bluuebunny/crossref_metadata_2025 huggingface dataset:
The Theory of the Earth's Orientation, with Some New Results for Nutation
Authors: Wahr John M.
Published: June 2019

The rotation of the earth is variable at a number of time scales, from a few days to thousands of years and longer. Variations occur in the rotation rate, in the geographical position of the rotation axis (referred to as polar motion), and in the position of the axis relative to inertial space (referred to as nutation and precession). The interpretations of the various observations have implications for the dynamical behavior and structure of the earth's deep interior, and for various aspects of meteorology and oceanography. These are reviewed below. Also included is an Appendix describing a model of the diurnal resonance in nutation for a non-hydrostatically pre-stressed earth.
An Elasto-viscous Model of the Earth
Authors: Losito S. , Pernice B. , Picca D. , Verrone G.
Published: June 2019

A two-symmetric-rigid-rotators model of the Earth has been studied, under the hypothesis of elasto-viscous coupling. The free Eulerian equation of motion has been solved in the linear approximation related to small wobbling amplitudes. Under these hypotheses, polar motion is stable, and the angular velocity of the Earth is the sum of three vectors rotating with different frequencies and damped amplitudes. One of these terms turns out to be retrograde with a quasidiurnal frequency and could be identified with a similar term appearing in liquid core models of the Earth. The other two terms are identified with the Chandler wobble and the annual term according to observational data. The elastic coupling produces, in the time variation of L.O.D., a periodic term whose frequency is about one year. It could be hypothesized that the “decade fluctuation” could be partially attributed to the free oscillation of the Earth.
On the Error Equation for Determining Earth Rotation Parameters with VLBI Network
Authors: Xia Y. F.
Published: June 2019

In this paper, the partial derivative matrix in the error equation for determining earth rotation parameters (ERP) with the VLBI network is described, and the effect of the functional parameter constraint on solving ERP is discussed.
Stability of Determining the Earth Rotation Parameters (ERP) with VLBI
Authors: Luo Shifang , Zheng Dawei
Published: June 2019

By using the observations of IRIS network, the stability of determinang ERP with VLBI is studied. It is concluded that the uncertainties from initial values of ERP, the errors of other parameters are at the same level as the formal errors in determination of ERP. The geometric effect on determination of ERP is important and appears as systematic errors. Geometric uncertainty on polar motion is greater than that on UT1. and specially much worse for the continenal network. The stability of determining ERP with VLBI can be improved either by increasing new stations at reasonable location in a VLBI network or by increasing new networks.
QUASAT
Authors: Schilizzi R. T.
Published: June 2019

QUASAT was proposed to ESA as a joint ESA-NASA medium scale mission for space VLBI in December 1982. At the time of writing, four and a half years later, QUASAT has passed through an Assessment Study in ESA, a pre-Phase A study in NASA and has just entered an ESA Phase A study in European industry, supported by JPL and Australian and Canadian space interests. If QUASAT is selected to fly at the end of the Phase A study, it will be launched in 1995–1996. The scientific goals, mission concept and current status are reviewed this paper.
VLBI Activities in Japan and a Projected Space-VLBI Program
Authors: Hirabayashi H.
Published: June 2019

There are three institutes in Japan that are equipped with VLBI facilities. Of these, Nobeyama Radio Observatory started VLBI experiments with astrophysical and astrometric interests. The Institute of Space and Astronautical Science, the Nobeyama Radio Observatory, and the Radio Research Laboratory collaborated with JPL and other institutions for this first space-VLBI test experiment, generating fringes on ground-TDRSS satellite baselines at S band. A working group formed by scientists of the above three Japanese institutes proposed to ISAS to launch a space-VLBI satellite. The project, called VSOP, for VLBI Space Observatory Program, is described here.
The Canadian Geophysical Long Baseline Interferometer
Authors: Yen J.L. , Leone P. , Watson G.A. , Wiedfeldt R. , Zao J. , Cannon W.H. , Mathieu P. , Tan H. , Popelar J. , Galt J. A.
Published: June 2019

A new VLBI system has been developed and put into operation. The system has three distinctive characteristics as compared with current VLBI practice, namely, the use of a wavefront clock at each station, the capability of a burst mode operation with high speed sampling, and the use of multiple 12 Mb/s data recording channels on commercial VCR's. The main use of the system is to be in geophysical investigations where these new features may prove of value.
Milliarcsecond Imaging with a Large Optical Array
Authors: Shao M. , Colavita M. , Johnston K. , Simon R. , Mozurkewich D. , Hutter D. , Staelin D. , Hines B. , Hughes J. , Kaplan G. , Hershey J. , Townes C.
Published: June 2019

Considerable progress has been made over the last few years in ground based optical interferometery. The Mark III optical interferometer at Mount Wilson which at this time is operating with a 12 meter north south baseline is capable of measuring amplitude and phase. Experience gained from this facility will allow for the development of a large optical array. Preliminary plans are described for a facility that will be capable of milliarcsecond imaging of celestial objects at wavelengths from 0.4 to 10 microns. If funded this array will be in operation by 1993.
The Itapetinga Contribution on North-South High Resolution VLBI Experiments
Authors: Kaufmann P. , Abraham Z. , Bakor Y. , Georges C. B. , Scalise E. , Schaal R. E.
Published: June 2019

