bluuebunny/crossref_metadata_2025_split huggingface dataset:
A 3-D hydrodynamical simulation of wind accretion
Authors: Boffin Henri M.J. 
Published: June 2019

Using a 3-dimensional hydrodynamical code, based on the Smooth Particle Hydrodynamics (SPH) method, we have simulated the case of wind accretion. Several configurations were studied:
-homogeneous mass flow-wind with a density gradient-wind with a velocity gradientIn each case, the accretion rate as well as the accreted angular momentum has been calculated. Comparisons were done with other simulations published in the literature and especially with the two currently existing 3-D simulations. According to one of these group (Matsuda et al., 1990) resolution is very critical in determining the accretion rate as well as to determine if the flow is stationary or not. We have done simulation at various resolutions and we confirm its importance for the deduced accretion rate. However, our simulations do not seem to show any non- stationary behaviour.
Eclipse Observations of the Extreme Solar Limb at Submillimeter Wavelengths
Authors: Clark T. A. 
Published: June 2019

This paper will review the use of solar eclipses in the study of the extreme solar limb at sub-millimeter and millimeter wavelengths. This approach has been used to overcome the severe limitation imposed by diffraction upon the resolution attainable by direct solar limb scans at these wavelengths. Strong absorption by water vapor in the Earth's lower atmosphere has necessitated the use of telescopes at high altitude sites or in jet aircraft. Data from several of these experiments will be reviewed, including those from the recent James Clerk Maxwell Telescope observation at a wavelength of 1.3 mm of the eclipse of 11 July 1991. In view of the success of recent measurements in improving the spatial resolution with this technique, several of the ultimate limitations placed upon it by lunar surface roughness and by diffraction at the lunar limb are outlined.These observations have demonstrated the inadequacy of present phenomenological solar atmospheric models at sub-millimetric source heights. Newer models have been developed to fit the observed extension, brightening and detailed structure of the solar limb by attempting to include the structure of the chromospheric network and its spicular field, and their relative success in doing so will be discussed.
Dust in Dense Clouds
Authors: Tielens A. G. G. M. 
Published: June 2019

Recent observational and theoretical studies of dust in dense clouds are reviewed with an emphasis on the growth of dust grains through accretion and coagulation. IR reflection nebulae around protostellar objects are useful probes of grain sizes in dense clouds. For example, detailed studies of the IR reflection nebula surrounding OMC 2-IRS 1 show that the (scattering) grains are much larger (Ã 5000 å) than in the diffuse interstellar medium. Likewise, the presence of a weak shoulder at 2.95 μm on the 3.08 μm feature in BN indicates the importance of scattering by icy grains and implies a very similar increase in the grain size.Theoretical studies of grain surface chemistry predict the possible presence of three distinctly different grain mantle components in dense clouds depending on the physical conditions in the gas phase. These are: 1) A hydrogenated mantle dominated by H2O and CH3OH; 2) An inert grain mantle dominated by CO and O2; and 3) An oxidized grain mantle dominated by CO2. Although the importance of H2O dominated grain mantles was known for 10 yrs, the presence of CH3OH was only recently confirmed. Furthermore, recent studies of the solid CO band have revealed the presence of at least two distinctly different interstellar grain mantle components along the line of sights towards most stars: One dominated by polar and one by non-polar molecules. Although specific identification of the molecules mixed in with the CO in these components is difficult, it is quite possible that the former component is dominated by H2O and the latter by CO itself, as suggested by theoretical models. Finally, the photochemical evolution of icy grain mantles is briefly reviewed and it is suggested that the resulting complex molecular mantles may evolve into amorphous carbon mantles in the diffuse ISM.Grain-grain collisions can lead to large modifications of the interstellar grain size distribution. At high velocities (v ≳ 1 kms−1) shattering into many small fragments will be important, while at low velocities (v ≲ 10 ms−1) coagulation dominates. Both processes can play a role in dense molecular clouds. The sticking of grains at low velocities is discussed in some detail and it is concluded that coagulation in molecular clouds is only important if the colliding grains are covered by icy grain mantles.Thus, a model for interstellar dust is proposed in which small (≲ 500 å) silicate and carbonaceous grains are “glued” together in large (Ã 3000å), open conglomerates by a polymerized, all enveloping grain mantle. This structure resembles that of certain interplanetary dust particles collected in the upper stratosphere.
Space Distribution of the Richest Abell Clusters of Galaxies
Authors: Kalinkov M , Kuneva I 
Published: June 2019

