The galactic x-ray source Circinus X-1 is notable for its periodic flares at x-ray, optical, infrared and radio wavelengths. The flares recur at 16.6 day intervals, but there is considerable variability from one cycle to another. (See Dower et al, 1982, and references therein for a detailed review of the properties of Circinus X-1).

The main purpose of this talk is to outline a few of the problems in our understanding of thermal radio stars that may be resolved by the large arrays. Several radio telescopes in operation (such as the VLA and MERLIN arrays) or under construction (such as the Australia Telescope) have both the required resolution and sensitivity.

SS433 has been under intensive study for the past five years in almost all wavelength bands of the electromagnetic spectrum. This peculiar object is generally regarded (Beer 1981) as being a binary system composed of a main sequence star losing mass via Roche lobe overflow to a massive accretion disk associated with a compact object, probably a neutron star. The binary period is 13.1 days. Supercritical accretion onto the disk causes about 10−6 M⊙/year of ionised matter to be ejected in the form of jets with a relatively constant velocity of 0.26 c along the disk axis. The disk (or the inner part of it) precesses with a period of about 164 days, although there is evidence that this may not be constant. The half angle of the precession cone is ~20° and its axis lies at an angle of ~80° to the line of sight. The main sequence star loses mass at a rate of 10−4 to 10−6 M⊙/yr into a stellar wind with the result that a relatively dense environment surrounds the binary system.

A series of MERLIN observations at λ6 cm of the peculiar object SS433 were made during the spring/summer of 1982. The maps obtained by hybrid mapping show that an elongated structure was formed, extending to ~1 arcsec in length before evolving into two symmetrically placed knots of radio emission straddling a central unresolved core. The structures can be compared with the loci of radio emission expected on the twin jet model for SS433. The knots C and D clearly visible on the map of 820514 (= JD2445104, Figure 1) fit the locus well and were ejected on JD2445085 ±2 at the same time as a strong outburst in the X band total power (Johnston et al. 1983). Knots A and B were ejected earlier but do not lie on the expected locus, even if allowance for the nodding motions proposed by Katz et al. 1982 is made.

There is now growing evidence that the cosmic jet phenomenon manifests itself in a remarkable way in regions of active star formation embedded in dense molecular clouds. The first indications for oppositely directed, supersonic outflows from young stars were provided by molecular line observations (most notably of CO) which detected spatially separated regions of redshifted and blueshifted emission in association with embedded infrared sources. About twenty sources of this kind have been identified so far, and more are continuously being discovered; they typically have radii ∼1018 cm, velocities ∼10–50 km s−1, dynamical ages ∼104 yr, and energies ∼1046-1047 erg s−1 (see Bally and Lada 1983 for a review). Statistical arguments indicate that energetic outflows of this type are probably a common feature in stellar evolution, and that they occur in both massive and low-mass stars. Direct evidence that the outflows in many cases are highly collimated was subsequently provided by the detection of high-velocity Herbig-Haro objects (optical emission clumps with typical masses ∼10−5M⊙) along the axes of the bipolar CO lobes. Proper-motion measurements are now available for a number of these objects (e.g., Herbig and Jones 1981), and they invariably reveal that the velocity vectors (of typical magnitudes 200–400 km s−1) point away from the central star. The clumps are often found to consist of many sub-condensations which move independently with disparate speeds, but which nevertheless travel in the same general direction with an angular spread ≲ 10°. Finally, radio continuum observations (e.g., Cohen et al. 1982) and deep CCD images (e.g., Mundt and Fried 1983) have shown that the collimation of the outflows is already well established on scales of ≲ 1015 cm.

Fine scale electron density fluctuations in the interstellar medium (ISM) are manifest as scintillations and temporal broadening of pulsar signals and as angular broadening of galactic and extragalactic sources. Although scattering off the fluctuations is often a nuisance for conventional studies of radio sources, analysis or searches for interstellar scintillations (ISS) can lead to information about the ISM or radio sources that otherwise would not be obtainable. For example, the length scale probed by ISS in the ISM is typically 1011 cm and the characteristic angular size for quenching ISS is less than 1 μarc sec.

