The advantages of studying samples of supernova remnants in external galaxies will be discussed. A summary of how the remnants can be used to infer starformation rates in starburst galaxies will be given, as will the use of these remnants to probe the interstellar medium of these galaxies on parsec scales. Widefield EVN and Global VLBI measurements of remnants in the nearby starburst in M82 will be described in detail, and recent expansion velocity measurements described. The similarity between the strongest compact source in M82 and the objects recently discovered in Arp220 will be noted. The sensitivity limitations of this work will be discussed and the prospects for studies of more distant objects using Square Kilometer Array considered.

Images are presented of the starburst galaxy M82 from a 20-station global VLBI experiment with an angular resolution of up to 3 mas. These images resolve several of the sources into a number of components and with future VLBI experiments the expansion of these components may be tracked on a ∼2 year timescale. Expansion of one of the brightest remnants in M82 has already been measured on a ∼10 year timescale using the European VLBI Network (EVN) alone and an expansion rate of 10,000 km s−1 calculated. Furthermore, limits on the deceleration of this remnant have been deduced. Further VLBI experiments will be able to better constrain the deceleration of the remnants on a ∼10 year timescale as they continue to interact with the interstellar medium in M82.

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

VLA A-array 21cm atomic hydrogen (HI) absorption observed against the central region of the starburst galaxy M82 and MERLIN HI absorption measurements which show HI absorption against 26 of the supernova remnants are discussed. These observations have been compared with the molecular (CO) and ionised ([NeII]) gas distributions and used to constrain the dynamics and structure of the ionised, neutral and molecular gas in this starburst.

Twenty-two consecutive VLBI images of supernova 1993J in the galaxy M81 taken over 7 years show, in unprecedented detail, the dynamic evolution of the expanding radio shell of an exploded star. High precision astrometry using phase-referencing shows that the supernova expands isotropically, and that its geometric center has a formal proper motion of 190±110 km s−1 w.r.t. the core of M81. Systematic changes in the images most likely reflect a pattern of inhomogeneities in the medium left over from the progenitor star, or possibly instabilities in the expanding shell. As the shockfront sweeps up the medium, it is progressively decelerated, and after 7 years it has slowed to less than 1/2 its original expansion velocity. SN1993J is likely now entering the early stages of the adiabatic phase common in much older supernova remnants.

The standard model for radio supernovae considers the synchrotron radio emission to be only partially absorbed by ionized thermal electrons in the circumstellar wind of the progenitor star. However, for the best studied radio supernova, SN 1993J, we present evidence of synchrotron self-absorption based on modelfits to all available radio data. We show that while external absorption is the main absorption mechanism at early times, synchrotron self-absorption is the dominant mechanism at late epochs.

Since our unexpected discovery of 12 compact radio sources in one of the nuclei of Arp220 we have been monitoring the structure using global VLBI and the VLBA. With several epochs of data we can now demonstrate that the fluxes of the radio sources are, in general, decreasing sytematically but not smoothly. The sources also have steep spectra. Both facts support our earlier supposition that these are highly luminous radio supernovae. We initially predicted a radio supernova rate of approximately 2yr−1 Our most recent observations indicate that the occurrence rate of luminous RSN is several times lower than predicted, thus our model of the Arp220 starburst must be revised.

The study of supernovae (SNe) and their environments in host galaxies at the highest possible angular resolution in a number of wavelength regimes is providing vital clues to the nature of their progenitor stars. We are observing SNe in the radio using the Very Large Array (VLA) and VLBI, in the X-rays with Chandra, and in the optical at the subarcsecond resolution of HST.

We report N-body simulations of the birthrate and distribution of supernovae in spiral galaxies. The simulations assume that stars form in GMCs with mass greater than 105M⊙ and that a Miller-Scalo IMF results. We assume that the resulting supernovae disrupt the GMC into smaller clouds; these clouds aggregrate to form new GMCs via inelastic collisions. Imposing a spiral potential, we find that supernovae form throughout the disk, but concentrated in the spiral arms. Using conditions appropriate for the Galaxy, a set of simulations with different initial random distributions of molecular clouds predicts 755τ5 supernova remnants (SNR) should exist in the Galaxy (where τ5 is the ratio of the lifetimes of SNRs to 105 yr). The predicted number of remnants and their spatial distribution can be compared to observations.

The positions (the coordinates and the offset from the nucleus of the host) of extragalactic supernovae (SNe) are very important for further research into their nature. The positions reported in the literature are, however, sometimes erroneous. Here we give the precise coordinates of three SNe based on the recently-developed star catalogs. The standard deviations of the positions are expected to be sub-arcsecond, which can be better than the offsets from the ambiguous nuclei of the host galaxies.