We discuss four samples from the Abell (1958) catalog of clusters of galaxies. Our samples are drawn out from the Abell sample and all clusters have richness 2 and 3. With Ho = 100 km/s/Mpc and qo = +1, we examine the following volumes, defined for both galactic hemispheres:
(i)RG 3: b 40° for 0° < 1 360° and b 30° for 90° < 1 240°, distance 300 < R < 750 Mpc, N = 35 clusters, 30 of which with known redshift;(ii)RG 3: b −35° for 15° < 1 232°, 150 < R < 600 Mpc, N = 15 (13);(iii)RG 2: b and 1 as (i), 60 < R < 525 Mpc, N = 110 (55);(iv)RG 2: b and 1 as (ii), 120 < R < 600 Mpc, N = 102(14).
A Program For A New Reduction Of Plates Of The Astrographic Catalogue
Authors: Röser S. , Jahreiß H. 
Published: June 2019

The Astrographic Catalogue is the major source of accurate first epoch coordinates of stars down to about 11 mag all over the sky. Therefore it could be the ideal tool for the determination of proper motions in this range of magnitudes - see also for example Fresneau (1985). There were mainly three reasons which prevented in the past a frequent use of this catalogue:
1)Apart from the „french“ zones (Lacroute 1981) the AC is not available in machine readable form.2)Only measured plate coordinates x,y and plate constants mostly derived with the AGK1 as reference catalogue have been published.3)The lack of easy to use software for a new reduction.
The main features of the 1.56-m telescope and its status quo
Authors: Lai Wan , Neng-hong Zhu , Lan-juan Wang , Zheng-hua Yang , Yi-jin Zheng 
Published: June 2019

This telescope was designed for determining the trigonometric parallaxes of faint stars. The otpical configuration is a Ritchey-Chrétien system and has only a f/10 Cassegrain focus for photographic work.The mounting is a combination of fork and yoke frame which can reduce the flexure due to gravity. A high speed digital servo-system was adopted for controlling and driving the telescope. At present some photographs of Comet Halley have been obtained.
Desorption Mechanism of Gases from Interstellar Grains and PAH Molecules
Authors: Léger A. 
Published: June 2019

Many gases, including CO, should be condensed at the temperature of interstellar grains in the absence of efficient desorption mechanisms. We consider such mechanisms and find that impulsive heating by X rays and heavy cosmic rays (iron group) are the most efficient ones.For CO rich mantles, we conclude that species as CO, N2, O2 and probably H2CO, NH3, H2O should be desorbed in most cases including dense and obscure regions. for mantles composed mainly of refractory ices (H2CO, NH3, H2O) we expect very little desorption.The case of PAH molecules that may offer a larger surface for condensation than grains is also discussed.
CO observations of gas surrounding B335 (IR) and L1551-IRS5
Authors: Walmsley C. M. , Menten K. M. 
Published: June 2019

Recent submillimeter observations of infrared sources embedded in dark clouds have uncovered the existence of a few highly compact objects. Two examples of this phenomenon are the infrared sources observed towards B335 and L1551 (Keene et al. (1983), Davidson and Jaffe (1984)). One can estimate for these regions gas masses of approximately one solar mass, densities of at least 106 cm−3, and diameters less than 1017 cm. Both infrared sources are associated with bipolar outflows observed in CO with mass loss rates of order 10−6 solar masses per year. It is reasonable in both cases to suspect the existence of matter in some form of disc around the infrared source.One might also expect to observe molecular counterparts to the submillimeter compact objects. In an effort to do this, we have mapped the C18O J = 1 → 0 transition in a small area surrounding the infrared sources in B335 and L1551 using the IRAM 30-m telescope. The beamwidth was 22 arc sec. which corresponds to 5 1016 cm at the distance of L1551 and 9 1016 cm at the distance of B335. In each case, we do find a compact structure which we believe to be the molecular counterparts of the compact submillimeter sources. We estimate the abundance ratio CO/H2 to be of order 10−4 in L1551 and at least 10−5 in B335. Further details of this work are contained in an article submitted to Astronomy and Astrophysics.
An extended nebulosity surrounding the S Dor variable R 127
Authors: Appenzeller I. , Wolf B. , Stahl O. 
Published: June 2019

