bluuebunny/crossref_metadata_2025 huggingface dataset:
X-ray Irradiated Accretion Disk and Bimodal States
Authors: Hoshi R. , Inoue H.
Published: June 2019

It is well known that black hole candidates, Cyg X-1 and GX 339-4, have distinct high and low states, known as bimodal states. Detailed spectroscopic studies of these X-ray sources have revealed high and low states corresponding to optically thick and thin states of the surrounding accretion disks.
Quasi-Periodic Oscillations in Low-Mass X-ray Binaries
Authors: van der Klis M.
Published: June 2019

The properties of the rapid, persistent quasi-periodic oscillations (QPO) discovered with EXOSAT in the X-ray flux of at least 7 bright low-mass X-ray binaries are described. Particular attention is given to the various relations observed between QPO frequency and X-ray intensity, the link between QPO and the low-frequency noise in the X-ray intensity and the bimodal properties of in particular Sco X-1, GX 5-1 and Cyg X-2. The merits of the hypothesis that the QPO indicate the presence of a neutron star with a magnetosphere are considered.
On the Long Term Stability of Neutron Star Magnetic Fields
Authors: Eichler David , Wang Zhengzhi
Published: June 2019

It is known that fluid object that it magnetized with a purely poloidal field is unstable to perturbations that overturn the field lines in one hemisphere relative to those in the other. We have shown that this instability occurs even when the entire object is encased by a solid crust, as conjectured by Flowers and Ruderman (FR) nearly a decade ago. We have derived the exact dispersion relation that displays this instability for an infinite, cylindrical geometry, which will be published elsewhere. The timescale for the instability is roughly the ohmic dissipation timescale of the crust.
Inverse Compton Scattering in Strong Magnetic Fields: Applied to the Radiation Mechanism of PSR 0531+21
Authors: Chen H. , Wu X.J. , Qiao G.J.
Published: June 2019

In strong magnetic field near pulsar's surface, the quantum effect for electrons is quite complicated. The classical approximation may lose resonance feature, which gives much smaller cross-section.

In this paper, we performed numerical integrations to get the total cross-section and the power spectrum of single electron. Thus considering the resonance, the inverse Compton scattering could be an efficient mechanism in strong magnetic fields. We have carefully calculated the power spectrum of single electron travelling through the isotropic thermal fields.
The Neutron Stars with Magnetic Charges
Authors: Wang Y.-J. , Peng Q.-H.
Published: June 2019

From KNK metric, we have shown that there may exist some kind of celestial bodies which can not collapse to black holes provided that the proportion of the content of the magnetic monopoles to the nuclei ξ = Nm/NB exceeds a critical value ξc (l-a2/Rg2), in which ξc =√G mB/gm ∼ 4.4 ×1021, a = J/Mc, Rg =GM/c2.
Some Constraints on Neutron Star Properties from Gamma Ray Burster Observations
Authors: Hurley K.
Published: June 2019

The results of recent soft X-ray and optical searches for quiescent gamma ray burster counterparts are used to constrain the properties of the neutron stars responsible for bursters. Ages are restricted to the range 2×105 y and above based on temperature upper limits and theoretical cooling curves, or 107 y and above if bursters have evolved from pulsars. Velocities are greater than 20 km/s if the neutron stars are unmagnetized. Practically no main sequence star could have escaped detection in the optical/IR searches, so if the neutron stars are in binary systems, the companion is most likely a degenerate, low mass, low temperature object.
Production and Interaction of High Energy Neutrinos in Close X-ray Binaries
Authors: Harding A. K. , Barnard J. J. , Stecker F. W. , Gaisser T. K.
Published: June 2019

Reports of air showers with E > 1015 eV from Cygnus X-3, LMC X-4, Vela X-1 and Hercules X-1 have been interpreted as requiring production of neutral secondaries by cosmic rays accelerated by the compact partner in these systems. If neutral pions are the source of photons that produce the observed air showers, then charged pions must also be produced, and they will give rise to neutrinos. We consider limits that may be placed on binary systems like Cygnus X-3 in which a neutron star is a strong source of ultra-high energy (UHE) particles that produce photons, neutrinos and other secondary particles in the companion star through nuclear interactions. The highest energy neutrinos (> 1 TeV), which have the largest interaction cross sections, are absorbed deep in the companion. From a detailed numerical calculation of the hadronic cascade induced in the atmosphere of the companion star, we estimate the neutrino production spectrum from an isotropic flux of monoenergetic 1017 eV protons and we estimate the resulting neutrino absorption in the stellar core. In the case of Cyg X-3 and LMC X-4, the cosmic-ray luminosities required to produce the observed gamma rays would result in energy deposition from neutrino absorption exceeding the intrinsic stellar luminosity of the companion. Over a timescale of 104−105 yr, the star would absorb its own binding energy and be disrupted. On shorter timescales, the energy deposition will cause significant expansion of the star, perhaps leading to quenching of high-energy signals from the source. From these results, we conclude that systems requiring intense UHE proton fluxes are either very young or the companion star is not the site of observed gamma-ray production. Alternatively, if the gamma-ray source is highly variable, the proton flux requirements would be lower, providing some relaxation of the above constraints.