Scattering of radio waves off inhomogeneities in electron density in the interstellar medium can produce an apparent broadening in the angular diameter of an intrinsically compact background radio source. The magnitude and distribution of this effect at low galactic latitudes (|b|<5°) is not well known, although several cases suggest substantial broadening in certain directions, such as the Cygnus X region (Anderson et al. 1972), and the galactic center (Davies, Walsh, and Booth 1976). Large scattering in the plane is consistent with the scintillation properties of pulsars seen through substantial thicknesses (≳ 1 kpc) of the galactic disk.

Synthesis maps of stellar OH maser emission have revealed that the OH lies in expanding spherical shells typically about 1016 cm in diameter. From the maps and the expansion velocity, derived from the OH spectrum, stellar mass loss rates may be determined. Typical values are 10−5 M⊙/yr. An important application of the stellar OH masers is in the estimation of stellar distances.

This is a brief report on a sensitive search for excited OH absorption and emission sources made with the Effelsberg 100 m telescope towards several compact HII regions. Our data are discussed elsewhere (Guilloteau et al., 1983) and we mainly present here our II3/2, J = 5/2 and 7/2 OH narrow emission results (masers). In the J = 5/2 state, 14 sources were observed with a typical 1 σ sensitivity of 0.2 Jy. Among 13 positive detections, 4 and may be 5 are new maser sources: G43.1, W48, K3-50, ON3 and G34.3 (?). With the exception of W3 (OH) the 6035 MHz line is always much stronger than the 6030 MHz line. Comparison with spectra published by others suggests time variability in M17, W51 and Orion A. In W3 (OH) a weak broad emission was also detected at 6049 MHz. In our j = 7/2 observations the peak to peak noise was the range 0.1 to 1 Jy. No masers were detected even in the strong 6035 MHz sources associated with M17 and W33 cont. Thus the F = 4−4 emission line in W3 (OH) seems to be really exceptional (Baudry et al., 1981).

MERLIN observations of the 1612 and 1665 Mhz OH masers from the supergiant IRC+10420 reveal several major departures from the usual morphology of OH/IR stars.

IRC+10420 is one of the most unusual OH maser sources yet found. It is an F8 supergiant with strong OH maser emission at 1612, 1665 and 1667 Mhz. A north-south optical nebulosity with an angular size of 2 arcsec has been detected (Thompson and Boroson,1977) and its peculiar IR properties are reminiscent of Eta Carina. These characteristics make IRC+10420 unique among Type II OH/IR stars.

A technique for mapping spectral line sources using closure phase VLBI techniques is described, and some initial results on circumstellar shell sources are presented. The maps show an unresolved point source on the nearside of the shell, but no corresponding source on the far-side of the shell. This is interpreted in terms of amplification of the stellar thermal emission. Preliminary results are also presented on another source, OH17.7, which show correlated flux outside the velocity range of the known OH masers, and it is suggested that this represents high velocity (~600 km s−1) violent motions around the central star. If confirmed, this may represent a new class of source, representing violent phenomena associated with red giant stars.

The absolute positions and the arcsec structure of OH maser clouds surrounding 20 Mira variables and late-type supergiant stars have been measured using the Very Large Array in a spectral line mode at 1612 MHz. The stars observed are listed in Table 1 which indicates that the angular radii θ of the maser clouds range up to 4″. The linear radii R range from < 100 AU for the Mira variable U Ori to 104 AU for the supergiant IRC+10420 and are correlated with the stellar mass loss rates.

VLBI observations of stellar masers provide important constraints on models for maser excitation and for the dynamics and physical conditions in circumstellar envelopes. The spatial distribution and apparent sizes of Si0 masers toward several late type variable stars and toward the star-forming region in Orion have been measured using the 75 km baseline between the 37 m telescope of Haystack Observatory and the 14 m telescope of the Five College Radio Astronomy Observatory. The fringe spacing is 0.02 arcsec and the transitions observed were the v = 1, J = 1 - 0 and v = 2, J = 1 - 0 lines at 43 GHz. The data were obtained in September 1979 and April 1982.