Using the CASPEC echelle spectrograph of the European Southern Observatory, La Silla, Chile, we obtained new high resolution spectrograms of the LMC S Dor variable R 127 in the blue and red spectral range.The red spectrogram, which contains the [N II] 6548 and 6533 and the [S II] 6717 and 6731 lines shows the presence of a well resolved extended gaseous nebula around R 127 (see Figures 1 and 2). The nebula (which is also detected at the Balmer lines) shows blueshifted and redshifted emission (projected) on the position of the stellar continuum, and no wavelength-shift at the maximum (East-West) distance from the star. Hence, the nebulosity appears to be an expanding shell, reminiscent of the nebula around the galactic extreme supergiant AG Car. The angular diameter (or East-West extension) of the nebula around R 127 is of the order 4“, corresponding to ≈1 pc at the distance of the LMC. The expansion velocity of the R 127 nebula is found to be 28 km s−1 from our spectrograms. Hence, assuming a constant expansion velocity we derive a kinematic age of the R 127 nebula of ≈2 · 104 years. This corresponds closely to the expected lifetime of the S Dor evolutionary phase.A more detailed description of our results will be published in the proceedings of the Workshop on “Instabilities in Luminous Early Type Stars” (C. de Loore, H. Lamers, eds.) Lunteren 1986.
Results of the Timing Analyses of X-ray Pulsars Observed by Hakucho and Tenma
Authors: Nagase F. 
Published: June 2019

A dozen of X-ray pulsars have been observed with the Japanese X-ray astronomy satellites Hakucho and Tenma between 1979 and 1984. The obsetvations revealed remarkable pulse period changes both in disk-fed and wind-fed pulsars. From the histories of the pulse period changes so far measured, we are able to classify the X-ray pulsars into three categories:
(1)Pulsars which exhibit a steady spin-up with a constant rate of period change (e.g., 4U 1626-67 and GX 1+4).(2)Disk-fed pulsars in which considerable period changes are superposed on the secular trend of spin-up (e.g., Her X-1 and Cen X-3).(3)Wind-fed pulsars whose pulse periods fluctuate randomly about the average pulse period on time scales from days to years (e.g., Vela X-1 and GX 301-2).Among the dozen X-ray pulsars observed by Hakucho and Tenma, Vela X-1 was most extensively studied. Timing studies of Vela X-1 revealed an episode of secular spin-down of the pulsar between 1979 and 1982, and large rates of short-term period fluctuations superimposed on the secular trend. Recent studies of the wind-fed pulsar suggest that the angular momentum transfer expected from the wind accretion is too small to account for the observed rates. Spectroscopic studies of Vela X-1 by Tenma observations provided a variety of phenomena related to the binary system and detailed properties of matter surrounding the system, especially using an iron emission line fluoresced thereby as a probe.Formation of a disk in the wind-fed pulsar is proposed to accommodate a variety of phenomena observed from Vela X-1. A disk is expected to be formed through the interaction of the spherically accreting matter with the rotating magnetosphere, under the assumption that the closed magnetosphere extends to the corotation radius. Application of this model to Vela X-1 requires a large magnetic moment of μ ≈ 1032 gauss cm3 (1013 ≲ B ≲ 1014 gauss, depending on the stellar parameters), but once the large value is accepted, this model fits well with the large rates of period change and phenomena which would otherwise be regarded as strange. If this model works for Vela X-1, it will be also applied to some of other wind-fed pulsars.
Millisecond Pulsar Formation and Evolution
Authors: van den Heuvel E.P.J. 
Published: June 2019

