We summarize the results and interpretation of a four station transcontinental VLBI experiment of the luminous water masers in the nearby galaxy NGC 4258. At a distance of 5 Mpc, the longest baseline of the experiment provides spatial resolution of less than 1016 cm. The strongest maser emission was detected on all baselines, and was found to consist of at least two features separated by about 0.1 mas (1016 cm). Weaker features are possibly spread over a region up to 1 mas in size. These results provide evidence that supports the scenario described by Claussen and Lo (1986) which suggests that the very luminous water masers reside in molecular gas that immediately surrounds the central, active nucleus.

On 1984 October 6 we conducted a 3-station intercontinental Mark II VLBI experiment in order to study the very luminous water vapor maser source in the nucleus of the galaxy NGC 3079, which was detected first by Haschick and Baan (1985) using the Haystack Observatory 36.6 m antenna. The cross correlation spectrum for the longest Owens Valley to MPI baseline is presented in Figure 1 and shows the phase variation across the width of the brightest feature at 955.7 km s−1 to be less than 10 degrees of phase.

VLBI synthesis observations of the 2Π1/2, J = 1/2 excited-state of OH at 6.3 cm (ΔF = 1–0 = 4765.562 MHz) have been made towards four compact HII regions. Detailed maps have been produced for W3(OH) where three groups of sources are distributed over a region ∼ 0.01 pc in size. The brightest sources are shown to be saturated masers with TB ≳ 109 K. In ONl a weak feature gives fringes with a ∼ 0.05″ lobe spacing.

Results of further calculations are presented to explore the non-linear, Zeeman overlap effect as the cause for the circular polarization of astrophysical masers. Emphasis is placed on the regime in which the Zeeman splitting is small and on the variation of the polarization with maser saturation.

Natural masers, which emit powerful narrow-band radiation in molecular microwave lines, were discovered about 20 years ago. They draw the astrophysicist's attention in two ways: 1) as a unique physical phenomenon and 2) as very informative probes of physical conditions in those astrophysical objects (very young and very old stars) with which they are intimately connected. In this paper, the first aspect of masers will be stressed, though some consideration will be given to the astrophysical aspect as well. The limited space here will not allow a complete discussion. For a more systematic account, refer to reviews on astrophysical masers by Litvak 1974; Strelnitskij 1974; Kegel 1975; Cook 1977; Moran 1976; Reid and Moran 1981; Elitzur 1982. I shall limit myself only to several topics of general character, which, to me personally, seem to be particularly important for understanding the fundamental principles of cosmic maser physics. I shall then try to illustrate these general principles by the example of H2O masers in star formation regions. These masers, in my opinion, are the most intriguing of all the cosmic masers known at present. And (to make the subjectivity of the report complete!) I shall lay special stress on the results obtained by our group in the last few years.

We report here on some preliminary results from a multi-epoch monitoring project of the OH and H2O masers in the source W43A. Our results suggest that W43A is a very young stellar object which has just entered a strong stellar wind stage.

The dynamics of the molecular envelopes surrounding ultracompact H II regions has been the subject of much debate. Since the envelopes often exhibit molecular line emission, line observations have been the primary method of probing these circumstellar shells. The existence of expansive motions in the molecular clouds surrounding massive newly formed stars has been firmly established. Proper motion studies of galactic water masers indicate general expansion, whereas high resolution CO studies often show collimated bipolar outflows centered on continuum H II regions. Maps of the OH maser emission, often associated with these regions, show the masers scattered, apparently randomly, in both position and velocity across an area a few arcseconds in size. Typical velocity ranges for the associated OH maser emission are on the order of 10 km s−1. A number of models of the dynamical and physical conditions of the masing envelope have been developed to explain the properties of specific regions, e.g., collapsing remnant accretion envelopes, rotating disks or tori of neutral material, and expanding shells of shocked material. There is evidence from comparing the velocities of the OH masers to that of recombination lines associated with the underlying H II regions that in most sources the OH masers are formed in material still accreting toward the central object (Garay et al., 1985) (however see Welch and Marr, 1986). Mirabel et al., 1986, have reported the detection of (thermal and subthermal) high velocity OH outflow in several regions of star formation, demonstrating that OH molecules, at least in some instances, are also involved in collimated flows from central stellar objects.

Information on the structure of the molecular flow within 1″ of IRC-2, in Orion-KL, is sparse. Measurements of the continuum at 7.8μ and 12.5μ show a disk of size and suggest that the center of the disk may be dust free (Lester et al. 1985). Aperture synthesis mapping of water maser shell features (Sylber 1986) has provided information on the scale. Smaller scales can be studied by mapping SiO maser emission. We observed the 43 GHz, v=1, J = 1 → 0, transition of SiO using a 2 station interferometer with a 74 km baseline between Haystack Observatory, Westford, MA and Five College Radio Observatory, New Salem, MA. The fringe spacing was 20 milliarcseconds (mas) and the velocity resolution was 0.25 km-s−1. Our results provide the highest resolution view to date of what is likely to be the inner of IRC-2.

