K. J. Johnston

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.

TANAMI: tracking active galactic nuclei with austral milliarcsecond interferometry - I. First-epoch 8.4 GHz images

We introduce the TANAMI program (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) which is monitoring an initial sample of 43 extragalactic jets located south of -30 degrees declination at 8.4 GHz and 22 GHz since 2007. All aspects of the program are discussed. First epoch results at 8.4 GHz are presented along with physical parameters derived therefrom. We present first epoch images for 43 sources, some observed for the first time at milliarcsecond resolution. Parameters of these images as well as physical parameters derived from them are also presented and discussed. These and subsequent images from the TANAMI survey are available at this http URL We obtain reliable, high dynamic range images of the southern hemisphere AGN. All the quasars and BL Lac objects in the sample have a single-sided radio morphology. Galaxies are either double-sided, single-sided or irregular. About 28% of the TANAMI sample has been detected by LAT during its first three months of operations. Initial analysis suggests that when galaxies are excluded, sources detected by LAT have larger opening angles than those not detected by LAT. Brightness temperatures of LAT detections and non-detections seem to have similar distributions. The redshift distributions of the TANAMI sample and sub-samples are similar to those seen for the bright gamma-ray AGN seen by LAT and EGRET but none of the sources with a redshift above 1.8 have been detected by LAT.

Astrometric Detection of a Low Mass Companion Orbiting the Star AB Doradus

We report submilliarcsecond-precise astrometric measurements for the late-type star AB Doradus via a combination of VLBI (very long baseline interferometry) and HIPPARCOS data. Our astrometric analysis results in the precise determination of the kinematics of this star, which reveals an orbital motion readily explained as caused by gravitational interaction with a low-mass companion. From the portion of the reflex orbit covered by our data and using a revised mass of the primary star (0.76 M☉) derived from our new value of the parallax (66.3 mas < π < 67.2 mas), we find the dynamical mass of the newly discovered companion to be between 0.08 and 0.11 M☉. If accurate photometric information can be obtained for the low-mass companion, our precise mass estimate could serve as an accurate calibration point for different theoretical evolutionary models of low-mass objects. This represents the first detection of a low-mass stellar companion using VLBI, a technique that will become an important tool in future searches for planets and brown dwarfs orbiting other stars.

VLBI Observations of Southern Hemisphere ICRF Sources. I

We present 8.4 GHz very long baseline interferometry (VLBI) observations of 69 southern hemisphere extragalactic sources in the International Celestial Reference Frame. These are the first in a series of observations intended to image all such sources at milliarcsecond resolution in order to determine their continued suitability for reference-frame use based on intrinsic structure. We use the resultant images to calculate a core fraction, that is, the ratio of core flux density to total flux density, for all observed sources. The resulting distribution, with a mean value of 0.83, suggests that most sources are relatively compact. However, just over half the observed sources show significant extended emission in the form of multiple compact components. These sources are probably poorly suited for high-accuracy reference-frame use unless intrinsic structure and potential variability can be taken into account. Our observations represent the first large, comprehensive VLBI imaging survey in the southern hemisphere, significantly extending the existing limited VLBI surveys and, along with some well-known objects, containing many sources that have never been imaged at milliarcsecond resolution. The overlap with Very Long Baseline Array images of sources between 0° and -35° declination helps determine the limits to imaging with the southern hemisphere–accessible telescopes.

Envelope structures and optical/radio positions of cool stars

Les resultats dobservations VLA sont presentes pour les masers OH associes avec les variables Mira IK Tau, U Her, R Aql, RR Aql et la supergeante S Per. Les positions optiques astrometriques de ces etoiles ainsi que U Ori sont presentees en vue de comparer les referentiels radio et optique. Les rayons lineaires des enveloppes OH setendent de 35 UA a 450 UA et des structures axisymetriques sont presentes dans toutes ces enveloppes. Un modele est propose dans lequel la matiere secoule de letoile dans une configuration ellipsoidale en expansion radiale. Ce modele permet dexpliquer les nombreuses complexites dans les distributions angulaires et les structures des profils des masers OH circumstellaires qui ne peuvent pas etre expliquees par les modeles standards denveloppes spheriques en expansion

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.

VLBI Observations of Southern Hemisphere ICRF Sources. II. Astrometric Suitability Based on Intrinsic Structure

We present 8.4 GHz very long baseline interferometry (VLBI) observations of 48 southern hemisphere extragalactic sources from the International Celestial Reference Frame. These are the second in a series of observations intended to image all such sources in the southern hemisphere at milliarcsecond resolution and bring the total number of observed sources to 111. We use these data, together with previously published data, to quantify the magnitude of the expected effect of intrinsic source structure on astrometric bandwidth synthesis VLBI observations by calculating a structure index for the sources; the structure index yields an estimate of their astrometric quality. Approximately 35% of sources in our sample have a structure index indicative of compact or very compact structures. The remaining two-thirds of our sources are less compact and should probably be avoided in astrometric and geodetic VLBI experiments requiring the highest accuracy unless intrinsic source structure can be accounted for in the astrometric/geodetic analysis.

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.