The evolutionary history of binary radio pulsars, including the two millisecond binary pulsars, is reviewed. There are two groups of binary pulsars, the PSR 1913+16-group, which descended from massive X-ray binaries, and the PSR 1953+29-group, which descended from fairly wide low-mass X-ray binaries. The neutron stars in the second group probably formed by the accretion-induced collapse of a massive white dwarf. The companion stars in both groups of systems are expected to be dead stars, i.e. white dwarfs or neutron stars.The large total number of millisecond binary pulsars in the galaxy (∼ 104), indicates that magnetic fields of neutron stars do not decay below a value of order 109 G. Possible explanations for this phenomenon are discussed.Coalescence with a close degenerate companion provides a viable model for the formation of the single millisecond pulsar.
The Formation of Single and Binary Nuclei of Planetary Nebulae
Authors: Yungelson L. R. , Tutukov A. V. 
Published: June 2019

The present birthrate of binaries may be written as
where M1 is the mass of the primary in M⊙, 0 < q ≤ 1 — mass ratio of components, A — semimajor axis of the orbit in R⊙. The Eq.(1) implies that all stars are born as binaries with 10 ≤ A/R⊙ ≤ 106, and that one binary with M1 ≥ 0.8M⊙ is formed annually in the Galaxy. We study numerically evolutionary scenarii of binaries within abovementioned range of M1, A,q. As PN formation events we consider all ejections of common envelopes by close binaries and ejections of envelopes by red giants, after which one may expect a formation of a hot (Te ≥ 30 000K) star, surrounded by a nebula. Altogether about 20 different single and binary cores of PN can be formed. Combining the scenarii data with the Eq. (1) one can estimate the birthrates of most numerous kinds of PNN (single and with main-sequence, white dwarf, giant and relativistic companions) listed in the Table.
VLBI Relativistic Time Delay Model with Picosecond Precision
Authors: Yang Zhigen , Zhao Ming 
Published: June 2019

The VLBI relativistic time delay model of transformation is reformuled with a precision of better than 1 ps, which is given as followswhere is geocentric newtonial potential, and are the barycentric velocity vector in B-frame and the geocentric velocity vector of antenna i. Ŝ is the unit vector of the direction from the barycenter of solar system to the source. c is the speed of light in vacuum. , where is the geocentric baseline vector. δtv can be expressed as

in which

and
where Δtatm, Δtion and Δtaxo are the tropospheric, the ionospheric and the axio offset refraction delays respectively, and Δtgrav is called the gravitational time delay. A straightforward differentation of expression (1), the equation of d(dτ)/dt can be obtained. The included in the can be expressed as
The orders of magnitude of the various correction terms in expression (4) and (5) are estimated respectively. Conclusion: expression (4) and (5) should be taken into account in the VLBI relativistic model of transformation for the 1 ps precision. Equation (1) and the expression of its differentation differ from all the models which have been published earlier.
Is the Vorticity Vector of the Galaxy Perpendicular to the Galactic Plane?
Authors: Miyamoto Masanori 
Published: June 2019

The modern astrometric catalogue ACRS invites attempts to re-examine the systematic stellar motions, together with the luni-solar precessional correction and the fictitious equinoctial motion correction to the FK5 system, and gives encouraging results.On the basis of the three-dimensional Ogorodnikov-Milne model for the overall pattern analysis of the proper motions, the systematic stellar velocity field of about 30000 K-M giants chosen from the ACRS is first examined in the heliocentric distance interval 0.5 to 1.0 kpc. We have found in an iterative fashion a solution for the K-M giants that yields neither deformation-nor vorticity-field other than the classical ones (the Oort constants A and B). The important point to note here is that the generally accepted idea such that the K-M giants are a steady-state constituent of the galaxy is compatible with the luni-solar precessional correction proposed by the VLBI and LLR observations. The K-M giants give the rational set of corrections to the FK5 system: the luni-solar precessional correction Δp = −0″.27 ± 0″.03 / cent and the equinoctial motion correction including the planetary precessional correction Δe + Δλ = −0″.12 ± 0″.03 / cent. Thus, the precessional correction previously proposed with the modern techniques has been confirmed by the pattern analysis of the proper motions.Next, applying the corrections obtained above, we have performed the overall pattern analysis of the proper motions of about 3000 O-B5 stars, supergiants, and bright giants, which are chosen again from the ACRS, and considered as an entity of the galactic warp. It is found that the kinematics of these stars is quite different from that of K-M giants. These stars show additional shears and rotations around two mutually orthogonal axes lying in the galactic plane, besides the classical ones. The present finding implies that the young stars are streaming around the galactic center in a tilted sheet (the warp) with the velocity of 225 km/s, and the sheet itself is simultaneously rotating around the nodal line of the warp (galactic center — sun — anticenter line) with the angular velocity of 4 km/s/kpc in increasing sense of the present inclination of the warp.
Data filtering in statistical studies of flares recorded in sky fields of stellar aggregates - demonstrated with the example of the

