bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Gravitational Field of the Galaxy in the Z Direction
Authors: Griffin R. F. 
Published: March 2016

This is a progress report of a project designed to find the component, perpendicular to the Galactic plane, of the gravitational potential of the Galaxy. The principle is to measure the radial velocities and distances of a large number of K-giant stars near the North Galactic Pole. My student G. A. Radford is masterminding the project; collaborating with us are Drs. J. E. Gunn of the Hale Observatories and L. Hansen and K. Gyldenkerne of Copenhagen.We have measured the radial velocities of all the HD stars of type KO and later, and many of the G5 stars, within 15°of the Galactic Pole, using the Cambridge photoelectric spectrometer. In addition, we have observed all the stars classified as K giants by Upgren in his declination zones 25° to 31°, using the spectrometer on the Hale telescope. There are about 900 stars observed altogether, including about 200 Upgren stars, running down to twelfth magnitude or so, which are not in the Henry Draper Catalogue. To determine the distances of all these stars we are now trying to determine the absolute magnitudes by narrow-band photoelectric photometry in the Copenhagen system. Most of the observations have been made, thanks largely to the very generous grants of observing time given by the Hale Observatories earlier this year; but the reductions have only been completed for about 300 stars (including 244 K giants) which were observed last year at Kitt Peak, and the present, very preliminary, discussion is based on those stars alone.
Concluding Remarks Part II
Authors: Blaauw A. 
Published: March 2016

In research in the galactic polar caps we may roughly distinguish two categories:I. Research on problems concerning the galactic population at large distances from the plane, including the density and velocity distribution and the field of force at large z.II. Research on problems concerning the population in the immediate neighbourhood of the sun; at high galactic latitudes it is easier to discriminate the nearest stars than at low latitudes - see, for in stance the investigations of the M dwarf population.Most of the papers presented in part II of this Joint Discussion deal with the first category, and of these, the distances reached are mostly within a few Kpc. Classical investigations, some of them dating from more than thirty years ago, have provided our current knowledge of the force K(z) perpendicular to the galactic plane up to about 1500 pc and the large scale features of the density distribution. What emerges from recent and current work, is attempts to improve the knowledge of K(z) and to extend it to greater distances; and refinement of the density and velocity investigations by more and more detailed discrimination of population types. The dominating problem is that of the separation of the stars according to two basic parameters: age and chemical composition. Kinematic and space distributional properties for subgroups according to these two parameters will be the necessary elements on which a theory of the (local) evolution of the Galaxy is to be built. The vague notion, that metal abundance may be taken to be an equivalent of age is abandoned and replaced by the recognition that at any epoch, star formation may have occurred with a considerable spread in the resulting stellar metal abundances.
Flows in Magnetic Flux Tubes
Authors: Durrant C. J. 
Published: March 2016

Harvey has reported in this discussion observations of mean downflows in magnetic elements that increase in speed with depth. It is therefore pertinent to ask what role such flows may play in the production and maintainance of the flux concentrations.Equilibrium ModelsThe first thing to be said is that such flows cannot directly influence the horizontal momentum balance, i.e. their dynamic pressure is negligible.A steady internal flow such as that proposed by Ribes and Unno (1975) for chromospheric concentration would require supersonic flows with very large curvature at photospheric levels. There is no evidence for transverse flows of such magnitude.
An Atlas of Ultraviolet Stellar Spectra
Authors: Code A. D. 
Published: March 2016

Ultraviolet stellar fluxes have been presented for 140 bright stars in the spectral region from 1200 A to 3600 Å in a graphical and tabular form (A. D. Code and M. R. Meade, 1976). The spectra represent a subset of 0A0-2 spectrometer data on file at the National Space Science Data Center. The monochromatic flux is given in units of ergs cm−2 sec−1 Å−1 with a spectral resolution of about 22 Angstroms in the region from 3600 Å to 1850 Å and approximately 12 Angstroms from 1850 Å to 1160 Å.The monochromatic flux, Fλ, is based upon the absolute calibration described by Bless, Code and Fairchild (1976).
The Role of General Relativity in Astronomy: Retrospect and Prospect
Authors: Chandrasekhar S. 
Published: March 2016