[See Gaisser et al. 1986, Ap. J. (Oct. 15), in press].
An Experiment for Observing VHE Gamma Ray Sources
Authors: Jang Y.L. , He C.X. , Xu C.X. , Yuan Y.K. , Li Y.G. , Chen D.B. , Zheng R.T. , Huo A.X. , Ye M.H. , Tang S.Z. , Zhang L.J. , Meng S.X. , Shi H.L. , Jiang S.Y. , Li Q.B. , Yang J. , Zhang H.Q.
Published: June 2019

An apparatus design is described. It is for detecting VHE gamma ray point sources by means of the atmospheric Cerenkov technique. Obviously, the improvement of flux sensitivity and discrimination between gamma ray and isotropic proton showers is still a key problem. Of course, it is necessary to set up more observatories and to track an object continuously with several facilities. With this in mind, we decided to develop an experiment for observing VHE gamma ray sources in China. As a first step, we will set up an apparatus which consists of three 1.5 m diameter searchlight morrors at Xinglong station of Beijing Observatory, Xinglong county, Hebei province (40°. 4N, 117°.5E, altitude 940 m). The observation will start in 1988. Then, the second apparatus will be set up at Delingha station of Purple Mountain Observatory, Delingha county, Qinghai porvince (37°.22N, altitude 3204 m). Both the sites are far from air and light pollution, and have suitable meteological condition for Cerenkov light detection as well as quite convenient facilities for transportation. Some probable technical improvements are also discussed in this paper.
A Clustering Method Applied to the Analysis of Planetary Nebulae
Authors: Yatsyk O.S.
Published: June 2019

Intensities of the observed spectral lines, radio fluxes and Hβ fluxes are used for the classification of planetary nebulae by centroid method of taxonomical analysis. Two variants of classification are proposed. The first one– in the three–dimensional space of relation of intensity of spectral lines He II λ 4686/HeI λ 4471, [OIII] λ 4959+5007/[OII] λ 3726+29, [OIII] λ 4959+5007/[OIII] 4363. The second variants is the classification in the three-dimensional space with cooordinates being radio flux, Hβ flux and [OIII] λ 4959+5007 intensity. The membership of classes (taxons) are presented. In the diagram joining the planetary nebula descriptors pairwise there are regions of complete (or predominant) of nebulae belonging to the same taxon, but there are also some regions of overlap. The corresponding taxons are not isolated but merge continuously into one another.
Observed Mass Loss from Central Stars of Planetary Nebulae
Authors: Perinotto M.
Published: June 2019

In the Introduction we recall the mass loss history of a progenitor of a planetary nebula (PN). Then we concentrate on the status of knowledge of fast winds in central stars of planetary nebulae (CSPN) : the detection and statistics, the observed edge velocities, relationships of the edge velocities with other stellar or nebular parameters. We then summarize the methods used to derive the mass loss rates associated to the fast winds, and review the determinations of the “observed” mass loss rates. The comparison with predictions from the radiation driven theory (RDT) is then discussed as well as possible lines for future improvements.
Spectroscopic Properties of the Nucleus of NGC 6826
Authors: Altner B. , Heap S.R. , Hubeny I.
Published: June 2019

We continue the trend of finding discrepancies in the values of central star masses derived from photospheric line profile fitting and other methods in our analyses of ground-based and UV spectra of the nucleus of NGC 6826.
Central Stars of Old Planetary Nebulae
Authors: Napiwotzki R.
Published: June 2019

During a program for the observation of central stars of old planetary nebulae 29 stars are classified until now. Most of them (22) are belonging to the hydrogen-rich sequence and resemble either high-gravity sdO stars or white dwarfs (14 DAOs, 3 DAs). 3 are hydrogen-deficient PG 1159 stars, and also 3 are hybrid type stars Further 3 CPN are close binaries, whose evolution has been certainly influenced by this circumstance. A complete list of so far observed objects is given in Napiwotzki (1992).
The Wind Temperature and C/He and O/He Ratios of the WC10 Central Star CPD-56°8032
Authors: Barlow M. J. , Storey R. J.
Published: June 2019

We have analysed absolutely calibrated 3500–8900Å AAT spectrophotometry of the WC10 central star CPD–56°8032. After dereddening by E(B–V) = 0.68, the few line-free portions of the stellar continuum are well fitted by the 18000K blackbody which corresponds to the nebular H I Zanstra temperature. The Wolf-Rayet wind terminal velocity is sufficiently low (210–230 km s−1) that it is possible to resolve virtually every fine structure component of every multiplet of C II, C III and O I listed by the NBS Multiplet Tables for this wavelength region.
The Shklovsky Paradox
Authors: Buckley David , Schneider Stephen E. , Van Blerkom David
Published: June 2019

Shklovsky estimated distances to planetary nebulae (PNs) based on an assumed constant ionized mass and the relationship between flux and radius under the assumption of a constant density, fully ionized shell. He found that a mass of ∼0.2 M⊙ yielded the best results. Estimates of the ionized masses of PNs with independently determined distances also rarely exceed a few tenths of a solar mass. This is surprising since many PNs are thought to derive from high mass progenitors (up to 8 M⊙). Recent optical work (Plait and Soker, 1990) and our own computer simulations show that this simple mass estimation method may severely underestimate the total ionized mass. This is because of a halo of lower density ionized material which often contributes only a small fraction of the PN luminosity even though it may contain many times the mass of the dense inner shell. The precipitous drop in surface brightness (both optical and radio) beyond the inner part of the ionized shell also lead to underestimates of the PN's actual ionized radius. Since the evolution of PNs is driven by the expansion of the nebular shell coupled with the evolution of the nucleus (PNN), we ran several simulations using a simple momentum conserving two-wind model as well as employing density profiles derived by more sophisticated energy conserving models, with a wide range of wind parameters and using two different models of PNN evolution. From our simulations (assuming a 4 M⊙ progenitor) we derive an apparent “Shklovsky Mass” – defined as the ionized mass that would be derived from the observationally determined fluxes and radii of our model PNs. While the total ionized masses and Strömgren radii of the model PNs varied widely depending on the PNN and wind parameters, the derived Shklovsky mass consistently remained below one solar mass. This result is almost independent of the total ionized mass or the mass of the progenitor envelope and is fairly insensitive to the wind parameters chosen as input to the models. The observed spread of masses (based on an error analysis of the work of Gathier, 1987) is similar to the mass dispersion in our models for PNs of moderate age. This may explain why the Shklovsky distance method has been found to agree well with kinematic distances (Schneider and Terzian, 1983) even though the fundamental assumptions may be inappropriate for the nebula as a whole.
Spatial Variations in UV-Optical Lines Across the Ring Nebula
Authors: Dufour R.J. , Quigley R.
Published: June 2019