Since the very beginning of the Itapetinga 45-ft antenna operations in 1971, there were plans to use it as a southern hemisphere VLBI terminal for high north-south resolution. The first successful VLBI tests were performed on water vapor lines in cooperation with Haystack Observatory in 1978 in one of the last runs using the MkI system. This cooperation led to the construction of a MkII terminal for Itapetinga years later (1981–1983). Astrophysical VLBI runs have been carried out since 1984, with the cooperation of Caltech, producing maps of unique resolution on stronger quasars, especially on 3C273 whose evolution has been followed since then. Recently, Bonn joined the collaboration in astrophysical VLBI programs. Itapetinga also participated successfully in geodetic surveys, and in the Vega-Venus mission, in collaboration with JPL and French organizations CNES, Observatoire de Paris and IGN. Plans for continuation and expansion of the Itapetinga VLBI participation in international efforts depend strongly on support to maintain and upgrade instrumentation at the Brazilian Radio Observatory.
Global Fringe Fitting for Space Interferometry
Authors: Jones Dayton L.
Published: June 2019

The technique of global fringe fitting has proven to be very valuable for very-long-baseline interferometry (VLBI). It allows weaker fringes to be detected than is possible with conventional single-baseline fringe fitting algorithms, and thus improves the sensitivity of present VLBI arrays. Global fitting was developed for VLBI by Schwab and Cotton (1983, Astron. J.88, 688), and has been incorporated into the NRAO Astronomical Image Processing System as program VBFIT. As Schwab and Cotton point out, this is a generalized from of self-calibration in which closure relations for residual fringe delay and rate (frequency) are satisfied in addition to those for fringe phase and amplitude. These additional closure relations allow station-dependent delays and rates to be solved for, using data from all baselines with sufficiently strong fringes. Then it is possible to calculate where fringes will be on any less sensitive baselines between these telescopes, so that very narrow search windows in delay and rate can be used.
Atmospheric Limits: A Review of the Effect of Path Length Variations on the Coherence and Accuracy of VLBI
Authors: Rogers Alan E.E.
Published: June 2019

Interferometer phase fluctuations produced by path length variations in the troposphere limit the coherence to between 100 and 1000 seconds at a 3 millimeter wavelength. An expression for the coherence is given using an Allan variance to characterize the atmospheric phase fluctuations. Methods of optimizing the fringe detection threshold and calibrating the fringe visibilities under poor conditions of poor coherence are outlined. The astrometric and geodetic accuracy of VLBI is limited by our ability to calibrate the atmospheric path. Atmospheric “self-calibration” techniques which use the elevation dependence of interferometer delay to solve for the highly variable “wet” component of troposphere are discussed. Various models for the elevation dependence of atmospheric path or “mapping function” are reviewed. The accuracy limits imposed by the atmosphere are discussed.
The Second Byurakan Spectral Sky Survey, Quasistellar Objects and Seyfert Galaxies
Authors: Stepanian J.A. , Erastova L.K. , Lipovetsky V.A. , Shapovalova A.I.
Published: June 2019

Survey of surveys, and the place of the Second Byurakan Survey (SBS) among them is shortly discussed. Deep low-dispersion surveys cover > 10000 sq.degrees, but they managed to study only 15% of this area relatively well.

For 450 SBS objects the slit spectra were obtained on 6 m telescope of SAO. The nature of 120 QS0s,40 Sy galaxies and more than 200 ELG are confirmed. The results of the slit spectroscopy in six SBS fields covering commonly the area of ∼100 sq.degrees are presented.

All surveys, except Byurakan Surveys are extremely poor with Sy galaxies, that is their distinction from other surveys. Weak (16<m<18m5) SBS Sy galaxies sufficiently well filled in the interval between QSOs and Sy galaxies, the efficiency of selection does not much depend on redshifts. There is quite good pass from Sy galaxies to QSOs.
The Spatial Distribution of Seyfert Galaxies
Authors: Fairall A. P.
Published: June 2019

For some years, this author has suspected that certain small regions of the southern sky were “good fishing grounds” for finding Seyfert galaxies. It is now possible to follow up this suspicion, not only on the sky (see also Fairall 1979, Petrosian and Turatto 1986) but in three-dimensional space (with redshift assumed to indicate distance). Figure 1 (overleaf) is a sample of the plots obtained from a database of some 6200 galaxies, south of Declination −17.5° for which redshifts are available - including 199 Seyfert galaxies (Fairall and Jones 1988). The data is not ideal, since it lacks statistical control; but experience has shown it nevertheless to be reliable for galaxies in general, whilst it is hoped that few, if any, higher-luminosity Seyfert nuclei remain undetected.
A 12μm Flux Limited Sample of Galaxies: Preliminary Results on the IR Luminosity Function
Authors: Spinoglio L. , Malkan M. A.
Published: June 2019

