a star according to this model and the first results obtained in the framework of a Russian-French collaboration in order to test the precision of the two methods. The aim is to give the best possible representation of the element distributions on the Ap stellar surfaces. The first method is the mathematical formulation proposed by Deutsch (1958-1970) and applied by Deutsch (1958) to HD 125248, by Pyper (1969) to α2CVn and by Mégessier (1975) to 108 Aqr. The other one was proposed by Khokhlova (1974) and used by her group.
Can Osawa'S Star HD 221568 be an Oblique Rotator?
Authors: Kodaira Keiichi
Published: July 2016

Radial velocity data of HD 221568 are critically reexamined. The revised data lead to a velocity curve essentially the same as obtained by Kodaira (1967). It is shown that the velocity variation of HD 221568 could be attributed to orbital motion, if an oblique rotator model is to be assumed for the cause of line-intensity variation.
On the Adoption of a Terrestrial Reference Frame
Authors: McCarthy Dennis D.
Published: July 2016

The report of the IAU Working Group on Nutation endorsed by Commissions 4, 8, 19 and 31 at the 1979 General Assembly points out that “… the complete theory of the general nutational motion of the Earth about its center of mass may be described by the sum of two components, astronomical nutation, commonly referred to as nutation, which is nutation with respect to a space-fixed coordinate system, and polar motion, which is nutation with respect to a body-fixed system …”. Unlike the situation for the space-fixed frame, there is not an adequate, formally accepted, body-fixed system for this purpose. The Conventional International Origin (CIO) as it is presently defined is no longer acceptable because of recent improvements in observational techniques. The effective lack of this type of terrestrial reference frame limits the complete description of the general nutational motion of the Earth. In the absence of a terrestrial reference frame suitable for specifying the orientation of the Earth, it is suggested that a body-fixed system could be represented formally in a manner analogous to that used to represent the space-fixed frame. This procedure would be quite similar to methods employed currently by the International Polar Motion Service and the Bureau International de l’Heure, and would allow for the use of observations from new techniques in the definition of a terrestrial reference frame to be used to specify the complete nutational motion of the Earth.
Critical Remarks on the Possibility of Determining Variations of the Geocenter Position Using Geostationary Satellite Observations
Authors: Klokoĉník J. , Kostelecký J.
Published: July 2016

The possibility of determining the variations of the Earth’s cejiter (expressed in terms of the geopotential harmonic coefficients , , or of some functions of them), using geostationary satellites, was discussed by Burŝa and Ŝíma (1978) after discerning the difference between observed and theoretical positions of libration points. Although their effect on geostationary orbits cannot be expected to be of high magnitude, the question of determination of these terms arose. It was suggested that the “resonant phenomenon” observed here might be analogous to the resonant effects in close satellite orbits, because the satellite is at the 1/1-resonance (1 nodal revolution per 1 sidereal day).
Concepts of Reference Frames for a Deformable Earth
Authors: Richter Burghard
Published: July 2016

In order to describe geometrical facts in geodesy, geodynamics, or astronomy, one must have suitable reference frames. This paper deals with some basic concepts for the definition of both terrestrial and celestial systems which make it possible to describe position and motion of mass elements of the earth or stars, respectively. It is suggested to use such frames with respect to which coordinate changes of points on the earth’s surface or of stars, respectively, become minimal. The rotation vector should be defined by applying the rotation operator on the velocity field.
Apport des Observations d’Occultations Stellaires en vue du Rattachement du Systeme Dynamique au Systeme Geometrique
Authors: Froeschlé M. , Meyer C.
Published: July 2016

The possibilities of tying the geometrical and dynamical reference frames by use of lunar occultations are reviewed. Actually, the best accuracy available is about 0”.4, essentially limited by stellar positions and limb corrections. One can expect an improvement of one order of magnitude with the accomplishment of POLO and HIPPARCOS Projects.
Development of a Radio-Astrometric Catalog by Means of Very Long Baseline Interferometry Observations
Authors: Fanselow J. L. , Sovers O. J. , Thomas J. B. , Bletzacker F. R. , Kearns T. J. , Cohen E. J. , Purcell G. H. , Rogstad D. H. , Skjerve L. J. , Young L. E.
Published: July 2016

The Jet Propulsion Laboratory of the California Institute of Technology has been developing a radio-astrometric catalogue for use in the application of radio interferometry to interplanetary navigation and geodesy. The catalogue consists of approximately 100 compact extragalactic radio sources whose relative positions have formal uncertainties of the order of 0”.01. The sources cover nearly all of the celestial sphere above -40° declination. By using the optical counterparts of many of these radio sources, we have tied this radio reference frame to the FK4 optical system with a global accuracy of approximately 0”.01. This paper describes the status of this work.
Beta Cephei and Related Stars
Authors: Percy John R.
Published: July 2016

Until recently, the β Cep stars were regarded as a small, homogeneous group of pulsating stars, well separated from the Cepheid instability strip in the H-R diagram. Now, the picture has been complicated by the discovery of other types of variables in and around the β Cep instability strip. These include (i) variable B supergiants, which may connect the β Cep strip to the Cepheid strip through the variable A and F super-giants (Maeder, 1980), (ii) the line profile variable stars or 53 Per stars, which extend to mid to late B types, and which may connect through the hypothetical Maia sequence to the δ Set strip (Smith, 1977), (iii) the short-period variable Be stars (Percy et al, 1980) and (iv) the ultra-short-period variable B stars discovered by Jakate (1979b). This variety of variables may reflect the variety of physical processes and pulsation mechanisms which are important in the B type stars.
The Light and Velocity Curve Bumps for BW Vulpeculae
Authors: Pesnell William Dean , Cox Arthur N.
Published: July 2016

BW Vulpeculae (HD 199140) is a well known Beta Cenhei star, with several unusual characteristics. It possesses the largest amplitude in both light and radial velocity variations, of any star in this classification. The most outstanding feature of the observations is a standstill or bump, on the light curve, accompanied by doubled lines in the radial velocity observations. In this paper, we have attempted to explain these phenomena with two different models. Firstly, the resonance mechanism, from Simon and Schmidt, and secondly, a non-linear calculation.