IUE spectra of the Ring Nebula (M57=NGC 6720) were taken 16–18 May 1991 using the large aperture (10 × 20 arc sec oval) at low dispersion. SWP and LWP spectra of seven locations were acquired at 10 arc sec intervals extending along a PA = 124° line and passing through the central star of the nebula. This direction was such that the long axis of the large aperture was aligned and overlapped, thus enabling the line profiles to be “spliced” together. This provided continuous spatial variation curves at approximately 1 arc sec spatial resolution over the entire diameter (80 arc sec) of the main body of the Ring Nebula.
Extended Nebulae Around WC11 Stars: IRAS 17514-1555
Authors: Pollacco D.L.
Published: June 2019

Imaging and long–slit spectroscopy of the nebula surrounding the WC11 star IRAS 17514–1555 are presented that allow, for the first time, the nebula to be studied without contamination from the underlying WC11 star. These data show that the nebula is photoionized and does not appear to have an obvious scattering component. The nebula consists of two shells: a low density outer region (diameter 18 arcsec) where ne ∼ 100cm−3 and an unresolved inner nebula (diameter < 1.5 arcsec) where ne > 104cm−3. The abundances in the nebula are similar to those found in Type I PN (He and N enriched). Simple models are constructed which demonstrate that the nebula is probably in equilibrium with the central star (T∗ ∼ 30,000 K). The nebular and stellar properties are compared to those of other late–type WC stars and the evolutionary status of IRAS 17514–1555 discussed.
Photodissociation Regions in Planetary Nebulae
Authors: Escalante V. , Góngora-T A.
Published: June 2019

The photodissociation rate of H2 molecules by UV photons from PN central stars is generally several orders of magnitude larger than the rate produced by the average interstellar field (Sternberg 1988, Escalante et al. 1991). Thus, in neutral envelopes of PN's, H2 molecules are destroyed quickly, and a photodissociation region forms around ionization bounded nebulae. Observations of H2 in PN's reveal that not all the hydrogen is photodissociated, and it has been suggested that this is due to the existence of dense disks around the ionized region (Zuckerman and Gatley 1988).
Axisymmetric Dust-Shells in Planetary Nebulae
Authors: Hopfensitz W. , Grewing M.
Published: June 2019

Aspherical mass loss is likely to be a common feature in the late stages of stellar evolution when the star arrives at the asymptotic giant branch. In the transition phase from the red giant to a proto-planetary nebula (PPN) the star developes an extended circumstellar dust-shell which is in many cases ellipsoid or disk-shaped. An algorithm for treating the radiative transfer problem in dense axisymetric dust shells is presented. The program has been used to determine the temperature distribution in disk-shaped dust envelopes in PNe. The implications of both composition and the spatial structure of the dust shell has been investigated. We confined the viewing angle to the equatorial plane to maximize the effects of the disk geometry. Our computations show that the observed morphology is mainly affected by the geometry of the envelope whereas the integrated spectrum is mainly determined by the characteristics of the grains themselves. The observed morphology in the optical and near infrared wavelength region is determined by scattered starlight in the thin polar region where the optical depth is low both to the the observer and to the central star whereas scattered light in the equatorial plane is blocked by the optical thick disk. This results in the familiar hourglass shaped morphology perpendicular to the orientation of the disk. Towards longer wavelengths the disk becomes more transparent thus the bipolar shape becomes less prominent. At wavelength longer than 5 μm the disk appears as a compact round source produced by scattered light at the inner rim of the disk. In the Mid and Far Infrared thermal emission of dust is the dominant emission process. Observational evidence for dust disks in PNe can be found in several wavelength regions. For example M 2–9 has been observed by Aspin (1984) in the optical region with a CCD Imaging Polarimeter. The polarisation maps show a high degree of polarisation in the lobes whereas emission from the equatorial plane is little or not polarized. In the NIR the change of the appearance from the bipolar to the compact round morphology predicted by our models is confirmed by the presented images in the J (1.2 μm), H(1.6 μm) and K(2.2 μm) filters obtained with the ESO InfraRed Array Camera. This confirms the general picture of M 2-9 as a proto-planetary nebula being surrounded by an extended, optically thick dust disk.
On the Possible Relationship Between the Photometric Parameters of the AGB Stars and Their Evolutionary Status
Authors: Kudashkina L.S. , Andronov I.L.
Published: June 2019

The statistical study of the Long-Period Variables, provided by various authors, show, that there is a relationship between the photometric characteristics and the physical properties. For example, the asymmetry of the light curve may be related with the strength of the shock wave. The presence of the maser-fed line emission characterizes the definite evolutionary stage of the Long-Period Variable.
Ice Mantle Formation in the Envelopes of OH/IR Stars
Authors: Charnley S.B. , Smith R.G.
Published: June 2019

We have computed ice column densities for a sample of O-rich late-type stars.