An all-sky 12μm flux-limited sample of 392 galaxies has been selected from the IRAS Point Source Catalog. More than 20% of the sample harbor active nuclei (with Seyfert 1 or 2 or LINER emission-line spectra). Thus one byproduct of this work is the definition of a large complete sample of bright active galaxies, with roughly equal percentages of Sy 1's, Sy 2's and LINERs. Since we now have virtually all (93%) the redshifts for the sample galaxies, the far-infrared luminosity functions of all classes of galaxies have been derived using IRAS coadded data. Since our luminosity functions for Sy 1 and Sy 2 are indistinguishable from those of the optically selected CfA sample, the 12μm selection appears to be an efficient and complete technique for finding active galactic nuclei. Optical spectrophotometry and near-IR photometry of the sample is being obtained to compute accurate bolometric luminosities.
The Evolution of Starburst Galaxies to Active Galactic Nuclei
Authors: Scoville Nick , Norman Colin
Published: June 2019

Recent observations of luminous IRAS galaxies (LIR > 1011 L⊙) have revealed that virtually all are extremely rich in molecular gas with H2 abundances 5–20 times that of the Milky Way, and deep CCD imaging indicates that most are recent galactic mergers. Interferometric observations at millimeter wavelengths for five of these galaxies demonstrate that approximately half of the interstellar matter is contained in the central kpc. This gas concentration can result in the formation of a massive central star cluster. The deep potential of the central star cluster and the high density of interstellar gas ensure that virtually all of the gas lost during late stellar evolution sinks to the center of the cluster, building up a central, massive black hole. For a coeval star cluster of 4×109 M⊙, a central mass of approximately 1.5×109 M⊙ will accumulate within approximately 108 years and accretion at an average rate of 7 M⊙ yr−1 over this time will result in a mean accretion luminosity of 1013 L⊙. This luminosity, radiated at X-ray and uv wavelengths from the inner accretion disk ionizes the mass loss envelopes of the surrounding red giant stars providing an origin for the broad emission line regions of QSO's. The large linewidths would then be due to orbital motion in the massive central star cluster.
Helium I λ10830 Observations of Seyfert 2 Galaxies
Authors: Cohen Ross D. , Rudy Richard J. , Rossano George S. , Puetter R. C. , Chapman Scott D.
Published: June 2019

Because of the metastability of the 23S level of He I, a variety of effects can change the line strengths from pure recombination values in Seyfert galaxies (see Feldman and MacAlpine 1978). This occurs because the population which builds up in the 23S level can be collisionally excited to the 23P level, enhancing λ10830. The expected ratios of λ10830/λ5876 can be altered by internal or external reddening and vary with temperature, density and optical depth.
The Flux and Spectral Variability of NGC6814 as Observed with EXOSAT
Authors: Mittaz J.P.D. , Branduardi-Raymont G.
Published: June 2019

We observed the Seyfert galaxy NGC6814 five times with EXOSAT between September 1983 and October 1985. For a more detailed description of the data see Mittaz and Branduardi-Raymont (1988). Between observations both the LE (0.02 - 2.5 keV) and ME (2 - 6 keV) fluxes vary in the same sense, NGC6814 being brightest during observation 1 and weakest during observation 3 when it was undetected by the CMA. Both variable luminosity and variable low-energy absorption can explain this.
Power-Law Spectra from Fermi Acceleration at Relativistic Shocks
Authors: Kirk J. G.
Published: June 2019

The theory of diffusive acceleration at shock fronts, which applies only if the fluid speed is nonrelativistic, yields a simple formula for the power-law index s of accelerated particles: s = 3r/(r – 1), where r is the compression ratio of the shock front. Although the acceleration process depends on there being effective pitch-angle scattering of the particles in both the upstream and downstream regions, no property associated with this process appears in the formula. Unfortunately, if the velocity of the fluid through the shock front is relativistic, as seems to be the case in the central engines of AGN's, and also in some hot-spots in the outerparts of their jets, this attractive property ceases to hold. To find the index s, it becomes necessary to develop specific models describing the scattering process. The physical reason for this is that the particle distribution close to a relativistic shock is anisotropic. The exact type of anisotropy depends on the properties of the pitch angle scattering and determines both the average energy gain per shock crossing, as well as the probability of escape downstream. In this paper, results are presented for the pitch angle diffusion resulting from scattering in a weakly turbulent plasma with a Kolmogorov spectrum of Alfvén waves moving parallel and antiparallel to the magnetic field. This kind of spectrum has been employed in nonrelativistic models of hot spots [1]. However, the results obtained tend not to vary too dramatically as a function of the turbulence spectrum, being less than about 0.2 in the resulting s (0.1 in the predicted synchroton spectral index) for turbulence spectra between k−1 and k−2 [3].
Bright Quasars as Rapid Precessors
Authors: Roos Nico
Published: June 2019

The formation and evolution of massive binary black holes in galactic nuclei is discussed in a galaxy merger model for the activation of galactic nuclei. Mass flow into a galactic nucleus induced by the merger causes the simultaneous tightening of a preexisting binary and an increase in the fueling level of the central hole(s). The model predicts that during the final, short and most luminous phase of an AGN the precession period of the central hole will rapidly become as short as 103 yr. Some observational implications are mentioned.
Discovery of a Probable Gravitational Lens
Authors: Meylan G. , Djorgovski S.
Published: June 2019

We report the discovery of a new probable gravitational lens system, associated with the quasar UM 425 = 1120+019, at z = 1.465. The optical image of the system consists of 4 or more components. The two well-observed brightest components have very similar spectra. The difference of the two spectra (scaled appropriately) is consistent with a spectrum of a galaxy at z ≃ 0.6. There is a large number of faint galaxies in the field, suggestive of a lensing cluster at that redshift.
Observational Studies of the Structure of Active Galactic Nuclei and QSOs
Authors: Miller J. S.
Published: June 2019