My last attendance at a meeting of the International Astronomical Union was forty-four years ago when it met in Paris in 1935. I do not doubt that my being asked to give an invited discourse at this meeting is a personal courtesy extended to me by your distinguished President recalling, perhaps, the years when he and I were colleagues together at the University of Chicago.I am aware that associated with my absence from these meetings for nearly half a century is the fact that during most of this period - if not all of it - my interests, at different times, have been outside whatever may have been the prevailing trends in the mainstream of astronomy. I am afraid that on this account, the point of view I shall present - retrospectively and prospectively - will not be in conformity with the trends currently prevailing. I must therefore begin by asking for your patience and for your forbearance.Dr. Blaauw, when he invited me to give one of the three discourses at this meeting, suggested that in selecting a topic I might wish to take into account the fact that this year is the centennial of Einstein’s birth. The subject of my discourse is in accordance with that suggestion.
A Formalism for Differential Rotation
Authors: Durney B. R. , Spruit H. C. 
Published: March 2016

One of the approaches for dealing with the differential rotation of the Sun is to separate the flow in the convection zone into an axisymmetric steady part (differential rotation plus meridional circulation) and a “turbulent” part comprising all smaller scales of motion. In the equations of motion and energy the effect of the small scales is then represented by a turbulent viscosity and conductivity. Theories of this type at present belong to either of two categories:a)Theories stressing the role of an anisotropic turbulent viscosity. Due to the presence of gravity as a preferred direction the velocity distribution in the turbulent flow is different in the horizontal and vertical directions. This implies that the turbulent viscosity is also anisotropic. Biermann (1951) showed that as a result, the convection zone cannot rotate as a solid body. Theories based on this idea were developed by Kippenhahn (1963) and Köhler (1970).b)Theories using a latitude dependence of the efficiency of turbulent transport of heat. In the deeper layers of the convection zone the influence of rotation on convection is strong because the convective turnover time is comparable to the rotation period. Since the angle between gravity and the rotation axis varies with latitude, this influence must also vary with latitude. This produces a temperature variation between the pole and the equator which drives a meridional circulation. Since the Coriolis force dominates over the viscous force in most of the convection zone, there is a strong differential rotation associated with this circulation (quasi geostrophic flow). This type of theory has been developed by Weiss (1965), Durney and Roxburgh (1971) and Belvedere and Paterno (1977).
Ultraviolet Spectroscopy of Chemically Peculiar Stars
Authors: Leckrone David S. 
Published: March 2016

This discussion focuses on high dispersion spectroscopic observations of chemically peculiar (CP) stars of the upper main sequence, classical HgMn, Si and SrCrEu stars, obtained with the IUE. Two subjects will be emphasized - the confirmation of composition anomalies previously identified in ground-based spectra and evidence relating to the validity of the diffusion theory for the production of chemical pecularities.The gallium anomaly occurs primarily in the hotter Mn stars (Cowley 1975). Model atmospheres analyses of Mn stars yield overabundances of Ga as large as 2×105 relative to the solar value (Heacox 1979). The identification of the Ga peculiarity and estimates of Ga abundances are based upon a few high excitation lines observed in the blue. Figure 1 illustrates the resonance line of GaII, λ1414.44, observed in six Mn stars. The feature is strong and broad in the hotter stars, к Cnc and μ Lep. It is present but weaker in 46 Dra (A+B). It is absent in the cooler stars г CrB, χ Lup and HR 4072. The sharp feature seen near λ1414.4 in the latter three stars is probably Mnll and is also seen in normal stars. The Gall λ1414.44 line varies in strength from star-to-star in the same manner as the blue Gall lines. The absence of the blue Gall lines in cool Mn stars cannot be an excitation effect, since the resonance line is also absent in these stars. Neither can the presence of the blue Gall lines in hotter Mn stars be due to non-LTE effects, because the resonance line is also abnormally strong in these stars. The IUE observations confirm the Ga peculiarity as a genuine abundance anomaly.
Stellar Coronae
Authors: Vaiana G. S. 
Published: March 2016