The apparent spot sizes of OH masers appear to be significantly broadened when seen through the inner galaxy or large extents of the galactic disk (Burke 1968). Bowers et al (1980) found evidence of small-scale structure (≲ 50 mas) in OH sources at distances of less than 5 kpc but this was characteristically absent in very distant sources (≳ 8kpc) at galactic longitudes 1 ≲ 40°. This result is typically explained in terms of interstellar scattering (ISS) by intervening diffuse HII regions.

We present preliminary results from a seven year VLBI study of the H2O maser outburst in Orion-KL. Our results reveal changes in the profile, in the spatial structure and in the polarization characteristics of the H2O masers. We interpret our data in terms of edge-on rings rotating about a protostellar object.

The spatial distribution and polarization characteristics of the SiO (v=1, J=1-0) maser emission from several late type stars have been observed. The spatial distribution, derived from VLBI observations, generally shows a number of emitting regions but no clear velocity pattern or geometry. Some of these regions have well defined polarization characteristics. The results of high spatial resolution polarization measurements of RCas are similar to the lower spatial resolution polarimetry performed on this source.

OH megamasers are believed to be active galaxies in which a substantial fraction of the OH gas in the disk of the galaxy is stimulated by the intense far-infrared flux from the active nucleus. The result is that the galactic disk acts as a maser amplifier, producing in the OH line an amplified image of the radio continuum source in the nucleus. Megamasers promise to be powerful tools for the study of active galaxies, provided we can determine what it is that turns an active galaxy into a megamaser. Here I examine the archetypal megamaser galaxy Arp220 and ask the question: what makes it different from other active galaxies?

VLBI OH maser observations simultaneously in right and left circular polarization at the 1665 and 1667 MHz transitions were conducted by taking advantage of the multichannel capability of the Mk III system. The OH maser maps of the right and left circular polarization at both transitions were obtained toward the OH maser sources NGC 6334N, NGC 7538N, NGC 7538S, and G45.07+0.13.

In recent years, observations of stars across the whole electromagnetic spectrum have yielded a wealth of information on their atmospheres and borne out new insights on their internal structures. Indirect evidences for chromospheres, coronae and winds were discovered in stars other than the Sun. For instance, ultra-violet and optical emission lines from collisionally excited atoms are evidences for chromospheric activity with temperatures between 104 and 2 × 105 K. Soft X-ray emission with thermal spectrum is evidence for hot coronae with temperatures between 106 and 3 × 107 K. The spindown of stars with age is best interprated by loss of mass and of angular momentum via magneticly coupled stellar winds originating in hot stellar coronae.

SS433 is thought to be a binary system from which two antiparallel beams are ejected at 0.26c. An excellent review of this peculiar object has been given by Margon (1984). The so-called “kinematic model” describes the precessing motion of the beams. They can be obcervef over at least 8 orders of magnitude in distance: from 1012 cm to 1020 cm.

An analysis of European VLBI data of nova RS Ophiuchi shows the presence of a linear jet-like structure which has a brightness temperature of 107K at 1.7 GHz. This ejection is shown to be two sided and the 1.7 GHz emission is non-thermal. The light curves show the presence of a second, thermal component emerging at higher frequencies at a later epoch.

The X-ray binary Cyg X-3 offers a rare opportunity to study the physical conditions of the formation of a relativistic jet. While the central engines of extragalactic jets are generally poorly understood, the infrared and X-ray data on Cyg X-3 allow detailed modelling of the conditions in the inner disk and corona, the source of the jet energy. Study of the periodic nature of particle injection into the jet may yield an important clue to the nature of the particle injection mechanism. And the time scale of jet evolution is much shorter than for extragalactic sources, allowing the possibility of observing a large number of flares to determine general characteristics.

We have measured the absolute and relative positions of the components of Sco X-1 (Fig. 1) with the VLA from 1980–1986 (Geldzahler, Fomalont, Hilldrup, and Corey, 1981, A.J., 86, 1036; Fomalont, Geldzahler, Hjellming, and Wade, 1983, Ap.J., 275, 802; Geldzahler and Fomalont, 1986, Ap.J., 313, 815). We have used these observations to test the dual-beam model of radio sources proposed by Blandford and Rees (1974).

Five stellar binary systems of RS CVn and Algol type have been observed on June 12, 1986, with the EVN (Effelsberg, Jodrell, Medicina, Onsala, Westerbork), and two US antennas (Green Bank, Haystack). Observations were carried out at 5 GHz, with 13 min of integration and with MARK III mode B.

An ‘interstellar interferometer’ related to a double imaging of the pulsar PSR 1237+25 by refraction in the interstellar medium has been used to resolve the pulsar magnetosphere for the first time. With an interferometer baseline ∼ 1 AU, we have measured a typical transverse separation between the emitting regions of PSR 1237+25 to be ∼ 108 cm, or an angular separation ∼ 0.02 μas.