Direct observational information on the structures of the central objects of active galaxies and quasi-stellar objects is hard to come by. Most of what we believe about these regions is inferred from data concerning the emission line spectra, the spectral energy distributions of the featureless continuum, and the luminosities and variability of the active nucleus. Data on the polarization of light from active galaxies, especially spectropolarimetric data which distinguishes between lines and continua, can offer additional insight into the nature of the central object, to some extent more direct information insofar as it can provide geometrical as well as physical information.
Stability of Emission-Line Clouds in Seyfert Galaxies
Authors: Mathews William G. , Veilleux Sylvain
Published: June 2019

Otherwise acceptable kinematic and dynamic models for emission line regions may be ruled out if the line-emitting clouds are unstable. We discuss the stability of radially-moving clouds to lateral gas flows. Such flows result from internal overpressures arising from the absorption of continuum radiation.
Anisotropic Continuum Emission in Seyferts
Authors: Ward Martin J.
Published: June 2019

The evidence for anisotropic continuum emission in some Seyferts, is increasingly convincing. However, the generality of this phenomenon has not yet been determined. I present new observations which suggest that its occurrence is widespread.
Theoretical Narrow Emission-Line Profiles of Active Galactic Nuclei
Authors: Contini M. , Viegas-Aldrovandi S.M.
Published: June 2019

In recent years, a large number of high resolution observations of active galactic nuclei (AGN) have become available. Most of them show the [OIII]5007 line profile (Heckman, Miley and Green 1984, Vrtilek and Carleton 1985, Whittle 1985a,b).
The Origin of the Coronal Lines in Seyfert Galaxies
Authors: Korista K. T. , Ferland G. J.
Published: June 2019

Photoionization model calculations suggest the possibility that the coronal line region in Seyfert galaxies may be the result of an interstellar medium exposed to and subsequently photoionized by a “bare” Seyfert nucleus. A series of models are presented in which a “generic” AGN continuum illuminates the low-density phase of the ISM of a spiral galaxy (Korista and Ferland 1988).
Reddening in the Narrow-Line Region of Active Galactic Nuclei
Authors: Yancoulova I.M. , Tsvetanov Z.I.
Published: June 2019

The best indicators of the reddening in the narrow-line regions of AGN are ratios of pairs of forbidden lines arising from a common upper level. The ratios of [SII] λ4072/λ10320 and [OII] λ7325/λ2470 are good examples, but they include lines in the far ultraviolet and infrared regions which are difficult to observe.
Longslit Optical Spectroscopy of Powerful Far-Infrared Galaxies
Authors: Armus Lee , Heckman Timothy M. , Miley George K.
Published: June 2019

It has been known since the IRAS mission that there exist galaxies with far-infrared luminosities of 1011–1012Lo, and LFTR/LB = 10–100. Through extensive modelling and observations of HII-region/molecular cloud complexes in the Galaxy, this infrared radiation is believed to be thermal emission from heated dust grains (c.f. review by Stein and Soifer 1983). While starburst models are consistent with the data over a large range in wavelength, direct evidence for sizeable populations of young stars is scarce, and in many cases the presence of an active nucleus either cannot be ruled out, or is required on the basis of energy considerations. In order to better understand the energy source responsible for heating the dust, we have undertaken a spectroscopic survey of galaxies chosen to have far-infrared spectral energy distributions similar to the prototypical class members Arp 220, NGC 6240, NGC 3690, and Mrk 231. It was required that between 25μ and 60μ, α ≤ −1.5, and that between 60μ and 100μ, α ≥ −0.5, where Sv α vα.
Faint IRAS Number Counts and Galaxy Evolution
Authors: Lonsdale Carol J.
Published: June 2019

We present results of number count studies of IRAS deep Pointed Observations and the new Faint Source Survey. Since number counts are dominated by the population near the knee of the luminosity function, the results refer to starburst and active galaxies with typical luminosities L60 ∼ 1010.5–11L⊙ and Lir/Lb, ∼ 3, and not to ‘normal’ spirals or the ‘ultraluminous’ IR galaxies.
NGC 613: Nuclear Region Narrow Band Imagery
Authors: Dottori H. A.
Published: June 2019