The standard theory of stellar coronae requires the presence of vigorous surface convection. In consequence, the expectation of such a theory is that stellar x-ray emission — if due to a corona — should be limited to a subset of stars (principally those of main sequence spectral types F and G), and therefore should be relatively rare. This theory also makes detailed predictions about coronal heating, which are subject to test if spatially resolved coronal data are available. We are now in a position to subject the standard coronal scenarios to observational scrutiny on both counts: Skylab and later observations have supplied us with spatially resolved data of the solar corona, while the succession of high-energy x-ray astronomy satellites, culminating with EINSTEIN, now gives us a long-awaited glimpse of stellar x-ray emission throughout the K-R diagram.I will maintain that these new data imply that coronal x-ray emission dominantly derives from plasma structure confined by stellar surface magnetic fields; that coronal heating is likely to be non-acoustic in character and involves the confining magnetic fields; that stellar x-ray emission is not well correlated with the level of surface convection activity. These results of course cast serious doubt upon the viability of the standard theory of stellar coronal formation. In the following, I will try to very briefly summarize the solar and stellar data, to present the context in which they were initially obtained, and very briefly sketch the new coronal picture we are pursuing. The results presented here are excerpted from lectures presented by R. Rosner and myself recently at Erice, Italy (viz. Vaiana 1979) and from the preliminary results of the EINSTEIN Stellar Survey (Vaiana et al. 1979). The latter, part of a larger effort in x-ray astronomy led by R. Giacconi, involves the work of many people, including F.R. Harnden, L. Golub, P. Gorenstein, R. Rosner, F. Seward, K. Topika at CFA, as well as a number of EINSTEIN guest investigators.
Types of Stellar Instabilities
Authors: Ledoux P. 
Published: March 2016

Aside from violent phenomena, regular forms of motions originate often in instabilities and the linear theory with terms ∞ exp (st) yields already significant information. The system, here a spherical star, will be the seat of an instability if R (s) > 0. In general, s will be complex as both conservative (adiabatic) and non-conservative (non-adiabatic) factors are present. However if the latter (small) are neglected, the eigen-values s2 often denoted -σ2 are real. If at least one σ2 < 0, then the star is dynamically unstable.Radial perturbations. If an appropriate average value Γ1 > 4/3, then all σ2 are positive. If Γ < 4/3 (formation phase: ionization; late evolution: nuclear equilibrium; degeneracy in white dwarfs and neutron stars or radiation in very large masses plus general relativistic effects) the fundamental eigenvalue of only becomes negative.
Planetary Nebulae Formation
Authors: Tuchman Y. , Sack N. , Barkat Z. 
Published: March 2016

It is a well accepted idea that Planetary Nebulae ( P.N. ) formation is due to mass ejection from red giant envelopes.According to the original model, as was proposed by Roxburgh and Lucy in 1967, and established about a year later by Paczynski and Ziolkowski (1968), red giant envelopes become dynamically unstable above a certain boundary luminosity, while their total energy, including the ionization energy, which should be available upon expansion, is positive. This energy is sufficient for ejecting the entire envelope with a velocity similar to those observed in P.N.In the light of these facts, it was quite natural to look for “one shot” ejection mechanisms in unstable red giant envelopes, using full non-adiabatic dynamical codes. This has been done several times since the adiabatic analysis carried out by Paczynski and Ziolkowski (1968), but with minor success, at least in those cases where natural initial conditions were used.It was found that as soon as the radial expansion of the envelope becomes comparable to its initial radius, the entire radiative zone, which is located above the partial ionization zone, turns to be almost completely transparent. Thus, most of the released recombination energy is radiated directly out of the star, instead of pushing mass shells outwards. Since this energy leakage is very fast, motion is quickly reversed without resulting in any mass loss. Nevertheless, it was pointed out by Smith and Rose (1972) and later by Wood (1974) that there is a possibility for some mass ejection due to shocks generated within the contracting envelope.
X-Ray Properties of Galactic Nuclei
Authors: Weedman Daniel W. 
Published: March 2016