Water ice is observed in the outflows from several late-type stars (e.g. Smith et al. 1988) and it is of some observational and theoretical importance to understand how common ice mantles may be and how their formation depends upon the physical conditions in the envelope, such as the radial distributions of density and temperature. We have developed a simple theory of the gas-grain interaction in circumstellar shells (Jura & Morris 1985; Charnley et al. 1992) and used it to compute ice column densities (Nice) and mantle thicknesses (d) for a sample of well-studied OH/IR stars (Herman & Habing 1985). The models are most sensitive to the dust and gas mass loss rates, as well as to the inner radius of the envelope, Ri, which is poorly determined. Representative results are presented in the Table below where Ri was estimated according to the prescription of Herman et al. (1986).
On the Evolution of Proto-Planetary Nebulae
Authors: Hu J.Y. , Jiang B.W. , Slijkhuis S.
Published: June 2019

The ages of PNNe in our sample (Hu et al., 1990) were derived from the inner radius of the dust shell using the radiative transfer model fitting with SED and expansion velocities of envelopes obtained from radio molecular lines observations (if without radio data a Vexp of 15 km/sec was assumed). The objects are classified into the following groups:

1.

invisible;

2.

M-type;

3.

F (or G)-type; and

4.

F (or G)-type with Ra emission.
Mid-Infrared Spectroscopy of Four 21μm Emission Band Sources
Authors: Justtanont K. , Barlow M. J. , Skinner C. J.
Published: June 2019

We report 10 and 20μm spectroscopic observations of four C–rich post–AGB objects which exhibit the unidentified emission feature at 21μm. The observations were carried out in October 1990 and May 1991 using CGS3 on UKIRT. The spectral resolutions were 70 for the wavelength range of 7.4–13.3μm and 80 for the region between 15.4–24.1μm. Three of the sources reported here are from the list of Kwok, Volk & Hrivnak (1989), i.e., IRAS 04296+3429; IRAS 07134+1005 and IRAS 22272+5435. Figure 1 shows the full spectrum of IRAS 04296+3429 and IRAS 22272+5435. The 10μm spectra of these objects exhibit UIR bands whose peaks all fall longwards of the usual peak wavelengths associated with such features. This may be related to the fact that they are the lowest excitation objects so far found to exhibit UIR emission bands. We also found narrow emission features superimposed on the long wavelength wing of the 21μm emission bands of IRAS 04296+3429 and IRAS 22272+5435. The fourth object we observed, SAO 163075, was found to also exhibit a (weak) 21μm emission feature. However, there is no PAH features in the 10μm region, apart from the plateau at 12μm.
A Spectroscopic Search for Hot (B-Type) Post-AGB Stars
Authors: Conlon E.S.
Published: June 2019

At Queen's University, we have been undertaking a spectroscopic programme to elucidate the nature of faint blue stars at high galactic latitudes. We have identified approximately 50 that appear to spectroscopically identical (even at high resolution and signal-to-noise) to normal young Population I B-type stars in the galactic disc. However, we have also found seven faint objects () that were previously classified as Population I on the basis of photometry and/or low resolution spectroscopy; careful model atmosphere analyses of high resolution spectra now indicate that they have non-Population I compositions. Their derived atmospheric parameters are coincident with theoretical post-AGB evolutionary tracks and thus together with their peculiar composition, they would appear to be hot evolved post-AGB objects.
Angular Momentum Loss in Post-Main Sequence Stellar Evolution Through the PN Stage
Authors: Villata M.
Published: June 2019

A simple analytical model can explain the large angular momentum loss which a star suffers in its post-main-sequence evolution up to the white dwarf stage.
The Magnetic Fields in the Envelopes of Proto-Planetary Nebulae
Authors: Hu J.Y.
Published: June 2019

It is possible that the magnetic field plays important role in the formation of planetary nebulae(Poscoli, 1992). In order to measure the strength of magnetic field in the envelope of protoplanetary nebulae(PPNe) we have used the Max-Planck-Institut fur Radioastronomie 100-m telescope at Effelsberg to obtain the high frequency resolution and high signal-to-noise ratio 1612 MHz spectra of PPNe, IRAS08005-2356, 18276-1431, and 20406+2953 in both circular polarization. The nature of PPN of these objects are confirmed by Slikhuis et al.(1991), Le Bertre(1987), and Hu et al.(1992) based on the extensive optical, infrared and radio molecular line observations.
On Bipolar Jet Formation in Planetary Nebulae
Authors: Pascoli G.
Published: June 2019

Bipolar outflows or polar knots are rather ubiquitous phenomena. A wealth of observational data is now available on bipolar flows associated with young stellar objects or stellar formation regions (Snell, Proc. I. A. U. Symp. Nr. 115, 1987), or nuclei of active galaxies (Asseo and Sol, 1987, Phys. Rep. 148, 307).
A Detailed Study of the Galactic Planetary Nebula G 258-15.7
Authors: Leisy P. , Dennefeld M.
Published: June 2019

