C. de Vegt

A Radio Reference Frame

A catalogue is presented based on the radio positions of 436 extragalactic sources distributed over the entire sky. The positional accuracy of the sources is better than 3 milliarcsec (mas) in both coordinates, with the majority of the sources having errors better than 1 mas. This catalogue is based upon a general solution of all applicable dual frequency 2.3 and 8.4 GHz Mark-III VLBI data available through the end of 1993 consisting of 1,015,292 pairs of group delay and phase delay rate observations. Details and positions are also given for an additional 124 objects that either need further observation or are currently unsuitable for the definition of a reference frame. The final orientation of the catalogue has been obtained by a rotation of the positions into the system of the International Earth Rotation Service and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy.

A radio optical reference frame. I - Precise radio source positions determined by Mark III VLBI - Observations from 1979 to 1988 and a tie to the FK5

Observations from 600 Mark III VLBI experiments from 1979 to 1988, resulting in 237,681 acceptable pairs of group delay and phase delay rate observations, have been used to derive positions of 182 extragalactic radio sources with typical formal standard errors less than 1 mas. The sources are distributed fairly evenly above delta = -30 deg, and 70 sources have delta greater than 0 deg. Analysis with different troposphere models, as well as internal and external comparisons, indicates that a coordinate frame defined by this set of radio sources should be reliable at the 1 mas level. The right ascension zero point of this reference frame has been aligned with the FK5 by using the optical positions of 28 extragalactic radio sources whose positions are on the FK5 system. Because of known defects in the knowledge of astronomical constants, daily nutation offsets in longitude and obliquity were determined relative to an arbitrary reference day in the set of experiments. 30 refs.

Accurate Optical Positions of Extragalactic Radio Reference Frame Sources

Optical positions on the 50 mas level in the Hipparcos system have been obtained for 327 extragalactic, radio reference frame sources for both hemispheres from a 2-step procedure. Positions of secondary reference stars in the 10 ≤ V ≤ 14 magnitude range were obtained from photographic plates taken at the Hamburg astrograph (Northern Hemisphere) and the US Naval Observatory astrograph at Black Birch (Southern Hemisphere) using Hipparcos stars for astrometric plate solutions. Positions of the optical counterparts of compact, extragalactic, radio reference frame sources were then obtained from CCD direct imaging at the KPNO and CTIO 0.9 m telescopes, using the secondary reference star positions and correcting for field distortions. Several previously unknown counterparts could be identified, and several previously suspected identifications turned out to be empty fields. CCD images are available on the World Wide Web to serve as finding charts. A comparison between the optical and radio positions reveals systematic offsets of 10 to 40 mas per coordinate as a function of the observing run. Corrections for Galactic rotation and solar motion have been attempted because of a lack of individual proper motions for the secondary reference stars. Neither 0.9 m telescope is an astrometric instrument, and limitations are clearly seen in remaining systematic errors. Orientation angles between the radio system and our optical data are zero within an accuracy of 4 mas (standard error). A significant reduction of the systematic errors will be achieved with new wide-field CCD imaging currently been taken in parallel to new 0.9 m observations, to provide more secondary reference star positions with higher precision at a common epoch, which will allow extended modeling of the mapping properties of 0.9 m observations.

Astrometric Positions and Proper Motions of 19 Radio Stars

We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine the astrometric positions of 19 radio stars in the International Celestial Reference Frame (ICRF). The positions of these stars were directly linked to the positions of distant quasars through phase-referencing observations. The positions of the ICRF quasars are known to 0.25 mas, thus providing an absolute reference at the angular resolution of our radio observations. Average values for the errors in our derived positions for all sources were 13 and 16 mas in α cos δ and δ, respectively, with accuracies approaching 1–2 mas for some of the stars observed. Differences between the ICRF positions of the 38 quasars and those measured from our observations showed no systematic offsets, with mean values of -0.3 mas in α cos δ and -1.0 mas in δ. Standard deviations of the quasar position differences of 17 and 11 mas in α cos δ and δ, respectively, are consistent with the mean position errors determined for the stars. Our measured positions were combined with previous Very Large Array measurements taken from 1978 to 1995 to determine the proper motions of 15 of the stars in our list. With mean errors of ≈1.6 mas yr-1, the accuracies of our proper motions approach those derived from Hipparcos and, for a few of the stars in our program, are better than the Hipparcos values. Comparing the positions of our radio stars with the Hipparcos Catalogue, we find that at the epoch of our observations, the two frames are aligned to within formal errors of approximately 3 mas. This result confirms that the Hipparcos frame is inertial at the expected level.