Preparing this review was my just punishment for stating only two years ago - in another review (Weedman 1977) - that Seyfert galaxies are not strong X-ray sources. I said that because, as recently as three years ago, NGC 4151 was the only Seyfert galaxy known as an X-ray source. Now we have 36 Seyfert 1 galaxies, along with 12 other galaxies with strong emission-line nuclei, that are X-ray sources. And this is all without even having HEAO-2 data at our disposal yet. The study of active galactic nuclei with X-ray astronomy is progressing so rapidly that a reviewer feels almost hopeless. The best I can do is summarize what is known as of the summer of 1979 and give a simple overview of how X-ray and other properties relate.Some excellent reviews of the X-ray properties of Seyfert and other emission-line galaxies already exist. I especially recommend that by Andrew Wilson (1979). He provides very complete references as of a year ago, but X-ray astronomy is progressing so rapidly that he then had only somewhat more than half the active nuclei now in Tables 1 and 2. It was the group working with the Ariel V SSI that made the initial comprehensive X-ray studies of Seyfert galaxies (Ward et al. 1977, Elvis et al. 1978). The UHURU results for Seyfert galaxies followed soon after and are summarized by Tananbaum et al. (1978); the HEAO-A-2 survey results are now in press (Marshall et al. 1979) I have tried to incorporate these and other recent results in Tables 1 and 2.
Recent X-Ray Observations of Seyferts and Emission-Line Galaxies
Authors: Griffiths R. E. 
Published: March 2016

It has been just a few years since Type 1 Seyferts were established as a class of X-ray sources with luminosities in the range 1042 - 1045 ergs s-1 by Elvis et al. (1978) using data from the sky survey instrument on Ariel V, and by Tananbaum et al. (1978) using data from UHURU.The average error-box sizes for X-ray sources identified with Type 1 Seyferts in the 2A catalog (Cooke et al. 1978) is ˜ 0.4 sq. degrees, and ˜ 1.0 sq. degrees for those in the 4U catalog (Forman et al. 1978). Improvement in these positions has been made over the past two years by the modulation collimators on board the satellites SAS-3 and HEAO-1. In particular, the HEAO-1 scanning modulation collimator has been used to position a total of 20 X-ray sources, confirming the identification in each case, with the possible exception of Mkn 279 (Dower et al. 1979, Griffiths et al. 1979a). Of the 37 X-ray sources which were discovered prior to the launch of the Einstein Observatory and which have been associated with Type 1 Seyferts, 21 have been positioned to ˜ 1 arc minute, representing an improvement by factors of ˜ 20 to 100 over the previous 2A and 4U error box sizes. Some examples of the error boxes and identifications confirmed with the HEAO-1 scanning modulation collimator are shown in figs. 1 and 2. In fig. 1 both NGC 7213 (Philips 1979) and MCG - 2 - 58 - 22 (Ward et al. 1978) were discovered to be Seyferts by optical spectroscopy of candidate objects in the error regions of the corresponding X-ray sources. NGC 7213 is a Seyfert nucleus in a galaxy of Type SO (Philips 1979). In fig. 2, NGC 931 was likewise discovered to be a Seyfert as a result of its X-ray emission (Ward and Wilson 1978).
Energy Transport in Radio Sources
Authors: Scheuer P. A. G. 
Published: March 2016

Dr. Pacini has dealt with the innermost parsec; my job is to comment on the next 999,999. I have four comments.1. A reminder of the importance of using the bulk kinetic energy of whatever beam or jet there is - forms of energy that exert pressure will get lost on the way out because of adiabatic expansion (Longair et al. 1973).2. What is the asymmetry of jets trying to tell us?
Optical Observations of Radio Jets
Authors: van Breugel Wil , Miley George , Butcher Harvey R. 
Published: March 2016

Over the past several years a considerable body of evidence has accumulated, suggesting that extended radio sources are powered quasi-continuously from the nuclei of their parent galaxies. This view is supported by the recent discovery that several radio galaxies have narrow radio jets which connect their active nuclei with the large radio lobes and which often extend for several tens of kiloparsecs. Because of their presumed association with the energy transport outward from the active nuclei, radio jets are at present being intensively studied with high-resolution radio techniques.The closest galaxy known to have a radio jet is the giant elliptical M87 (e.g., Wilkinson 1974), and in this case there is a well-known optical counterpart (e.g., Curtis 1918; de Vaucouleurs, Angione and Fraser 1968), This optical jet is highly polarized (Baade 1956), implying that at least part of the emission is non-thermal. This and the good agreement between the optical and radio structure suggests that these features are closely related.
The European Astrometry Satellite, Hipparcos
Authors: Høg Erik 
Published: March 2016