The galactic Planetary Nebula G 258-15.7 is a large, bright nebula well suited for a detailed study. Known since Wray (1966), its morphology presents several blobs and ansae, generally associated with type I nebulae, and could be described as “late-butterfly” type according to the classification by Balick (1989). The central star has been classified as hydrogen-deficient by Mendez et al. (1985). Spectroscopy of the two main blobs shows a clear overabundance in He and N, with a marginally significant difference between the two sides. The most striking feature is the jet-like structure appearing on the [OIII]/Halpha picture (Fig. 1), the “jets” being located within the main blobs seen on the monochromatic images. A detailed appraisal of all the data will be presented in a subsequent paper.
The Central Region of the Planetary Nebula A58
Authors: Pollacco D.L. , Hill P.W. , Clegg R.E.S.
Published: June 2019

We present images and high–resolution spectra of the hydrogen deficient knot at the centre of the old planetary nebula A58. The spectra confirm that this region contains essentially no hydrogen, as previously suspected. Emission lines from the knot are broad (FWHM ∼ 180 and 270 km/s for [NII] and [OIII] lines respectively) and are blue–shifted by ∼100 km/s relative to the systematic velocity.
Observational Studies of Close Binary Central Stars of Planetary Nebulae: HFG 1 and a 63
Authors: Malasan Hakim Lufthfi , Yamasaki Atsuma
Published: June 2019

We have made spectroscopic and photometric observations of two close binary central stars of planetary nebulae HFG 1 (V 664 Cas) and A 63 (UU Sge), using the UH 2.2–m telescope (Mauna Kea Observatory) and the NAO 1.9–m and 0.9–m telescopes (Okayma Astrophysical Observatory).
Imaging and Spectroscopy of Abell 63 (UU SGE)
Authors: Walton N.A. , Walsh J.R. , Pottasch S.R.
Published: June 2019

UU Sge, the eclipsing binary central star (Bond et al, 1978) of the low-surface-brightness planetary nebula (PN) Abell 63, has been observed spectroscopically in the visible throughout its 11.2 hour period and especially during the minimum. A spectral determination of the binary system has been made. The primary hot central star is an ‘O’ type PN nucleus of temperature ≈40,000 K, consistent with the low excitation of the nebular spectrum (e.g. no He ii 4686Å nebular emission detected). From the spectrum at minimum light, the secondary star appears to be a cool dwarf star around G7. Measurement of the magnitude of the secondary during the eclipse of the primary enabled the distance to the PN to be directly determined as 3.6 kpc. For this distance the luminosity of the hot star is approximately 4 320 L⊙, in good agreement with evolutionary tracks for (single) PN nuclei. Deep CCD images of Abell 63 show it has a ‘butterfly’ morphology implying that the close binary central system may have had a strong effect on the nebula shaping. The paper describing this work has been submitted (Walton et al, 1992).
On the Dereddening of Symbiotic Stars
Authors: Raykova D. , Raytchev B.
Published: June 2019

In studying low dispersion spectra of symbiotic stars and following the usual dereddening procedure we obtain from different pairs of the Balmer lines Hα, Hβ and Hγ considerably different values for the interstellar absorption C at Hβ. There is a tendency the values to be Cαβ > Cαγ > Cβγ. For one of the objects we obtained Cαβ = 2.03, Cαγ = 1.51, Cβγ = 0. The mean values for a sample of 13 objects with measured Hγ are Cαβ = 1.82, Cαγ = 1.63 and Cβγ = 1.13. That could be caused by processes taking place in the emitting gas and neglected in the theory as well as by nonlinearity of the detector system. Selfabsorption in the Balmer lines and collisional excitation could affect in different ways the Balmer decrement.
Diagrams for Observational Testing of Evolution of Planetary Nebula Nuclei
Authors: Tylenda R.
Published: June 2019

Confrontation of theoretical models with observations is the main way of progress of our understanding of the nature of astrophysical objects. This paper discusses different diagrams used for observational testing of theoretical evolutionary models of planetary nebula nuclei (PNNi). Particular attention is paid in indicating and discussing sources of errors and uncertainties which may result in wrong conclusions drawn from the diagrams. Finally, results of recent studies comparing the theory and observations for the Galactic PNNi are critically reviewed.
White Dwarf Central Stars
Authors: Liebert James W.
Published: June 2019

Planetary nebula nuclei on lower luminosity, diagonal tracks in the HR Diagrams are degenerate dwarf stars close to their final radii. For several reasons, these stars have until recently been difficult to identify and study. With the advent of new techniques and technologies, both hydrogen–rich and hydrogen–poor atmospheric sequences have been found.
On the Relation of Core Mass by Chemical Composition in PN
Authors: Pottasch S.R.
Published: June 2019

The attempts in the literature to find a relation between planetary nebula core mass and the nebular abundance ratios N/O and He/H are described. A critical discussion concludes that the evidence for such a relation is weak.
Hydrogen and Helium Burning Evolutionary Tracks
Authors: Wood P.R. , Vassiliadis E.
Published: June 2019

We discuss the effect of metallicity on the luminosities of planetary nebula nuclei (PNNi), and we use theoretical stellar evolution calculations to predict the fraction of PNNi that enter the planetary nebula domain burning helium rather than hydrogen. Both these factors will clearly influence the planetary nebula luminosity function.
Planetary Nebula Evolution Traced by Distance-Independent Parameters
Authors: Zhang C.Y. , Kwok S.
Published: June 2019