The peculiar nuclear region (Hummel et al., 1987) of the southern barred spiral NGC613 has been imaged with a CCD attached to the 90 cm CTIO reflector and integration time of one hour, using narrow band filters at Hα, Hβ, [OIII], [OII] and continua at 3765 Å(C[OII]), 5200 Å(CEβ) and 6476 Å(CHα), which allow to obtain pure line frames. Stone and Baldwin (1983) spectrophotometric standards have been observed for absolute calibration. The CHα (fig. 1) and CHβ (fig. 2) images show the well known 2 blobs structure, the SE of which is the nucleus. The ratio of both frames reveals an up to now unknown ring like structure (fig. 8) of 1400 × 2300 pc. The monochromatic color c(CHβ-CHα) reach 0.6 mag on the ring and 0.0 to 0.2 mag in its inner zone, where most of the gas emission is produced. The ratio Hα/Hβ (fig. 7) shows that the ring is practically absorption free, and that the dust is strongly concentrated in a zone of 400 pc, shifted 500 pc to the N of the nucleus, absorving up to 6 mag. in Hβ. Comparison with star monochromatic colors (O'Connell, 1973) lead to conclude that the ring colour is produced by red stars, with a mean spectral type G8 to K1, while in the central region it is A5 to F0. This ring is probably similar to that of HII regions observed in NGC 1097 (Osmer et al. 1974), but with the ionizing stars having evolved to RSG. The Hα pattern (fig. 3) presents only one maximum, displaced about 300 pc to the SE of the nucleus, being the weakest isophotes distorted to the W. The Hβ one (fig. 4) shows also one maximum, but the isodensities are elongated due to the extinction. The [OII] (fig. 6) shows a two blob like structure; and the [OIII], with the richest feature distribution, shows emission minima coincident with the [OII] blobs. The mean Hα/Hβ ratio is about 3.9, but mean extinction correction can be meaningless due to the distribution of dust. The ratios [OIII]/Hβ = 0.47 and [OII]/Hβ = 0.27, when compared with Stasińska's (1982) photoionization models, indicate higher than solar metallicity and Teff ≈ 40,000K for the ionizing source.
Low Activity Nuclei with Strong [NII] Lines
Authors: Bergmann Thaisa Storchi , Pastoriza Miriani G.
Published: June 2019

We present results from the study of six low activity galactic nuclei spectra with strong [NII] λλ6548 + 6584 emission lines in order to investigate if this characteristic is due to a peculiar chemical abundance. The selected galaxies NGC1358, NGC1386, NGC4941 and NGC6300 present Seyfert 2 (Sy2) activity, NGC3312 is a LINER and NGC7743 can be classified as intermediate between the two categories. All of them are nearby early type galaxies, whose nucleus present a prominent absorption spectrum with no direct evidence of a non-stellar continuum. Observations were made with the Tololo 1m telescope and 2DFRUTTI detector in the spectral region λλ3700 - 7000 A, with 5A (FWHM) resolution.
Superluminal Motion in CTA 102
Authors: Wehrle Ann E.
Published: June 2019

Sholomitskii (1965) discovered that the flux density of the quasar CTA 102 varies at low frequencies on a timescale of a few months. Low-frequency variability can be explained by “superluminal flux variation” (Romney et al. 1984): If the intrinsic brightness of a component moving in a relativistically beamed source varies by only a few percent, the observer sees its flux density change by a much larger factor δ3-α when the optically thin blob moves almost directly toward the observer. Such a relativistically beamed source is likely to exhibit superluminal motion if studied with sufficient resolution and sensitivity. Superluminal motion in CTA 102 was discovered by Bååth (1987) who concluded on the basis of maps made at three epochs at a frequency of 932 MHz that two components were separating at a rate of 0.65 milliarcseconds (mas) per year. Using a redshift z = 1.037 and H0 = 100 km s−1 Mpc−1, q0 = 0.5, this expansion speed corresponds to (18 ± 4)h−1c. The extraordinarily high speed led us to make VLBI images of the source at a higher frequency in order to increase the resolution and make a more precise determination of the speed.
Future AGN Research II
Authors: Shields Gregory A.
Published: June 2019

The “standard model” of AGN involves a supermassive black hole accreting gas, possibly through a flattened disk. This model enjoys widespread popularity among astronomers, but convincing proof has been elusive. Recent observations of “dormant” black holes in nearby galactic nuclei encourage the idea that black holes lie at the centers of AGN. An understanding of the nature of the accretion flow seems most likely to come from Doppler shifts of spectral features. The thermal continuum from the inner disk offers an intriguing opportunity. Theoreticians should not neglect the study of the thin disks that should exist for some AGN parameters, as these relatively “tame” objects may yield the most reliable observational tests. The broad emission lines may represent the debris of tidally disrupted stars.
A Historical Introduction to the Search for Extraterrestrial Life
Authors: Papagiannis Michael D.
Published: June 2019

The possibility that life, primitive or advanced, might exist in other places of the Universe has occupied the thoughts of scientists and laymen for thousands of years. It is only in the last 25 years, however, that we have entered the experimental phase of the search for extraterrestrial life, where considerable progress has already been made.
A New Objective for the Allegheny Observatory 30-Inch Refractor
Authors: Gatewood George D. , de Jonge Joost Kiewiet , Stein John , DiFatta Charles
Published: June 2019

As a follow-up to the development of astrometric detectors of sufficient accuracy to detect extrasolar planetary systems, the Allegheny Observatory is modernizing their 30-inch Thaw Refractor. Changes include a new objective lens and the transition of the observational bandpass from blue light to red. The new system will have sufficient accuracy to detect jovian planets orbiting any of several dozen of the Sun's nearest stellar neighbors.
Absolute Astronomical Accelerometry: A New Tool for Planetary Searching
Authors: Connes Pierre
Published: June 2019

