bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modern Developments of the Meridian Circle (Invited Paper)
Authors: Høg Erik 
Published: July 2016

The successful improvements of meridian instrumentation during the past twenty years have mainly come through electronic devices and computers being applied for facilitating the data handling and for increasing the accuracy. Photographic recording of the circle and of the star has played an important role here, but has for both tasks gradually lost its attractiveness as direct photometric-electronic methods have become available. At the same time new types of the telescope system have been introduced without convincing results as yet, but this field still holds promises for the near future.The problems of the refraction, the meridian building, the foundation of the instrument and the site selection have been treated in recent years and these efforts will be doubly repaid when the telescope and micrometers become more nearly perfect. Altogether, it need not be long before a few partly automatic instruments produce observations with a mean error of 0″.15 and a systematic error of 0″.03 at a rate of 300 observations per observing night. In addition the limiting magnitude can be mv = 11 or 12, thus 2 mag. fainter than for visual observations. These goals are conservative – most of them have already sometimes been surpassed – and they should be compared to a present day good visual meridian circle giving a mean error 0″.30, a systematic error 0″.10 and 120 observations per night.The relative roles of meridian and photographic astrometry must be defined anew in the light of the great improvements of both methods.
The Influence of the Reduction Model on the Systematic Accuracy of Adjusted Parameters
Authors: Eichhorn H. , Rust A. E. 
Published: July 2016

Everybody who deals with the numerical determination of parameters from observations is familiar with the phenomenon that the ‘external’ errors of these parameters are invariably larger than their ‘internal’ ones. This phenomenon is traditionally explained by the presence of systematic errors.Since it is virtually impossible to use a completely rigorous reduction model in any astrometric problem, an ‘interpolation’ model must normally be employed in which there are fewer parameters (unknowns) than in the rigorous one. It is shown that under these circumstances, the rms errors of the adjustment parameters are always underestimated, which creates the illusion that the results computed with these parameters are more accurate than they actually are.
Spatially Resolved X-Ray Spectra of Coronal Active Regions
Authors: Catura R. C. , Acton L. W. , Joki E. G. , Rapley C. G. , Culhane J. L. 
Published: July 2016

SummaryX-ray spectra from a number of coronal active regions were obtained during ATM support rocket flights carried out by the Lockheed group on June 11 and December 19, 1973. Multi-grid collimators were used to provide fields of view of 40″ diameter and 90″ diameter for a number of scanning crystal spectrometers and a bent crystal spectrometer which employed a position sensitive proportional counter to register the diffracted spectrum. A solar image was produced on film and on a TV camera on board the rocket with the aid of a 1 Å Hα filter. A small part of the X-ray collimator was used to generate a multiple spot diffraction pattern which was superimposed on the Hα image and the composite picture was transmitted to the ground. Pre-launch calibrations allowed the spot corresponding to the X-ray collimator axis to be identified and so the collimator pointing direction on the solar disc was controlled from the ground by means of commands sent to the rocket.
Time Variations in Coronal Active Regions
Authors: Krieger A. S. , Chase R. C. , Gerassimenko M. , Kahler S. W. , Timothy A. F. , Vaiana G. S. 
Published: July 2016

SummaryThe AS&E X-ray telescope experiment on Skylab has obtained images of the solar X-ray corona with a variety of time resolutions ranging from 21/2 s to the regular 12 ± 2 h synoptic observation rate. The form and brightness of coronal active region structures are seen to vary on time scales ranging from seconds, for flare associated changes, to several solar rotations for long term evolution of the regions. The extrapolation of photospheric magnetic fields into the corona, using the potential field approximation, results in a good morphological agreement between the form of the computed coronal field lines and the structure of many of the active regions observed. Thus, in general, the coronal active region structures follow potential field lines and the long term evolutionary changes can be explained on the basis of the spreading of the fields. Short term changes in active region structure frequently take the form of selective brightening or dimming of pre-existing loops due to changes in the pressure of the emitting coronal plasma. In these cases, variations in the non-potential component of the coronal fields supporting and containing the plasma are implied.
On the Thermal Structure of the Flare-Produced Plasma
Authors: Craig Ian 
Published: July 2016