Making use of the results from recent infrared and radio surveys of planetary nebulae, we have selected 431 nebulae to form a sample where a number of distance-independent parameters (e.g., Tb, Td, I60μm and IRE) can be constructed. In addition, we also made use of other distance-independent parameters ne and T∗ where recent measurements are available. We have investigated the relationships among these parameters in the context of a coupled evolution model of the nebula and the central star. We find that most of the observed data in fact lie within the area covered by the model tracks, therefore lending strong support to the correctness of the model. Most interestingly, we find that the evolutionary tracks for nebulae with central stars of different core masses can be separated in a Tb-T∗ plane. This implies that the core masses and ages of the central stars can be determined completely independent of distance assumptions. The core masses and ages have been obtained for 302 central stars with previously determined central-star temperatures. We find that the mass distribution of the central stars strongly peaks at 0.6 M⊙, with 66% of the sample having masses <0.64 MM⊙. The luminosities of the central stars are then derived from their positions in the HR diagram according to their core masses and central star temperatures. If this method of mass (and luminosity) determination turns out to be accurate, we can bypass the extremely unreliable estimates for distances, and will be able to derive other physical properties of planetary nebulae.
O VI Central Stars of Planetary Nebulae: NGC 2371
Authors: Stanghellini L. , Kaler J.B. , Shaw R.A.
Published: June 2019

We performed detailed spectral analysis of the planetary nebula NGC 2371 and its nucleus. The central star of NGC 2371 is a member of the 0 VI PNNi class, and it shows luminosity variations (Bond & Ciardullo 1990, ASP Conf. Ser. vol 11, Confrontation between Stellar Pulsation and Evolution, C. Cacciari & G. Clementini (eds.), p. 529) that could be associated to nonradial pulsations. From the spectrum of NGC 2371 we calculate the nebular parameters and the abundance of the most prominent ions. The nucleus is hot (Teff ≥ 120,000K) and luminous (V=15.4), it is probably close to the blue bend of the post-AGB evolutionary sequence, and shows prominent O VI emission with (so far) unique double narrow-broad feature, as shown in the Figures. This feature can not be related to the shock front of the fast wind into the planetary nebula. This star has been analyzed together with the other O VI PNNi known. A preliminary correlation between the stellar parameters and the total O VI strength have been derived, and the locus of NGC 2371 on the logTeff – logL/L⊙ plane have been studied in relation to the locus of the post-AGB nonradial instability strips. These results, together with a study on the other O VI central stars of planetary nebulae, will be published in the near future.
Discovery of a Planetary Nebula Associated by the White Dwarf GD 561
Authors: Napiwotzki R. , Schönberner D.
Published: June 2019

The search for old planetary nebulae around hot white dwarfs, PG 1159 stars, and sdO stars is subject of intensive studies (e.g. Kwitter et al. 1989). We report here on the discovery of such a nebula around the white dwarf GD 561.
Kinematics of Disk Planetary Nebulae
Authors: Maciel W.J. , Dutra C.M.
Published: June 2019

In the past few years, it has become clear that planetary nebulae (PN) are a true stage in stellar evolution, especially regarding their chemical composition. This fact led to the introduction of the classification scheme developed by Peimbert, which has been applied to a sample of galactic objects for which a detailed amount of data exists.
Determination of Element Abundances in Planetary Nebulae from Recombination Line Spectra
Authors: Nikitin A.A. , Kholtygin A.F. , Sapar A.A. , Feklistova T. Kh.
Published: June 2019

Carbon, nitrogen and oxygen abundances have been found from the recombination lines for 63 planetary nebulae.
Hipparcos, Three Years After Launch
Authors: Kovalevsky J. , Froeschlé M.
Published: June 2019

In a first part, the present status of the HIPPARCOS mission is described. Despite the degradations and failures of gyroscopes, it is still hoped that a 4 1/2 mission duration will be reached. The first-year of data has been reduced by both FAST and NDAC consortia. For the best 46200 observed stars, the distribution of standard errors in positions has a maximum of 1.5 mas in latitude and 1.8 mas in longitude and the mean standard error for parallaxes is of the order of 3 mas. The comparison of results obtained by both consortia shows that the differences are small and quite consistent with the announced internal precisions. Magnitude measurements are precise to 0.02 magnitude for a 4 second observation. The precision to be expected for double star observations is also given. The main new result is that the magnitudes of the components are obtained with a few hundredths of a magnitude precision. This allows to devise a new method of mass determination based upon the parallax and a recalibrated mass-luminosity diagram. The parallax dependence of the results is much more favourable than in the case of the classical determination of masses using orbital motions.
Analysis of Hipparcos Measurements: Positions, Proper Motions, Parallaxes
Authors: Walter H.G. , Froeschlé M. , Falin J. L. , Hering R. , Kovalevsky J. , Lenhardt H.
Published: June 2019

Using the first 12 months of HIPPARCOS mission data already reduced, a synthesis solution was obtained by both CERGA and ARI teams. The reduction includes two steps. In the first one the origins of individual reference circles are determined, thus constituting the HIPPARCOS celestial reference frame. This solution uses a subset of some 30 000 bright and evidently single stars in order to minimize possible corruptive effects on the system. In the second step, the abscissa measurements of all observed stars are substituted in the system and then processed star by star in a least squares adjustment providing the astrometric parameters (positions and parallaxes).

