The paper describes the methods and observations by which the Hipparcos Catalogue was linked to the International Celestial Reference System (ICRS). The contributions of several groups represented in the authorship of this paper, using a variety of techniques, were synthesised in order to determine the global orientation and rotation (spin) of the coordinate frame defined by the Hipparcos data with respect to extragalactic sources. The following link techniques were used: interferometric observations of radio stars by VLBI networks, MERLIN and VLA; observations of quasars relative to Hipparcos stars by means of CCDs and photographic plates. and by the Hubble Space Telescope photographic programmes to determine stellar proper motions with respect to extragalactic objects (Bonn, Kiev, Lick, Potsdam, Yale/San Juan); and comparison of Earth orientation parameters obtained by VLBI and by ground-based optical observations of Hipparcos stars. Although vastly different in terms of instruments, observational methods and objects involved, the various techniques generally agree to within 10 mas (milliarcsec) in the orientation and 1 mas/yr in the spin of the system. Two different numerical methods are described for the systematic comparison and synthesis of the link observations. The methods give very similar solutions, and a mean value was adopted for the definition of the system of positions and proper motions in the Hipparcos Catalogue. As a result, the coordinate axes defined by the published catalogue are believed to be aligned with the extragalactic radio frame to within +/- 0.6 mas at the epoch 1991.25, and non-rotating with respect to distant extragalactic objects to within +/- 0.25 mas/yr.

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

We present 2.7 and 8.1 GHz radio light curves for 46 compact radio sources observed with the Green Bank interferometer during the period 1979-1987. Flux densities for the period 1979-1985 were reported in a previous paper. The flux densities reported here extend the daily monitoring an additional 2 years. This marks the end of the dual use of the interferometer by the US Naval Observatory to measure Earth rotation parameters and by the Naval Research Laboratory to study compact source variability. The interferometer is currently dedicated to searching for extreme scattering events (ESEs). ESEs may be found in the radio light curves of 0300 + 470, 0333 + 321, 0954 + 658, 1502 + 106, 1611 + 343, 1821 + 107, and 2352 + 495 during the period 1979-1987, including three events observed in 0300 + 470 and 0333 + 321 during the additional 2 years reported here.

A cooperative program has been established to produce a reference frame of at least 400 suitable extragalactic sources. This is an extension of collaborative efforts (Johnston et al. , Russell et al. , Ma et al. , and Reynolds et al. ) reported at this meeting. In Phase One, a catalog of radio source positions will be constructed using original VLBI observations already obtained by NASA, NGS, NRL, JPL and USNO. Observations will be re-reduced in a consistent system to avoid some of the more serious problems associated with the formation of compilation catalogs. In Phase Two, the USNO will maintain the system, including the monitoring of source structure and/or variation. During both phases cooperation among the various agencies will continue. A list of sources will be made available to optical observers so that the frame will have benchmarks in both the optical and radio regimes.

Les observations RX, IR et radio du systeme Wolf-Rayet WR 140, WC 7 + O 4-5, realisees de 1979 a 1989 sont presentees. La photometrie IR, les courbes de lumiere, la periode, les elements orbitaux et les parametres physiques sont analyses.

The results of a VLA program are reported to examine the radio continuum emission from 11 rapidly-rotating cool giant stars, all of which were originally believed to be single stars. Six of the 11 stars were detected as radio sources, including FK Com and HR 9024, for which there exist multifrequency observations. HD 199178, UZ Lib (now known to be a binary system), and HD 82558, for which there is only 6-cm data. The radio properties of these stars are compared with those of the active, rapidly rotating evolved stars found in the RS CVn binary systems.

The authors present flux densities of SS 433 at 2695 and 8085 MHz spanning the period July 1979 - December 1985. The variations may be characterized as a clustering of flare events separated by periods of quiescent emission. A harmonic analysis reveals no significant periodicities in either the entire 7 yr data set or in subsets corresponding to active and quiescent time frames. There is, however, broadband power from roughly 50 to 200 days which fades during 1985. Moreover, the authors find that the temporal power distribution of SS 433 at both frequencies strongly resembles 1/f noise. Finally, a comparison with 408 MHz data (Bonsignori-Facondi et al., 1986) shows variations in the light curves to lag those at 2695 and 8085 MHz by approximately 4 days.

view Abstract Citations (13) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evidence for Refractive Scintillation of Two Extragalactic Sources Seen through the Galactic Disk Dennison, Brian ; Fiedler, R. ; Johnston, K. J. ; Spencer, J. H. ; Waltman, E. B. ; Angerhofer, P. E. ; Florkowski, D. R. ; Josties, F. J. ; Klepczynski, W. J. ; McCarthy, D. D. ; Matsakis, D. N. Abstract Extragalactic radio sources were qualitatively examined to determine if there are any cases in which apparent variations at 2.7 GHz are uncorrelated with variations at 8.1 GHz, as might be expected for extrinsically induced refractive scintillation. Two sources exhibiting this behavior were identified, NRAO 150 and 2013 + 370, both of which happen to lie within 2 deg of the Galactic plane. The calculated refractive scintillation properties, based on intrinsic VLBI sizes and scattering measurements in the Galactic disk, are in reasonable agreement with the observed fluctuations. The light curves of these sources are interpreted as significantly affected by refractive scintillation in the interstellar medium. The findings suggest that extragalactic sources seen through extended paths in the Galactic disk may be modulated by refractive scintillation even at gigahertz frequencies. Publication: The Astrophysical Journal Pub Date: February 1987 DOI: 10.1086/164955 Bibcode: 1987ApJ...313..141D Keywords: Astronomical Spectroscopy; Extragalactic Radio Sources; Milky Way Galaxy; Refracted Waves; Scintillation; Interstellar Matter; Light Curve; Astrophysics; INTERSTELLAR: MATTER; RADIO SOURCES: GENERAL full text sources ADS | data products SIMBAD (2) NED (2)

In 1985 April, the WC7+abs star HD 193793 was observed, using UKIRT, to have brightened significantly in the infrared owing to the formation of a new dust shell. Examination of infrared photometry of this star since 1979 and previously published data indicates that the dust formation occurs at intervals of 7.9 years. Phasing the published radial velocities of the absorption line component with this period confirms that it is a member of an eccentric (e = 0.7−0.8) binary system having periastron passage shortly before dust formation. The X-ray spectrum also changed between 1984 and 1985 in becoming significantly “harder” while the non-thermal radio source disappeared, both changes indicating greater extinction. This suggests a model wherein the source of the non-thermal radio and X-ray emission moves deep into the Wolf-Rayet wind.