SummaryRecent flare studies have shown that soft X-ray data are not compatible with simple isothermal models of the source (Herring and Craig, 1973; Craig, 1973; Neupert et al., 1973). With this in mind, the emitting flare plasma has been represented by the temperature-emission measure distribution function,

where ζ(T) is the differential emission measure (cm–3 per 106 K), T is the electron temperature in units of 106 K, T0 is a low temperature cut-off for the distribution, αi are real positive numbers, and Ai are positive coefficients determined from data (for appropriate values of T0 and αi) by a least squares fitting procedure. Such a distribution is suggested by results obtained by the present author using simple delta-function representations for ζ(T) (with n ≤ 4); these discreet multi-temperature models usually indicate that the emission measure decreases with increasing temperature. Also, as discussed by Brown (1974), a power law distribution for ζ(T) is consistent with the observed bremsstrahlung emission in the hard X-ray (> 10 keV) domain. In attempting to find a suitable form for the differential emission measure, a simple empirical function of the type assumed by Chambe (1971) for active regions was also tried, but the fit, as evidenced by the χ2 test was unsatisfactory.
Relationship between Hard and Soft Solar X-Ray Sources Observed by OSO-7
Authors: Datlowe D. W. , Hudson H. S. 
Published: July 2016

SummaryThe UCSD experiment on the OSO-7 satellite has provided hard and soft X-ray observations of a large number of solar flares. Of these a sample of 123 had sufficiently large fluxes to permit analysis of their spectra (Datlowe et al., 1974). The locations of these flares upon the solar disk have been obtained by comparison with Hα flare listings. We find that the soft (5.1–6.6 keV) X-ray bursts, above a threshold of 1000 photons (cm2 s keV)–1, have a relatively flat distribution from center to limb. The frequency of occurrence of hard (20–30 keV) X-ray bursts, as normalized to the longitude distribution of the soft X-ray bursts, shows a statistically insignificant excess of 19±34% in the longitude range 80–90°. Furthermore, the limb flares exhibit a small but statistically significant spectral softening.
Implications of NRL/ATM Solar Flare Observations on Flare Theories
Authors: Cheng C. C. , Spicer D. S. 
Published: July 2016

SummaryDuring the Skylab mission, many solar flares were observed with the NRL XUV spectroheliogram in the wavelength region from 150 to 650 Å. Because of its high spatial resolution (∼2″) the three-dimensional structures of the flare emission regions characterized by temperatures from 104 K to 20 × 106 K can be resolved. Thus the spatial relationship between the relatively cool plasma and the hot plasma components of a flare, and the associated magnetic field structure can be inferred. For example, the Fe XXIV plasma (T≃20 × 106 K) observed near the soft X-ray maximum during the 2B (M2) disk flare of 1973, July 15 is elongated perpendicular to the neutral line shown on the photospheric magnetogram, while lines of lower ionization temperatures such as He II–Fe XVI show the familiar double ribbon structures on either side of the neutral line (Widing and Cheng, 1974). The disk flare of 1973, September 5 shows the same spatial structures. The flare was observed at 1831 UT at the X-ray maximum, and shows that the hot Fe XXIV cloud is located spatially between the ribbons of the He II, Fe XIV–Fe XVI emissions. As the flare cools at 1837 UT, the Fe XXIV cloud disappears while the gap is filled with emissions from ions of lower ionization potentials, and exhibit loop structure. Many other flares also show similar spatial distributions in the XUV emissions. The linear dimensions of the Fe XXIV plasmas as measured from the photospheric plates range from 7000 km to 14000 km, and the heights of associated loops range from 10000 to 30000 km. From the spatial distributions of the XUV emissions of the many flares, and the comparisons with the magnetograms, we concluded that the magnetic field configuration for the flares we observed are simple bipolar magnetic flux loops with the hot flare plasma located near the top and the cool plasma component on the footprints of the loop.
Measurements of Isotopic Abundances in Interstellar Clouds
Authors: Penzias Arno A. 
Published: July 2016