The results of sphere solution and astrometric parameter determination are discussed. In this context error ellipses and internal errors are calculated. External errors are derived by comparison with suitable astrometric catalogues. Especially, for estimating the external errors of the HIPPARCOS measured FK5 stars the instrumental system was tied to the FK5 system by rigid rotations followed by comparisons of the positions with those of FK5. The mean internal errors of the HIPPARCOS positions of FK5 stars is 1.5 milliarcseconds (mas) while the standard deviation of the differences HIPPARCOS minus FK5 reaches 90 mas. For the 5022 measured parallaxes the mean internal and external errors are 3.8 mas and 16.7 mas, respectively. — The results of precision and comparisons with external data show that the accuracy predicted for the final catalogue can be reached.
Perturbing Effects in Past Optical Astrometric Data. Statistical Modelisation and Separation
Authors: Bougeard M. L.
Published: June 2019

Past optical astrometric observations are to be re-reduced in order to get a new evaluation of the Earth Orientation Parameters in the more accurate Hipparcos reference frame. Among the selected instruments is the Paris astrolabe, considered here. In this Paper, through multivariate statistical procedures, we deal with the preliminary step of the new evaluation which consists in detection and separation of sources of significant inconsistency and outstanding errors in the Paris reductions so far performed. The analysis is performed over two test periods — one per instrumental setting-that are compared in particular in terms of observer effects, magnitude, colour and sidereal time effects. The Paper also gives a synthetic overview on the statistical methods we used to obtain the main results so that they can be applied to other astrometric data.
Very-Long-Baseline Interferometry Applied to Geophysics
Authors: Carter W. E. , Robertson D. S.
Published: June 2019

Very-long-baseline Interferometry (VLBI) has opened for study a broad new spectrum of geophysical phenomena including: direct observation of the tectonic motions and deformations of the Earth's crustal plates, observations of unprecedented detail of the variations in the rotation of the Earth, and direct measurement of the elastic deformations of the Earth in response to tidal forces. These new measurements have placed significant constraints on models of the interior structure of the Earth; for example, measurements of the variations in the Earth's nutation have been shown to be particularly sensitive to the shape of the core-mantle boundary. The VLBI measurements will allow us to construct a global reference frame accurate at the centimeter level. Such a frame will be essential to studying long-term global changes, especially those changes related to sea-level variations as recorded by tide gauge measurements.
The Mark IV VLBI Data-Acquisition and Correlation System
Authors: Whitney Alan R.
Published: June 2019

Modern VLBI observations for both astronomy and geodesy continue to demand the utmost in sensitivity. Of the methods potentially available for increasing the sensitivity of continuum VLBI observations, increasing the recorded bandwidth is generally the most cost effective.

Over the past two years a broadly-supported program has been underway at Haystack Observatory to increase the sensitivity of the Mark IIIA VLBI system by more than a factor of 2. The result is an upgrade to the existing Mark IIIA data-acquisition system, dubbed Mark IV, which increases the maximum data rate to 1024 Mbits/sec, more than quadrupling the maximum data-rate of the Mark IIIA.

A new correlator, based on a new custom VLSI correlator chip is also being designed to support the 1 Gbit/sec data rates from the Mark IV data-acquisition-system. An international collaborative effort is being mounted to help defray the high costs of development.
Astrometry and Geodesy from VLBI
Authors: Ma C. , Russell J. L.
Published: June 2019

Dual frequency Mark III VLBI observations acquired since 1979 by several geodetic and astrometric observing programs have been used to establish precise celestial and terrestrial reference frames. The program to establish a uniformly distributed celestial reference frame of ∼400 compact radio sources with optical counterparts was begun in 1987. Some 700 sources have been considered as part of this effort and a preliminary list of ∼400 has been observed. At present, 308 sources have formal 1σ errors less than 1 mas in right ascension and 308 have similar precision in declination. The astrometric results include some data acquired for geodetic purposes. The geodetic results using data to September, 1992 include the positions of 105 sites with formal 1σ horizontal errors generally less than 1 cm at 1992.6 and the velocities of 64 sites with formal 1σ horizontal errors generally better than 2 mm/yr.
Determining Ionospheric Total Electron Content in Using Space VLBI
Authors: Zhi-Han Qian , Yong Zheng
Published: June 2019

For the first time, a new concept of space VLBI's application is presented, in which space VLBI technique is used to sound ionospheric electron content along the ray path.
Precession and Stellar Kinematic Parameters from the Proper Motions of the AGK3U
Authors: Xue Wei , Wenjin Jing , Tongqi Xu
Published: June 2019

Precessional corrections and stellar kinematic parameters have been derived from the proper motions of 142,171 stars in the updated version of AGK3 (AGK3U). The sky has been divided into 1332 small areas, in which mean proper motions for each component are formed for different magnitude intervals, with and without exclusion of fast stars. Solutions were performed for various kinds of the mean proper motions. Maximum-likelihood algorithm is used to take into account the ellipsoidal distribution of residual velocities. The results for precessional correction are, Δn = 0″.47 ± 0″.03 /cy, Δk = −0″.35 ± 0″.02 /cy, and the results for Oort's constants are, A = 9.5 ± 1.5 km/s/pc, B = −7.5 ± 1.5 km/s/pc.
Long-Term Variations of Earth's Orbital Geometry and its Effect on the Climate System
Authors: Hua Yingmin
Published: June 2019

