The high resolution obtained through the use of VLBI gives an unique opportunity to directly observe the interaction of an expanding radio supernova with its surrounding medium. We present here results from our VLBI observations of the young supernovae SN 1979C, SN 1986J, and SN 2001gd.

Global VLBI 1.7GHz datasets from 1990 and 1998 have been used to study the evolution of SNR 41.95+575 in M82. An expansion velocity of around 1800 km/s is found with an implied birth around 1915. A young 41.95+575 would resemble radio supernovae seen in Arp220, although it could also be an evolving accretion disc system

We conducted an observation of weak HI and OH absorption features in the parsec-scale nuclear region of NGC 3079 using a global VLBI network; the VLBA, the VLA and the Green Bank 43-m telescope of NRAO. One mas corresponds to 0.076 pc in NGC 3079.

We have carried out VLBA observations of the OH absorption lines toward the nucleus of the active galaxy, NGC 5793, which is known to have a megamaser in its center. We have also successfully made its continuum image at the L band. It reveals the typical “core-jet” structure on parsec scales. Uniform values of optical depth at each point against the background radio continuum source shows that the column densities of the OH absorbing cloud uniformly distributes around the radio compact nucleus.

NGC 3079 has very luminous water megamaser from the nucleus, the peak of the spectrum being blueshifted by 180 km s−1 from the systemic velocity of the galaxy (Vsys = 1131 km s−1) (Henkel et al. 1984, Haschick & Baan 1985). Core-jet like continuum structure is also found in the nuclear region (Irwin & Seaquist 1988). No velocity drift for main features of water maser (VLSR = 941–975 km s−1) has been shown (Nakai et al. 1995). However, the drift was recently detected for the maser of 1190 km s−1 (Nakai 1997). HI and OH absorptions are detected in the nucleus (Haschick & Baan 1985, Irwin & Seaquist 1991). Thus this galaxy is very unique object to investigate water masers, continuum structure and absorption features all together with VLBI.

Residual delay errors in VLBI data limit the image sensitivity and dynamic range attainable using current calibration techniques such as self-calibration. Applying phase corrections derived from regular observations of nearby calibrators (phase-referencing), the residual delay errors on a target source can be reduced, increasing coherent integration times. In this paper we present the results of a series of observations made with the VLBA to examine the effectiveness of phase-referencing for improving instrumental phase stability. Optimum observing strategies when studying weak (~ mJy) sources are discussed.

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.