While an examination of the available data reveals some seemingly contradictory results, a general framework having the following outlines can be put forward:1. With the exception of the two galactic center sources SgrA and SgrB, the relative isotopic abundances exhibited by the giant molecular clouds in our Galaxy exhibit few, if any, significant variations from the values obtained by averaging the data from all these sources.2. The 13C/12C and 14N/15N abundance ratios are ∼130% and ∼150%, respectively, of their terrestrial values throughout the galactic plane and somewhat higher, ∼300%, near the galactic center.3. The 16O/18O and 17O/18O abundance ratios are ∼130% and ∼160%, respectively, of their terrestrial values throughout the Galaxy, although the former may be somewhat lower near the galactic center.4. The S and Si isotopes have generally terrestrial abundances.
Non-conservative Evolution of Massive O-Type Close Binaries with Galactic and with Magellanic Cloud Chemical Abundances
Authors: Vanbeveren D. 
Published: July 2016

The general evolutionary pattern of massive O type close binaries evolving according to a case B mode of mass transfer, including mass loss by stellar wind prior to Roche lobe overflow (RLOF) at rates appropriate for O type stars, only marginally depends on the choice of the initial chemical composition whether the galactic or the MC abundances are used (the difference never exceeds 10%). The theoretical results are compared to the observations, O type binaries describing the evolutionary phase prior to RLOF, WR type binaries describing the helium burning phase after RLOF. The large mass loss by stellar wind in WR stars considerably affects the evolution during the latter phase. The comparison yields the following conclusions:a) from the ZAMS up to the WR stage, 50%-60% of the initial primary mass is leaving the system corresponding to at least 70%–80% of the total mass lost by the primary due to stellar wind and RLOF;b) during the WR phase the star is losing approximately half of its mass;c) the average mass ratio for binaries prior to the supernova explosion equals 3, i.e. the exploding star is 3 times less massive than its companion.
SPECTROSCOPY OF THE ORION MOLECULAR CLOUD CORE
Authors: Scoville N. Z. 
Published: July 2016

Recent infrared and radio spectroscopic data pertaining to the Orion BN-KL infrared cluster are reviewed. A new, high resolution CO map shows that the thermal structure over the central 10′(1.5 pc) in the Orion molecular cloud is dominated by energy sources in the infrared cluster and M42. Peak CO brightness temperatures of 90 K occur on KL and near the bar at the southern edge of M42.Within the central 45″ of the infrared cluster, both radio and IR data reveal a highly energetic environment. Millimeter lines of several molecules (e.g. CO, HCN, and SiO) show emission over a full velocity range of 100 km s−1. These supersonic flows can be modeled as a differentially expanding envelope containing a total of ~5 M⊙ of gas within an outer radius of r ≃ 1.3 × 1017 cm. Over the same area emission is seen from vibrationally excited molecular hydrogen at an excitation temperature of 2000 K. The high velocity mm-line emission and the NIR H2 lines are clearly related since they exhibit similar spatial extents and line widths. Comparison of the total cooling rate for all the H2 lines with the estimated kinetic energy and expansion time for the mm-emission region indicates that the H2 emission probably arises from shock fronts where the expanding envelope impinges on the outer cloud.Near IR spectroscopy also probes ionized and neutral gas closely associated with BN. Br α and Br γ emission is detected from an ultracompact HII region of mass MHII ≲ 10−4 M⊙. Full widths for the HII lines are ~400 km s−1. CO bandhead emission detected in BN at λ ≃ 2.3 μm is probably collisionally pumped in a high excitation zone (nH+H2 > 1010 cm−3 and TK ≃ 3000 K) at only a few AU from the star. The velocity of both the HII and CO emission is VLSR ≃ + 20 km s−1; thus BN appears to be redshifted by 11 km s−1 with respect to OMC-1.
Radio Polarisation Studies of Galaxies at z > 2
Authors: Athreya R.M. , Kapahi V.K. , Mccarthy P.J. , Van Breugel W. 
Published: July 2016