Pleiades
Authors: Szécsényi-Nagy G. 
Published: June 2019

Photometric data on flares of cluster flare stars published by different observers were - as a rule - collected during decades by different instruments and amidst various conditions. The uncritical use of these data in global statistical studies is usually unfair since such a collection is an indefinite mixture of high and low quality results. In order to make these investigations more reliable even the best and most complete photometric catalogues have to be filtered.The first step of the procedure described in the paper is rejecting all flare events having too small amplitudes - seeing that they are under-represented in the catalogue as opposed to medium and large amplitude flares. Then the photographic coverage of the aggregate has to be checked and all events observed in partially covered regions have to be excluded. Next any non-randomness in the time distribution of flares of each active object is to be unveiled and flares of the doubtful stars have to be omitted. Finally some objects are to be rejected for their individual characteristics.The usefulness of the method is demonstrated with the example of the Pleiades, one of the richest open clusters known. More than 3000 hours of effective observing time has been devoted to flare photometry in fields centered upon Eta Tauri and the outcome of the programme the CPFS - the most complete catalogue of flare stars of the region - contains data of 519 flare stars and their 1532 flare events. Earlier versions of this catalogue or subsets of its data have been used to estimate the probable number of flare stars in the Pleiades region. Results of these attempts range from 600 to 2700 partly owing to the unreliability and incompatibility of the data sets. The method presented here is able to give much more reliable estimates but of course in the limited largest common field (LCF) only. As a result of the filtering the LCF contains 77% of the objects and 67% of the flares listed in the CPFS. Time sequence analysis of the 1026 flare events observed on 402 flare stars and statistical modelling of the phenomena suggested in an earlier paper fit each other almost perfectly.The conclusion of the study is that the most likely number of flare stars in the reach of the telescopes used in this programme is between 520 and 600 in the LCF of the Eta Tauri fields. Their average flare frequency is about 1/2400h and the CPFS lists 70% of these objects. In order to be able to compare the Pleiades field flare stars and those of other aggregates the method of filtering will be carried out throughout the analysis of the most extended data sets available.
Earth Rotation Velocity in Relation with Different Reference Frames
Authors: Brumberg V. A. 
Published: June 2019

The high precision of present observations makes it reasonable to clear up a question about GRT (general relativity theory) corrections in the problem of Earth's rotation. The answer is that one may almost forget about GRT corrections when dealing in an adequate reference system (RS). The problem of Earth's rotation may be related to the relativistic hierarchy of RS started in (Brumberg and Kopejkin, 1989) and completed in (Klioner, 1993). Let letters B, G and T be related to barycentric, geocentric and topocentric RS, respectively. Let DRS and KRS be dynamically nonrotating or kinematically nonrotating RS, respectively. From the dynamical equations of rotation it follows that the most adequate system for studying the Earth's rotation is DGRS. Apart from the geophysical factors the rotation of the Earth in this system is fairly well approximated by the rigid-body rotation with some angular velocity . The same rotation of the Earth as considered in BRS and DTRS may be also approximated by the rigid-body rotation but with some additive relativistic corrections and with other angular velocities ωi and , respectively. Substituting these three rotation relations into four-dimensional BRS-DGRS and DGRS-DTRS transformations one may express ωi and in terms of and determine the additive relativistic corrections in BRS and BTRS. These corrections are of importance for treating kinematics problems in various coordinate systems and for obtaining physically meaningful solutions of the dynamical equations of rotation in the barycentric reference system.The complete text will be published in Journal of Geodynamics.