We construct two temporal models for SN1987A. The first model assumes spin independent trajectories for massless particles, while the second assumes spin dependent ones. The suggested scheme can be applied to any supernova event.

It is shown that if the recently discovered hypernova events are intrinsically energetic spherically symmetric explosions of massive stars than they can leave after themselves very large diameter (up to 500 pc) nonthermal radio shells with spectral properties like ordinary shell type radio supernova remnants (SNRs). The properties of such shells are modeled and compared with the statistics of large diameter SNRs.

The nuclear region of the starburst galaxy NGC 253 was observed in H92α (8.3 GHz) and H75α (15.4 GHz) recombination lines at sub-arcsecond resolution using the A-configuration of the Very Large Array (VLA). In the highest resolution observations at 15.3 GHz, we have detected a compact (< 5 pc), high density (> 5 × 104 cm−3) ionized region which is nearly coincident with the radio continuum peak near the nucleus. Modeling of RRL emission from this region shows that about 1000 O stars are needed to maintain the ionization. Assuming the Salpeter initial mass function, the implied overall stellar density corresponds to that of a super star cluster.

I briefly review the means by which VLBI observations can determine the position, proper motion, and parallax of a pulsar, consider a subset of the applications of such results, and highlight recent developments in pulsar gating at JIVE.

We present the results of recent Arecibo observations of the relativistic double-neutron-star binary PSR B1534+12. The timing solution includes measurements of five post-Keplerian orbital parameters, whose values agree well with the predictions of general relativity. The observations show that the pulse profile is evolving secularly at both 1400 MHz and 430 MHz. This effect is similar to that seen in PSR B1913+16, and is almost certainly due to general relativistic precession of the pulsar's spin axis. We also present high-quality polarimetric profiles at both observing frequencies.

We use observations and modelling of geodetic precession in the PSR B1913+16 system to obtain direct constraints on direction and magnitude of kicks imparted to the pulsar companion during the most recent supernova explosion. We find that the kick must have been directed almost perpendicular to the rotation axis of the progenitor star.

The past ten years has seen very significant advances in high resolution imaging arising from the construction and routine operation of Very Long Baseline Interferometer arrays in the US, Europe and Australia. Coupled with the launch of the first VLBI antennas into earth orbit, angular resolution below a milliarcsecond has been achieved on a large number of compact objects. Nevertheless, while the application of phase referencing to VLBI observations has increased the number of sources that can be imaged by VLBI, the astrophysical impact of interferometry on baselines greater than a few tens of km, has been severely limited by sensitivity. The next decade promises to bring about very significant increases in continuum sensitivity at high angular resolution through new developments in VLBI recording devices to achieve recording bandwidths of 4 to 8 Gbits per second, and by direct optical fibre-linked long baseline arrays. In the following decade, the Square Kilometre Array will herald a new era of high resolution radio astronomy with a factor of 100 sensitivity increase. In addition to vastly increasing the sample of non-thermal sources accessible to VLBI, the SKA will open up high angular resolution imaging to a new regime of astrophysics, by enabling milliarcsecond resolution imaging of thermal radio emission. At wavelengths of a few centimetres the SKA, in combination with next generation space VLBI missions, will allow direct imaging of the x-ray emitting accretion disks in galactic nuclei.

High angular resolution imaging at low frequencies (below ∼ 10 MHz) requires observations from above the ionosphere. A radio interferometer array in space will be able to open new vistas in solar, terrestrial, galactic, and extragalactic astrophysics. A space-based interferometer could image and track transient disturbances in the solar corona and interplanetary medium - a new capability which is crucial for understanding many aspects of solar-terrestrial interaction and space weather. It could also produce the first sensitive, high-angular-resolution radio surveys of the entire sky at low frequencies. The radio sky will look entirely different below 10-30 MHz because new emission and absorption processes become dominant at these frequencies. As a result, low frequency surveys from space will provide a fundamentally new view of the universe and an extraordinarily large and varied science return.

The Canadian S2 system gives a chance for Russian and some other radio telescopes in the world to be involved into international VLBI programs. Brief descriptions of previous S2 experiments and future possibilities are presented.

Space VLBI enables high angular resolution and high dynamic range imaging through an extension of ground-based VLBI. The TDRSS space VLBI experiments in the 1980s were followed by the first space VLBI imaging mission, VSOP, in the 1990s. The new century holds the promise of the VSOP-2 and ARISE missions, which aim for more sensitive, higher angular resolution and higher observing frequency capabilities. These missions will enable AGNs to be viewed much more clearly and will make broader science areas, including lower brightness sources, accessible. It is noted that in all space VLBI missions, international collaboration in global sense plays an important role.