Absolute astronomical accelerometry is a new proposed technique specifically optimized to detect small radial velocity changes (i.e. accelerations) of either the Sun or a star; it is intended for the two problems of stellar seismology and the search for extrasolar planetary systems. In both cases the computed performance is such that positive results should be obtainable in a large number of cases with a moderate size telescope. Essentially the method involves two separate and simultaneous servo loops. In the first a variable path-difference Fabry-Perot interferometer is adjusted so that its bandpasses track the fluctuations of the lines in the stellar spectrum. Then a tunable laser tracks the fluctuations of the FP and one has only to measure the beat frequency from a stabilized laser. The result is absolute i.e. obtained solely in terms of frequencies and the speed of light. All instrumental or spectral characteristics drop out; no calibration is required. Furthermore the method can be demonstrated to approach closely a so far never computed photon noise limit for radial velocities.
Infrared Spectral Identification of Complex Organic Molecules in Interstellar Grains
Authors: Greenberg J. Mayo , Schutte Willem
Published: June 2019

The chemical evolution of interstellar grains leads ultimately to a composition consisting largely of complex organic molecules. Comparison of infrared absorption spectra of laboratory produced analogue materials with astronomical observations confirm the presence of similar molecules in interstellar space. The abundance of this complex organic matter derived from the strength of the absorption bands is of the order of ten million solar masses and is almost certainly as large or larger than all conceivable planets.
Human Evolution: the View from Saturn
Authors: Landau Misia
Published: June 2019

The evolution of bipedalism is considered a critical step in human evolution. To discover how it occurred, and whether it could have occurred elsewhere in the universe, scientists must study the structure of their theories as well as fossils. In particular, they must determine whether such critical steps are a function of the “data” or of the (often unwitting) use of narrative for, it will be shown, scientists tend to make sense of the past by telling stories in which everything leads up to or away from being human.
A Search in the Infrared to Microwave for Astroengineering Activity
Authors: Slysh V. I.
Published: June 2019

Huge space power plants (Dyson Spheres) utilizing most of a star's energy should be detectable as infrared or microwave sources. A recent far infrared all-sky survey has revealed many sources with a spectrum peaking on this region which is characteristic of the thermal emission of the hypothetical Dyson spheres. The possibility of confusing them with thick circumstellar dust shells around evolved red giant stars is discussed. Microwave detection of cool extended Dyson Spheres by all-sky surveys searching for microwave background fluctuations is also considered.
An Analysis of the Elements of an all Sky Survey
Authors: Olsen Edward T. , Lokshin Anatoly , Gulkis Samuel
Published: June 2019

One component of the NASA search for microwave signals of extraterrestrial intelligent origin will be an all sky survey at a significantly low limiting flux over a broad frequency range. We are currently designing an overall strategy which will permit this survey to be: (1) carried out using existing antennas in less than 3 years of observation time, (2) uniform in sensitivity (within 0.5 db) over the celestial sphere for any given frequency, and (3) complete to 6×10−23 W/m2 or better over the frequency range 1.2GHz⩽ν⩽10GHz with a frequency resolution of 32 Hz.
On the Question of Interstellar Travel
Authors: Wolfe John H.
Published: June 2019

Arguments are presented which show that motives for interstellar travel by advanced technological civilizations based on an extrapolation of Earth's history may be quite invalid. In addition, it is proposed that interstellar travel is so enormously expensive and perhaps so hazardous, that advanced civilizations do not engage in such practices because of the ease of information transfer via interstellar communication.
On a Cosmic Alphabet
Authors: Hoang-Binh D.
Published: June 2019

The choice of frequencies for SETI has been discussed by many authors. We wish to point out that the line spectrum provided by the hydrogen (H) atom is very rich, and covers a large spectral domain, from the ultraviolet to radio wavelengths. The frequencies of H lines present one remarkable feature, which in our opinion makes them more attractive than those of, e.g. the OH or H2O lines, or those chosen arbitrarily in the water hole: they all relate to only one atom, the simplest and most abundant one in the universe. In terrestrial alphabets, one word is composed of one or several characters. We can always replace any set of characters by a set of H frequencies, and “write” messages with them. Since the H frequencies are universally known, a “cosmic alphabet” could be constructed with them. In a mono-frequency search for artificial radio signals, the frequency of the H166 alpha line, 1425 MHz, might be a good substitute for the frequency 1420 MHz of the H 21cm line. In a multi-frequency search, the frequencies of the prominent Hn alpha lines near H 21cm might be interesting. There are about ten of them in the water hole. Finally, owing to the physical link between them and the H 21cm frequency, those around this symbolic frequency appear most attractive.
Chemical Gradients in Globular Clusters
Authors: Freeman K. C.
Published: June 2019

First I will describe briefly some older work on chemical gradients in globular clusters. Then I will discuss some more recent data on the chemical and kinematical properties of 47 Tuc and Omega Cen, that leave us in little doubt about the reality of such chemical gradients.
X-Raying the Dynamics of Globular Clusters
Authors: Grindlay Jonathan E.
Published: June 2019

Recent studies of the x-ray sources in globular clusters have provided important new clues for both the dynamical processes in clusters and the stellar content and evolution of globular clusters. Very deep x-ray images of several globular clusters show evidence for diffuse x-ray emission from hot gas which may be related by a simple shock model to properties of both the cluster, such as its orbit in the Galaxy, and the interstellar medium in the halo of the Galaxy. The x-ray surveys conducted with the Einstein Observatory are reviewed and the results derived for the luminosity function, masses and nature of the compact x-ray sources are discussed. The evidence for the compact binary nature of the sources is now overwhelming, but long-term x-ray variability studies previously reported may suggest that some of the systems are in fact triple systems with distant companions. Possible relationships between the initial mass function, stellar density and cluster evolution are discussed, and our arguments that the ostensibly similar compact x-ray sources in the galactic bulge are remnants of a population of globular clusters disrupted by giant molecular clouds are updated.
A Deep Luminosity Function for 47 Tucanae
Authors: Harris William E. , Hesser James E.
Published: June 2019