Chinese Loess data over the past 2.4 million years are good proxy records for understanding past climate. Maximum entropy spectrum of magnectic susceptibility data at the locality of Luochuan, which is in Shaanxi province of China, have shown periodic fluctuations of about 21 kyr, 41 kyr, 100 kyr and 400 kyr. These periodic fluctuations are related to the long-term variations of earth's orbital elements of the obliquity, the climatic precession and the eccentricity respectively. The data series was separated into two parts to examine the variations of these fluctuations. The first part is from present to 1.1 Myr B.P.(i.e. million years before present), the second part is from 1.1 Myr B.P. to 2.2 Myr B.P. The periodic fluctuation of 93.4 kyr has the maximum amplitude in the first part of the data series, while the periodic fluctuation of 413 kyr has the maximum amplitude in the second part. The fluctuation about the 400 kyr was found in both parts of the data series.
50-Day Oscillation of the Earth Rotation and Solar Activity
Authors: Zhian Li , Li Chen , Qiao Lin
Published: June 2019

Djurovic et. al. detected 50-Day oscillation of the daily Wolf numbers and the daily apparent sunspot areas. We analysed the 50-Day oscillation and discovered the closer relations between the El Nino events and the 50-Day oscillation of solar activity and earth rotation. To verify the relation, in the present work, we collect longer and more data series. The result confirms that the El Nino events and the 50-Day oscillation of earth rotation are caused by the 50-Day fluctuation of solar activity.
Accuracy Assessment of Hipparcos Optical and VLA Radio Positions of Stars
Authors: Walter H. G. , Hering R. , Lenhardt H. , deVegt Chr. , Florkowski D.R. , Johnston K. J.
Published: June 2019

Optical positions of some 30 radio stars derived from 12 months of HIPPARCOS measurements are compared with their radio positions obtained with the Very Large Array (VLA). — Once the lengths of arcs between optical and radio positions of pairs of stars are calculated the differences of the arcs are formed. They provide an estimate of the coincidence of the optical and radio emission centres. — From the comparison of optical and radio positions infinitesimal rotation angles of the HIPPARCOS frame with respect to the VLA extragalactic reference frame are determined by rigid rotations. After taking account of the relative orientation of the frames the standard deviations of the remaining residuals are approximately of the order of the VLA observation errors, thus demonstrating the reliability of the HIPPARCOS results. However, they also indicate some data noise very likely caused by the low accuracy of optical proper motions used to bridge the HIPPARCOS-radio epoch differences up to 9 years, and possible discrepancies of radio-optical emission centres of some stars.
Priori Estimates of the FK5 Covariance Functions
Authors: Gubanov V. S. , Titov O. A.
Published: June 2019

Covariance functions of individual differences of positions and proper motions of stars in the FK5 and FK4 catalogues are given. The application of these functions for the study of the FK5 system and for its further improvement are discussed.
Realization of a Regional Terrestrial Reference Frame: A GPS Campaign
Authors: Wei Ziqing
Published: June 2019

A terrestrial reference frame can be defined by the coordinates of a set of stations on the surface of the earth. It is possible to realize a reference frame through the GPS positioning. In this sense two basic approaches are available to realizing a terrestrial reference frame using GPS: absolute and relative. By the absolute approach we mean the point positioning, whereas by the relative approach is meant the relative positioning. At the present day the absolute approach should realize a reference frame at a meter or submeter level precision. By contrast, the relative approach could realize a reference frame with a centimeter or even better precision.
Summary of a contribution on “the Magellanic Clouds”
Authors: Feast M. W. , Thackeray A. D. , Wesselink A. J.
Published: June 2019

Combined spectroscopic and photo-electric observations of bright stars in the two Magellanic Clouds show that:

1)

There is no marked difference between the stars observed in the two Clouds when compared with each other or with normal galactic supergiants, as judged by:

(a)

direct spectroscopic comparison,

(b)

Hertzsprung-Russell diagram,

(c)

U, B, V plot.

2)

Stars observed in both Clouds suffer a small amount of absorption, the precise amount depending on the intrinsic colours adopted. How much of the absorption takes place within the Clouds remains to be determined.

3)

A reddening path has been determined by comparison of Cloud stars (little reddened) and galactic supergiants (heavily reddened). The observations are consistent with a single reddening path and have not yet suggested any difference in the absorbing properties of dust in the Clouds and in the Galaxy. A conclusion on this latter point must await observations of heavily reddened stars in the Clouds.
The Manifold of Elliptical Galaxies
Authors: Djorgovski S.
Published: June 2019

Global properties of elliptical galaxies, such as the luminosity, radius, projected velocity dispersion, projected luminosity density, etc., form a two-dimensional family. This “fundamental plane” of elliptical galaxies can be defined by the velocity dispersion and mean surface brightness, and its thickness is presently given by the measurement error-bars only. This is indicative of a strong regularity in the process of galaxy formation. However, all morphological parameters which describe the shape of the distribution of light, and reflect dynamical anisotropies of stars, are completely independent from each other, and independent of the fundamental plane. The M/L ratios show only a small intrinsic scatter in a luminosity range spanning some four orders of magnitude; this suggests a constant fraction of the dark matter contribution in elliptical galaxies.
Compact Elliptical Galaxies
Authors: Nieto Jean-Luc , Prugniel Philippe
Published: June 2019

We summarize our present knowledge on low-mass high-surface brightness elliptical objects near massive galaxies that are often called M32-type or compact elliptical galaxies. The origin of the low mass of these objects is a controversial matter: is it intrinsic to their formation or produced, as classically believed, by tidal stripping from the massive neighbor? We present new observational data allowing to define better the characteristics of these objects and a simple theoretical model whose consequences support the idea that the precursors of compact ellipticals are related to the low-mass end of the luminosity function of elliptical galaxies.