Two projects are discussed in which a phase referencing technique is used to determine stellar parallaxes and/or proper motions by means of the circumstellar masers. The observations are special in the sense that both continuum and line sources need to be detected. To obtain stellar motions assumptions on the location of the maser with respect to the star need to be made.

We present the first 86 GHz spectral line VLBI map. Strong SiO masers in the late type star VX Sgr are mapped with a relative astrometry of 0.15 mas revealing an irregular ring of maser emission that echos structure seen at 43 GHz. A surprising feature of this map is the detection of a large (0.5 km s−1 AU−1 ) linear velocity gradient that can be interpreted as rigid rotation of the circumstellar envelope.

The VLBA map of the H2O maser IC 1396N, which was obtained during the VSOP pre-launch survey, shows an ordered position of maser spots. The maser spots are located along an almost straight line with the separation between extreme maser spots of 20 mas. The radial velocity varies along the line from by 3 km s−1. In total 9 maser features have been located on the line, all of them unresolved by the VLBA beam. This geometry is consistent with the disk model of masers. The mass of the disk is estimated between 10−7 and 10−4 M⊙, depending on the thickness and density. The disk must be supported by a central star with the mass of about 0.07 M⊙. The H2O maser in IC 1396N may be a proto-planetary disk around a low mass protostar in the process of accretion.

We report on water maser proper motion in the jet of the YSO IRAS 05413–0104.

VLBI of the H2O maser sources associated with outflows from young stars reveals, besides the regular velocity component (expansion and rotation), a random component suggestive of highly supersonic turbulence generated by the interaction of the star’s jets with the quiescent ambient gas. Our analysis of the geometry and velocity statistics in Sgr B2(M)–H2O demonstrates low fractal dimension of the turbulence and strong deviations of the velocity increments from Gaussian statistics—both indicating strong intermittency of the turbulent energy dissipation. These properties are discussed, along with the two-point velocity scaling law, and compared with the related properties of incompressible turbulence.

We have used 3 antennas of the EVN to observe in 6 star-forming regions simultaneously, and for the first time, the 2 main lines of the J=5/2 state of OH with right and left circularly polarized feeds. Maser features and Zeeman pairs are identified by searching for emission over adjacent channels, and adjacent positions (within one synthesized beam) in both polarizations after we had mapped and used one selected simple channel as a reference. The magnetic field strength is thus estimated from the Zeeman pairs identified in our OH maps. We briefly present results obtained for W3(OH), ON1, and W51.

In order to study the powering source behind KL nebula, we made a map of OH and H2O masers from VLA observations of this region, and compared their spatial distribution and velocity structure with that of the SiO masers. The distribution of the OH masers consists of a multi-stream structure and seems to be related to the Hot Core molecular emission. The low velocity H2O masers may be classified as two groups: i) an inner clump of maser spots surrounding the SiO maser disc and continuum source “I”, and ii) component associated with the Hot Core represented by HDO emission peaks.

We have used the VLBA to image the 12.2 GHz (20-3−1 E) masing transition of methanol toward the massive star formation region G345.01+1.79. The maser spots are distributed in a curved structure with a near monotonic velocity distribution along the curve. The cluster of maser emission covers an area of approximately 200 milli-arcseconds in right ascension and 70 milli-arcseconds in declination.

Comparison of the positions of the 12.2 GHz methanol maser spots in G345.01+1.79 as determined from the 1995 VLBA observations with 1988 Parkes-Tidbinbilla Interferometer observations shows that the relative positions of the maser spots detected in both epochs has changed by less than 5 milli-arcseconds during that interval. Assuming a distance of 2.3 kpc to G345.01+1.79 implies an upper limit on the relative tangential velocities of the maser spots of 7 km s−1.

Since the discovery of the 12.2 and 6.7 GHz methanol maser lines, these masers have been studied in great detail. Even in the earliest studies, it appeared that in some fraction of the sources, the maser spots were arranged in lines. This contrasts with the well-studied OH and water masers, in which the masers tended to be clustered almost randomly around a compact H ɪɪ region. Here I describe recent work to investigate the hypothesis that these lines represent edge-on circumstellar disks.

Using A-configuration of the BIMA-array with 0″.4 angular resolution maps were obtained of the 107 GHz methanol line in W3(OH). The 107 GHz masers have their counterparts in another methanol transition at 6.7 GHz. The strongest maser spots are unresolved with the BIMA-array and are less than , which corresponds to the lower limit of the brightness temperature 5×105 K. A model of Class II methanol masers emitted in the extended atmosphere of icy planets orbiting around the O-star which excites H ɪɪ region is proposed.