CCD photometry in B and V reaching B(lim) ≃ 25 has been employed to obtain the luminosity function and color-magnitude diagram for the main sequence of 47 Tuc. For 5<Mv<10 we find that its LF is essentially flat (Δlog n/Δm~0). The CMD is successfully matched by isochrones with [Fe/H] = −0.5 and t ≃ 15 × 109y.
BV Concentric Aperture Photometry of Globular Clusters
Authors: Peterson Charles J.
Published: June 2019

Globular clusters are among the most difficult types of objects to observe with standard photoelectric techniques. Among factors which may contribute to measurement error are the centering of apertures in the oftentimes diffuse cores of clusters, the random distribution of bright stars near the cluster centers, and the determination of an adequate sky correction in fields badly crowded by foreground stars. These problems have been well-discussed by a number of authors (see, e.g., Hanes and Brodie 1984, and the references they cite).
An Extension of the Search for Spectroscopic Binaries in M3
Authors: Pryor Carlton P. , Latham David W. , Hazen-Liller Martha L.
Published: June 2019

We have obtained 295 new radial velocities for the 112 giants in the globular cluster M3 previously observed by Gunn and Griffin. Our velocities have a typical accuracy of 0.8 km/s per measurement and have been combined with the Gunn and Griffin data in order to search for radial velocity variations over a time span of ten years. We find no convincing evidence that any of the giants observed are spectroscopic binaries with one notable exception, von Zeipel 164, which we believe is the first spectroscopic binary to be found in a globular cluster. Modelling of the velocity variations that would be expected in our data for a variety of binary populations confirms Gunn and Griffin's conclusion that binaries with separations of less than 10 AU must occur much less frequently among the giants of M3 than among the population I field stars.
Dynamical Evolution of Globular Clusters After Core Collapse
Authors: Heggie Douglas C.
Published: June 2019

This review describes work on the evolution of a stellar system during the phase which starts at the end of core collapse. It begins with an account of the models of Hénon, Goodman, and Inagaki and Lynden-Bell, as well as evaporative models, and modifications to these models which are needed in the core. Next, these models are related to more detailed numerical calculations of gaseous models, Fokker-Planck models, N-body calculations, etc., and some problems for further work in these directions are outlined. The review concludes with a discussion of the relation between theoretical models and observations of the surface density profiles and statistics of actual globular clusters.
Direct Fokker-Planck Calculations
Authors: Cohn Haldan
Published: June 2019

The past decade has seen the development of powerful numerical methods for studying star cluster evolution by direct integration of the Fokker-Planck equation. Cohn's basic algorithm for spherical systems of identical point masses and its application to the study of core collapse is reviewed. Merritt's extension of this method to treat systems containing a mass spectrum, and Goodman's extensions to include strong scattering and rotation are discussed. Results from direct Fokker-Planck computations of core collapse in single mass and multi-mass isotropic clusters and single mass anisotropic clusters are presented to illustrate the method. Calculations of pre- and post-collapse evolution with a central black hole and with heating by hard binaries are reported. Prospects for future development of the direct Fokker-Planck method will be discussed, with emphasis on the central goal of developing physically realistic models for interpreting Hubble Space Telescope observations of globular cluster structure.
Close Encounters
Authors: Goodman Jeremy
Published: June 2019

The mathematical treatment of encounters is discussed briefly, and it is pointed out that in a spherical system with an isotropic distribution function and equal-mass stars, the relevant equations can be analytically orbit-averaged. Even in small systems, close encounters have little effect on core collapse. Near a black hole, however, close encounters may cause an accumulation of stars on large, very eccentric orbits.
Direct N-Body Calculations
Authors: Aarseth Sverre J.
Published: June 2019

The main principles for direct integration of large point-mass systems are outlined. Most particles are advanced by the Ahmad-Cohen neighbour scheme, using fourth-order force polynomials and individual time-steps. Close encounters and persistent binaries are handled by two-body regularization, whereas extreme triple and quadruple configurations are treated as unperturbed systems by special regularization techniques. As an illustration of these methods, we discuss some recent results of an isolated system containing 1000 particles of unequal mass, with special emphasis on the post-collapse phase.
A Unified N-Body and Statistical Treatment of Stellar Dynamics
Authors: Lightman Alan P. , McMillan Stephen L. W.
Published: June 2019

We summarize the methods of a new “hybrid” computer code for stellar dynamics. All particles in the inner spatial region are followed exactly via a direct N-body code and all particles in the outer spatial region are treated statistically via a distribution function and Fokker-Planck type methods. An intermediate region, with features of both, allows exchange of particles and energy between the outer and inner regions. We apply our code to the period just before core collapse and just after and summarize the results.
Direct N-Body Simulations with a Recursive Center of Mass Reduction and Regularization
Authors: Jernigan J. G.
Published: June 2019