AbstractA description of the instrument is given. It is expected to obtain positions, annual proper motions and parallaxes for 100 000 stars in a 2.5 years mission. The accuracy (s.e.) will be for stars of mB < 11 and about at mB=14. An extension of the mission to 3.5 years would improve the annual proper motion to for stars brighter than mB = 9.
The Impact of Star Parallaxes and Very Accurate Proper Motions on Galactic Structure and Dynamics Studies
Authors: Lindblad Per Olof 
Published: March 2016

Assuming that we have the absolute parallaxes and homogeneous set of proper motions for a large number of stars as described by the previous speakers - how should we use them in studies of galactic structure and dynamics?Concerning the parallaxes I will be very brief - the subject has already been discussed by Dr. Pagel. Clearly, the main importance of the absolute parallaxes obtained is in the calibration of photometric or spectrophotometric absolute magnitude scales. For a sufficient number of stars observed in the Hyades cluster at a distance of 50 pc and in the Scorpius-Centaurus association the systematic error, which lies at the bottom of all subsequent calibration of distance indicators, may be reduced to the same order as the accuracy of photometric measurements.
Stellar Atmospheres and Chemical Compositions of Galaxies
Authors: Aller Lawrence H. 
Published: March 2016

AbstractThe chemical compositions of external galaxies are usually found from their HII regions or composite absorption line spectra of their nuclei. In the Magellanic Clouds, however, individual stars are observable, but objects heretofore studied necessarily have been luminous la supergiants which border on the brink of instability. The image photon counting system on the Anglo-Australian Telescope makes it possible to observe fainter, more stable Ib supergiants. Energy distributions and Balmer line profiles for these stars can be fitted with theoretical predictions by Kurucz. A joint effort by J.E. Ross and B.J. O’Mara of the University of Queensland, Bruce Peterson of the Anglo-Australian Observatory, and a group at the University of California, Los Angeles to analyze three lb supergiants suggests that metals of the iron group are depleted by a factor of 2-3.5 with respect to the normal chemical composition of our own galaxy.
Rotation in Close Binary Stars
Authors: Plavec Mirek J. 
Published: March 2016

In binary stars of short period, axial rotation of the components tends to be synchronized with the orbital revolution. Rotation of B. and A stars is therefore slowed down, while for the later-type stars, it is accelerated. This latter fact probably contributes to the phenomenon of the RS CVn stars. Notable deviations from synchronism among short-period systems are probably related to mass transfer between the components.Binary stars play a double role in astrophysics. On the one hand, they reveal properties and structures that would otherwise be almost unobservable, and therefore help considerably to understand the character and evolution of single stars. On the other hand, they create objects and phenomena that otherwise would not exist at all, thereby actually creating more problems than they help to solve. I think that the first aspect makes the binary stars extremely useful; the other aspect makes them extremely exciting and interesting. This dual role also appears in connection with rotation. The “revealing” property is best documented by the fact that the phenomenon of stellar rotation was discovered in binary stars, through the “rotation effect” distorting the radial velocity curve of an eclipsed star. A more recent example is the probable identification of a star considerably flattened and distorted by differential rotation, in agreement with the theoretical models advanced by Bodenheimer and Ostriker (1970). The star is the secondary component of BM Orionis, which is the faintest of the four stars forming the famous Orion Trapezium. BM Orionis was once suspected of harboring a black hole, since during the apparently total eclipse of the primary B3 star, is spectrum remained visible and no other spectrum could be detected (Doremus, 1970). A higher dispersion of our Lick spectra permitted us (Popper and Plavec, 1976) to detect weak lines of the secondary star, which is most likely a late A giant still contracting to the main sequence. The flat bottom of the light curve at the primary eclipse is best explained by a model first suggested by Hall (1971), involving a disk-shaped, differentially rotating star. It would be difficult to recognize the shape of such a star if it were not member of an eclipsing system.