The link of the Hipparcos and VLBI extragalactic reference frames has been achieved with a precision of 0.0005″ in global orientation at the epoch of the catalogue (1991.25) and of 0.0003″/yr in rate of rotation by VLBI observations of 12 radio-emitting stars.

The Radio Reference Frame, defined by the radio positions of a large number of extragalactic sources, marks a milestone in the production of a global, self-consistent, quasi–inertial celestial reference frame accurate at the milliarcsecond level. A brief review of the current status and future direction of the project is presented.

The NASA/Stanford Relativity Mission (Gravity Probe B) is to test the unverified “frame-dragging” prediction of general relativity through measurements of the precessions of orbiting gyroscopes. For mission accuracy goals to be met, the proper motion of a “guide star,” whose position will be used as an inertial reference, must be determined in an extragalactic reference frame with a standard error less than 0.5 mas/yr. We discuss our VLBI observations of the current guide-star candidates (radio stars HR 1099, HR 5110, and HR 8703) and our techniques for obtaining differential astrometric positions with the needed accuracy.

We use source models derived from VLBA observations of 169 extragalactic sources to quantify the expected effects of intrinsic source structure on astrometric bandwidth synthesis VLBI observations. We define a source “structure index” based on the median structure corrections and suggest that this index can be used as an estimate of the astrometric quality of the sources. A correlation between the compactness of the sources and their formal position uncertainties was found indicating that the more extended sources have larger formal position uncertainties.

We determine the relative separations of the sources in the triangle 1803+784/1928+738/2007+777 with submilliarcsecond accuracy from global 8.4 GHz VLBI observations. We remove the ionospheric contribution to the phase-delay observable using ionospheric total electron content estimates obtained from Global Positioning System (GPS) data. The triangular geometry provides a consistency check through sky closure.

We present first results of global VLBI astrometric pulsar parallax and proper motion measurements (phase-reference). The aim is to obtain information on pulsar motions and pulsar birthplaces. Proper motions could provide answers to questions like: How large are pulsar velocities at birth? How are these velocities produced and what is the final galactic pulsar distribution? Identification of birthplaces (with, e.g., an OB-association) provides information on the pulsar progenitor population (the fraction of pulsars born in binaries; the mass range of the progenitors etc.). We have a first epoch on three pulsars, selected on the basis of age (young < 3 Myr), flux density (relatively strong) and presence in the solar neighborhood (d < 3 kpc). Gating increases the SNR by typically a factor of 5.

The ALFA mission is designed to map the entire sky at frequencies between approximately 0.3 and 30 MHz with angular resolution limited by interstellar and interplanetary scattering. Most of this region of the spectrum is inaccessible from the ground because of absorption and refraction by the Earth’s ionosphere. A wide range of astrophysical questions concerning solar system, galactic, and extragalactic objects could be answered with high resolution images at low frequencies, where absorption effects and coherent emission processes become important and the synchrotron lifetimes of electrons are comparable to the age of the universe.

A high performance VLBI recorder project using advanced digital technology started in 1995. TOSHIBA GBR 1000 and VLBI interface define 32 parallel, 32 MHz clock standard digital interface and support transparent recording/play-back to extremely high-speed digital bit stream. This is the formatter independent scientific recording bind the UTC to observed data. This highly reliable, low-bit-error-rate (10−16) recording system is also expected to be used in other scientific field. For the first example to examine the recorder performance in VLBI, we had been prepared 1024 Mbps sampler (256/512/1024 MSps, 2 bit, 4/2/1 ch). And here, an experimental giga-bit correlator specification for these VLBI acquisition system is introduced.

We are developing a future space VLBI mission, ARISE, that would involve placing a VLBA-equivalent radio telescope in a highly elliptical Earth orbit. The telescope would operate at VLBI frequencies of 5, 22, 43, and 86 GHz, as well as being capable of single-dish operation at 60 GHz. Fringe-detection thresholds will be less than 10 mJy except at 86 GHz. Key scientific goals include dual-polarization imaging of blazars on the same physical scale as their γ-ray emission, astrophysical and astrometric studies of extragalactic water megamasers, and single-dish observations of O2.

A new version of the Space VLBI Assistance Software (SPAS) developed at the FÖMI Satellite Geodetic Observatory (Penc, Hungary) is described. SPAS version 2.0 will be a useful software tool for proposal preparation in response to the second Announcement of Opportunity of the VSOP satellite.