The H2O outburst in Orion KL had been observed by the VLBI technique in 1979–1983. The line profile is asymmetrical. The flux density and line width are variable and correspond to a partially saturated maser. The outburst region had a compact and a less compact one whose sizes are equal to 0.3 and 2.5 a.u. respectively. The fine structure of the profile corresponds to knots whose size is smaller 0.2 a.u. The brightness temperature of the region, Tb ≃ 1017K, implies a high emission directivity

Magnetic fields play a major role in the general dynamics of astronomical phenomena and particularly in the process of star formation. The magnetic field strength in galactic molecular clouds is of the order of few tens of μG. On a smaller scale, OH masers exhibit fields of the order of mG and these can probably be taken as representative of the magnetic field in the dense regions surrounding protostars. The OH molecule has been shown to emit highly circular and linearly polarized radiation. That it was indeed the action of the magnetic field that would give rise to the highly polarized spectrum of OH has been shown by the VLBI observations of Zeeman pairs of the 1720 and 6035 MHz by Lo et. al. and Moran et. al. VLBI observations of W3 (OH) revealed that the OH emission was coming from numerous discrete locations and that all spots fell within the continuum contours of the compact HII region. The most detailed VLBI aperture synthesis experiment of the 1665 MHz emission from W3 (OH) was carried out by Reid et. al. who found several Zeeman pairs and a characteristic maser clump size of 30 mas. In this work, we report the results of a 5 station VLBI aperture synthesis experiment of the 1665 MHz OH emission from W3 (OH) with full polarization information. We produced VLBI synthesis maps of all Stokes parameters of 16 spectral features that showed elliptical polarization. The magnitude and direction of the magnetic field have been obtained by the detection of 7 Zeeman pairs. The three dimensional orientation of the magnetic field can be obtained, following the theoretical arguments of Goldreich et. al., from the observation of π and σ components.

We report preliminary results of a long term spectral line VLBI experiment to observe internal proper motions of water maser sources in the vicinity of newly formed stars. This technique yields a picture of the three-dimensional kinematics of the region and a measure of the distance to the source. First results from the galactic center source SGR B2 are presented.

Radio astrometry is now the premier astrometric technique for measuring the positions of celestial objects. The precision with which absolute positions can be determined is approaching a few milliarc-seconds. The progress towards establishing an almost inertial reference frame based upon the positions of extragalactic radio sources is reviewed as of June 1983. The outlook for relating this reference frame to optical reference frames is also reviewed.

High precision measurements of the celestial coordinates of pulsars are desirable for a number of reasons. If carried out at several epochs, the measurements can yield angular proper motions; together with distance estimates based on dispersion measure, the proper motion of a pulsar reveals two of three components of its space velocity, and consequently provides important kinematic information on pulsar ages (see, for example, Manchester, Taylor and Van 1974; Lyne, Anderson and Salter 1982; and references therein). Direct measurements of annual parallaxes are also possible in principle, and are marginally feasible with present techniques for a few of the closest pulsars. Model independent distances obtained from parallax measurements, together with observed pulsar dispersion measures, yield the electron density along the line of sight to the pulsar. Knowledge of the interstellar electron density in the solar neighborhood provides a calibration of the dispersion-based distance scale that is complementary to the calibration derived from neutral hydrogen absorption measurements of more distant pulsars (Weisberg et al. 1980), and permits appropriate statistical analyses to be made of the local space density of pulsars and their birthrate (e.g. Taylor and Manchester 1977). Finally, pulsar astrometry can be expected to yield important information on the relative orientations of fundamental reference frames. In particular, pulse timing observations yield positions in a reference frame based on motions of the planets, while interferometric position measurements are based on an Earth-equatorial system. At present the relative orientation of these two coordinate systems is known to only accuracy, though the potential precision of both types of measurements is much higher.

The Hipparcos (H) reference system will contain 100 000 stars whose positions and proper motions will be known with an accuracy of 0″.002 and 0″.002/yr respectively. Because a residual rotation of the H system may exist and because a VLBI extragalactic frame is being constructed one must (i) determine an eventual rotation of the H rigid frame, (ii) link the H system to the VLBI extragalactic frame. VLBI observations at JPL (Fanselow et al., 1981) have provided a catalogue of about 100 quasars with internal accuracy of' ~ 0″.001.

Comparison of optical and radio positions in the northern hemisphere yields local systematic differences up to 0″.2, mainly due to combined systematic errors of current optical reference frame and contributing main catalogues. Interrelations of radio/optical frame and future developments are discussed.