We present some interesting results from a radio study of 17 radio galaxies at z >2, selected from a complete sample of Molonglo sources with S408 > 0.95 Jy for which optical identifications and spectroscopic redshifts are being obtained in a major observational programme (Kapahi et al. 1995).Observations and results : The 17 galaxies were mapped using the VLA at 1.4, 4.8 and 8.3 GHz with the highest resolution of ≃ 0.3″ at 8.3 GHz. The integrated spectra are generally quite steep and convex in many sources, steepening to α ≥ 1.2 (S ∝ v–α) at the highest frequency. Most of the sources have a double structure with sizes ranging up to 22″. Unresolved radio cores, coincident with the optical galaxies, are detected at the mJy or sub-mJy level in most cases. Several sources also show one-sided jetlike features. Surprisingly, most of the cores appear to have steep spectra between 5 and 8.3 GHz!
On the Problem of the Development of Astronomical Refraction Methodology on the Basis of New Technology
Authors: Sergienko V. I. 
Published: July 2016

The work on construction and operation of automated systems for collection and processing astrometric and meteorological information and also of automated television systems with remote control have placed before us a number of fundamental difficulties in that hundreds of correct solutions to these problems require a systematic approach.The most suitable cybernetics-mathematical composite for solving the problems of astronomical refraction may become the so-called hybrid-expert systems — a union of traditional expert systems with calculation-logical ones. In these systems logical-linguistic models are used together with mathematical ones.The basic problem of ARA-C consists in obtaining a model of the environment in which light propagates and also to give a notion of interaction between electrons (environment) and photons (light).It is proposed to use in this system a model of light interaction with environment built up on special formalisms of artificial intellect (logical-linguistic model). The basic principal in hybrid-expert system ARA-C operation must be mathematical modelling and calculation experimentation. In the proposed hybrid-expert system knowledge is presented on three levels—object, mathematical and programmed. Functioning of the lower levels of ARA-C is connected with formulation of the calculation problems. A calculation problem presented to a technical model determines a partial order on a twopart column of mathematical correlations.
Effect of the New Equinox Definition on the Zero-Point of Longitude of the Indian Calendar
Authors: Bhatnagar A. K. 
Published: July 2016

Indian calendars follow a sidereal system of astronomy taking a fixed initial point on the ecliptic as the origin from which the longitudes are measured. Its position for the official Indian Calendar has been defined by the Calendar Reform Committee (1955) as the point on the ecliptic whose true tropical longitude was 23°15′00″ as on 21 March 1956, 0h UT. Its position was determined upto the year 1984 in accordance with Newcomb's value for general precession using the relation
where T is in centuries of 36525 ephemeris days from 1900 January 0.5 ET. Recent changes in the location and the motion of the equinox with reference to the epoch J2000.0 have necessitated corresponding changes to be included in the determination of the mean and true positions of the above initial point. The new algorithm worked out is
where T is in Julian centuries of 36525 days from J2000.0.
On the Errors of the Ephemerides Derived From Optical Observations of Planets
Authors: Kharin A.S. , Kolesnik Yu.B. 
Published: July 2016

On the basis of about 40 000 optical observations of the Sun and major planets obtained with 33 meridian and photographic instruments during last 3 decades a comparative consistency analysis of old and new ephemerides with these observations has been made.For the inner planets significant improvement in RA is confirmed while in DEC it is less apparent. For outer planets the improvement is strongly marked in DEC for Jupiter and Saturn and espesially for Neptune in both coordinates. Significant systematic differences between meridian and photographic observations are detected.
On The lunar Secular Acceleration: A Possible Approach
Authors: Protitch-Benishek V. 
Published: July 2016

The secular quadratic term in the expression of the Moon's longitude has been introduced empirically after the conclusion that its mean motion is not constant (Halley, 1695).But, the explanation of this term and also of its numerical evaluation presented and still presents in our time great difficulties. All efforts, namely, to obtain an exact agreement between observed and theoretical value of Moon's secular acceleration were unsuccessful: the first of these two values exceeds always the second one by a very large amount. This discordance and unexplained residuals (O – C) in the mean longitude of the Moon gave rise finally to the statement that these are due to a retardation and irregularity in the Earth's rotation. But, after hardly a fifty years, this hypothesis revealed even more new difficulties and questions concerning also the problem of stability of the Earth-Moon system. It seems that there is a true reason for which this problem occurs as one of the unsolved problems of Celestial Mechanics (Brumberg and Kovalevsky, 1986; Seidelmann, 1986).
In-Orbit Status of the Hipparcos Astrometry Mission
Authors: Perryman M.A.C. 
Published: July 2016