THE VARIABLE RADIO SOURCE T TAURI

Centimeter-wavelength radio observations of the T Tauri system between 1983 and 2001 confirm that the radio emission from the optical star T Tau (T Tau N) has a spectral index of 0.6 and may be attributed to a stellar wind and nonthermal flares that are masked by the thermal emission. Right-circular polarization of 26% is detected in the 2 cm T Tau N emission. The infrared source (T Tau S) is confirmed to be nonthermal and variable in intensity, with a radio spectral index ≤0. T Tau S displays weak extended emission and strong, compact left-circular polarization at wavelengths of 3.7 and 2 cm; the percentage of left-circularly polarized emission is larger at the shorter wavelengths. The 2 cm radio and optical positions of T Tau N are found to be coincident to 13 mas. Relative motion between the two radio sources (T Tau N and S) does not agree in detail with that measured at a wavelength of ~2 μm. The difference in relative position is as large as 100 mas (15 AU) in declination in the 1980s. The recent IR detection of T Tau S as a binary star with a separation of 50 mas (7 AU) in 1997 December may explain this discrepancy. We suggest a scenario in which the radio emission detected toward T Tau S in our most recent observations (epoch 2001) is located at the position of the less luminous member of this binary system, T Tau Sb. A preliminary estimate of the orbital parameters of this system is an eccentricity of 0.7, semimajor axis 113 mas, inclination 69°, period 38.8 yr, and total system mass 5.3 M⊙. The circular polarization of the centimeter radio emission of T Tau N and T Tau S indicates that the emission from both components is gyrosynchrotron in nature. The opposite sense of circular polarization may indicate that it is dipole in nature.

A Radio-Optical Reference Frame.VIII.CCD Observations from KPNO and CTIO: Internal Calibration and First Results

In this pilot investigation, precise optical positions in the FK5 system are presented for a set of 16 compact extragalactic radio sources, which will be part of the future radio--optical reference frame. The 0.9 m KPNO and CTIO telescopes equipped with 2K CCDs have been used for this project. The astrometric properties of these instruments are investigated in detail. New techniques of using wide field CCD observations for astrometry in general are developed. An internal precision of 5 to 31 mas in position per single exposure is found, depending on the brightness of the object. The tie to the primary optical reference system is established by photographic astrometry using dedicated astrographs on both hemispheres. An accuracy of

The Second Cape Photographic Catalogue on the HIPPARCOS System

\approx 30

A radio optical reference frame. II ; Additional radio and optical source positions in the northern hemisphere

mas per source is estimated for the multi--step reduction procedure when based on the future Hipparcos catalog, while the FK5--based positions suffer from system errors of 100 to 200 mas as compared to the radio positions. This work provides a contribution to the international effort to link the Hipparcos instrumental coordinate system to the quasi--inertial VLBI radio reference frame. Precise radio and optical astrometry of a large sample of compact extragalactic sources will also contribute to the astrophysics of these objects by comparing the respective centers of emission at the optical and radio wavelengths.

A Catalog of Faint Reference Stars in 398 Fields of Extragalactic Radio Reference Frame Sources

The Second Cape Photographic Catalogue (CPC2) is an astrometric, photographic catalog covering the entire Southern Hemisphere to a limiting magnitude of about 10.5. The Hipparcos Catalogue has been used for a new, plate-by-plate, rigorous reduction. A significant improvement over the release 1 version of the data was achieved. With an average accuracy of 53 mas and a mean epoch of 1968, the CPC2 is a key catalog for proper-motion determination. This release 2 of the CPC2 contains high-quality positions of 266,629 stars and an appendix of 8040 other stars. Catalog reduction and construction details are given, as well as a description of the final product, which is available from the US Naval Observatory.

A radio/optical reference frame. 5: Additional source positions in the mid-latitude southern hemisphere

VLBI observations from 12 sessions have been made of a total of 103 sources north of {minus}2 deg declination. These data give the first extension to the radio/optical reference frame catalog already defined by the data in the NASA Crustal Dynamics Project database prior to July 1988. The new observations add 53 new sources to the radio reference frame catalog, improve the positions of 29 from the previous version, and show another seven sources to be unsuitable reference frame objects. Observations of 14 calibration sources tie the new positions to the existing catalog. The formal mean errors for the radio positions of the additional sources are about 0.8 mas in each coordinate. Optical positions for eight sources have been measured in the FK5 system, bringing the total number of sources in the catalog with accurate positions for their optical counterparts to 36. The new optical positions have an accuracy of about 50 mas. 21 refs.