We report on an investigation of the core collapse of an initially bound system of 1024 point masses based on a new numerical method which incorporates a recursive center of mass reduction of the N-body system and includes regularization of the two body encounters. A tree which organizes the point masses into a logarithmic hierarchy is the primary feature of the numerical algorithm. The spatial structure of the tree allows for rapid force calculation while the associated temporal structure allows for variable time steps. Simulations are calculated for several core collapse times in an effort to follow the end point evolution of the system. The point-like nature of the interactions are maintained throughout the simulations.
Multi-Component Models for the Structure and Evolution of Spherical Stellar Systems
Authors: Davoust Emmanuel
Published: June 2019

We present stationary analytical models for the structure of spherical stellar systems with several mass components. Evolution is simulated by increasing the central velocity of escape. We compare our models to the results of numerical simulations. An observable kinematical effect of mass segregation is pointed out.
The Role of Hard Binaries in Cluster Evolution
Authors: Giannone G. , Molteni D.
Published: June 2019

The presence of an initial binary population in a stellar cluster can have a dominant effect on the dynamical evolution of the whole system. We discuss simulations, carried out with a direct N-body method for several models with N = 300 equal mass objects, with 20% of all the stars as initial binaries having different binding energy. These calculations show that binaries with values of the binding energy, h, in the range 5–10 times the mean kinetic energy, x, are the most strongly interacting ones with the field stars and between themselves. The main result of these interactions is an enhanced expansion of the core and also of the whole cluster. The binary heating prevents the gravitational collapse for a time longer than the collapse time for systems without binaries. Energetic binaries with h greater than 25x concentrate at the center, due to mass segregation but these binary-binary interactions produce disruption and escape of the components, with smaller heating effect. Binary-binary interactions occur predominantly in the inner regions and are mainly responsible for the core heating and for the escaping stars. Single star-binary encounters, instead, occur more frequently in the outer regions, and only contribute to the strong expansion of the halo. In all the simulations there was no formation of new persistent hard binaries.
Numerical Simulations of Encounters of Hard Binaries
Authors: Mikkola Seppo
Published: June 2019

Results from numerical integrations of random binary-binary encounters have been used to obtain various cross-sections and outcome distributions for the four-body scattering. The initial orbital elements were chosen randomly except the Kepler-energies for which various selected values were used. Rough estimates for mass effects were obtained by simulating encounters of binaries with unequal component masses.

We developed a semi-analytical theory for obtaining the types and energies of the outcome configurations. The theory contains some adjustable parameters, the values of which we deduced by comparing the theory and experiments.

The energy transfer rate by collisions (=outcome is not two binaries) dominates over that due to fly-by's by an order of magnitude, provided that the binaries are hard. The formation of a hierarchical three-body system is fairly common. In a collision of energetically similar very hard binaries the probability is about 20 percent, while it is greater than 50 percent if the binding energies differ by a factor of more than four.
Monte Carlo Simulations of the 2+1 Dimensional Fokker-Planck Equation: Spherical Star Clusters Containing Massive, Central Black Holes
Authors: Shapiro Stuart L.
Published: June 2019

The dynamical behavior of a relaxed star cluster containing a massive, central black hole poses a challenging problem for the theorist and intriguing possibilities for the observer. The historical development of the subject is sketched and the salient features of the physical solution and its observational consequences are summarized.

The full dynamical problem of a relaxed, self-gravitating, large N-body system containing a massive central black hole has all the necessary ingredients to excite the most dispassionate many-body, computational physicist: it is a time-dependent, multidimensional, nonlinear problem which must be solved over widely disparate length and time scales simultaneously. The problem has been tackled at various levels of approximation over the years. A new 2+1 dimensional Monte Carlo simulation code has been developed in appreciable generality to solve the time-dependent Fokker-Planck equation in E-J space for this problem. The code incorporates such features as (1) a particle “cloning and renormalization” scheme to provide a statistically reliable population of test particles in low density regions of phase space and (2) a time-step “adjustment” algorithm to ensure integration on local relaxation timescales without having to follow typical particles on orbital trajectories. However, critical regions in phase space (e.g. disruption “loss-cone” trajectories) can still be followed on orbital timescales. Numerical results obtained with this Monte Carlo scheme for the dynamical structure and evolution of globular star clusters and dense galactic nuclei containing massive black holes are reviewed.

Recent dynamical integrations of the Einstein field equations for spherical, collisionless (Vlasov) systems in General Relativity suggest a possible origin for the supermassive black holes believed to power quasars and active galactic nuclei. This scenario is discussed briefly.
Can a Moderately Massive Black Hole Reverse Core Collapse?
Authors: Duncan Martin J.
Published: June 2019

Direct N-body simulations show that the heat flux from the cusp of stars surrounding an object of mass M ~ 10–100 M⊙ has a profound effect on the evolution of a cluster of N stars of mass m ~ 1 M⊙. A cluster containing an object which is ≳ 5% of its total mass expands, transferring most of the cluster binding energy to a few stars deep in the cusp within several central relaxation times. The results, together with analytic estimates, suggest that core collapse in a globular cluster will be reversed when the core mass is reduced to ~10 M. As is discussed below, this result does not depend on whether or not stars are disrupted by the black hole.