The Hipparcos satellite was successfully launched by Ariane 4 flight 33, from French Guyana, on 8 August 1989. However, the apogee boost motor, designed to place the satellite into its intended geostationary orbit, failed to function, and the satellite was left in a highly elliptical transfer orbit. After a perigee raising manoeuvre, using the hydrazine propulsion system, the orbital parameters are approximately: apogee 35900 km, perigee 526 km, eccentricity 0.72, inclination 6.8°, orbital period 10h 40m.Operational procedures have had to be revised during the first weeks of satellite operations, due to the non-nominal orbit. Commissioning and calibrations have taken longer than foreseen because of the operational difficulties and observational overheads, but routine observations commenced on 27 November. The essential differences between the nominal and the present missions are that observations can only be conducted for about 30-40 per cent of real time in the present mission, and the fact that the actual lifetime of the mission is still rather uncertain, and possibly somewhat shorter than anticipated. This paper gives a summary of the present satellite status, measurement results obtained so far, and in indication of the scientific goals that may be achievable with the present mission.
The Reference Frame Determined From the Observation of Minor Planets by Hipparcos
Authors: Morando B. , Bec-Borsenberger A. 
Published: July 2016

The observation of minor planets by Hipparcos offers the opportunity to obtain high precision positions for some minor planets. About fifty minor planets are on the programme. Their ephemerides had to be improved in order to reach a precision of 1 arsec and occultations by the Earth and the Moon had to be predicted.From the position of a minor planet on reference great circles at different times better values of the initial position and velocity will be deduced but the reduction of the observations of the minor planets have to take into account the displacement of the photocentre relative to the centre which is due to the shape, the phase effect and the scattering properties of the surface. For some very small planets considered as star like this diplacement will be small and the precise positions obtained will allow to position the dynamical reference system relative to the Hipparcos system. For the bigger minor planets the observations by Hipparcos may give informations on the shape and scattering properties of the surface.
Numerical Simulations of Solar Disturbances and their Interplanetary Consequences
Authors: Dryer M. , T. Wu S. , R. Detman T. 
Published: July 2016

Responses of the solar atmosphere and interplanetary medium to simulated solar disturbances were studied by time-dependent, MHD numerical simulations. This deterministic initial-boundary value problem was attacked in the classical way: a representative steady state is first established, then input parameters at the lower near-Sun boundary are perturbed. We discuss a number of 2- and 3-dimensional examples of coronal mass ejection (CME) simulations and some current controversies concerning the basic process of CME initiation. Footpoint shearing motion is tested to see whether it can provide a reasonable mechanism for CME development from arch filament configurations.We also demonstrate possible interplanetary consequences to CME-like disturbances by using 3-D simulations to determine the dynamic response of the solar wind to a plasmoid injection from an eruptive filament or prominence. We also discuss the separate possibility whereby a plasmoid may be generated in the interplanetary medium by a solar-generated shock that propagates through a heliospheric current sheet. Application of the 3-D model for the interpretation of interplanetary scintillation observations is also discussed.
Dual Frequency Variability Study of an Active Region
Authors: Shevgaonkar R. K. , Kundu M. R. 
Published: July 2016

A short-time variability study of a solar active region simultaneously at 6 and 2 cm wavelengths was carried out using the VLA. The observations show interesting uncorrelated brightness temperature variation at the two wavelengths. The observed low brightness temperatures indicate that the emission is mainly originating from the chromosphere - corona transition region.A transition region model with constant pressure and power-law temperature variation as a function of height has been assumed to analyse the data. The uncorrelated variation of the observed brightness temperature at the two wavelengths suggest different dominant emission mechanisms (bremsstahlung at 2cm and gyro-resonance at 6cm) operative at the two wavelengths. It is shown that an independent variation of a few percent in the magnetic field (900 ±45 G) and a factor of two variation in the density (2 to 4 × 1010 cm−3) over a time scale of few hours is required to explain the uncorrelated brightness temperature variations at the two wavelengths.