We report on observations of the kinematical structure of optical filaments in evolved supernova remnants, using an imaging Fabry-Perot interferometer. The radial velocity characteristics as seen in [OIII] λ5007 emission in one area in the Cygnus Loop are described, where four kinematically different components contributing to the emission can be recognized.

Repeated supernovae from an OB association will, in a few ×107 yr, create a cavity of coronal gas in the interstellar medium, with radius > 100 pc, surrounded by a dense expanding shell of cool interstellar gas. Such a cavity will likely burst through the gas layer of a disk galaxy. Such holes and “supershells” have been observed in optical and H I radio emission maps of the Milky Way and other nearby galaxies. The gas swept up in the supershell is likely to become gravitationally unstable, providing a mechanism for propagating star formation that may be particularly effective in irregular galaxies.

Superbubbles play an important role in determining the state of the ISM in both spiral and irregular galaxies. We are modeling supershell dynamics in both homogeneous and stratified atmospheres using ZEUS, a 2-D hydrocode. We find that when a superbubble blows out of a Gaussian atmosphere, the cold, dense shell is not greatly accelerated. In addition, we believe that we observe the Vishniac overstability in radiative, decelerating shells.

New high resolution radio continuum images of the nearby irregular galaxy IC 10 have revealed a large (> 250 pc) nonthermal source. The source is roughly circular with a spectral index of ∼ –0.5, and is most likely a very large supernova remnant. Its large size suggests that it is the result of several supernovae, and may be related to the supershells observed in our own and other galaxies.

We propose a model of expanding supershells regulated by star formation which is induced in HI superclouds by similar shells from a previous evolutionary stage. Compression of pre-existing cloudlets by such shells triggers the formation of massive stars which then explode at the end of their evolution as supernovae. Efficiency of induced supernovae formation must be less than 1% to fit the observational properties of supershells.

We report the detection of molecular clouds associated with three giant galactic H I shells.

The collective effects of supernova (SN) explosions in the nucleus of a starburst galaxy are studied. It is shown that a large wind bubble with the size of a few hundred parsecs in the nucleus of a starburst galaxy will expand for SN explosion rates of 0.1 per year or greater. The bubble gradually elongates due to density stratification. Finally, the shell breaks near the top and the hot matter flows through the cylindrical shell up to 1-2 kpc above the disk plane. We will discuss the X-ray emission in the halos of galaxies such as M82 and NGC 253 and the distribution of molecular gas in such galaxies in relation to the starburst phenomenon.

In this paper we find that the spatial distribution of SNRs shows a peak in the range R = 4–6 kpc; the total number of SNRs in the Galaxy is 749±104; and the birthrate of SNRs is one in every 18±3 years.

A statistical study of the correlation of Galactic supernova remnants with spiral arms and the disk is presented. SNR apparently have a larger radial scale length than disk stars. We estimate that only about 10 percent of the Galactic SNR have been detected.

A new determination of the SN-frequency in different morphological types of galaxies, obtained from the material of the Asiago SN-Search, is presented. From these values we derive a mean interval between SN events in our Galaxy consistent with values determined in different ways.

A physical model for bi-modal star formation and the structure of the interstellar medium and the self-regulating evolution of disk galaxies is presented. Stars heavier than about one solar mass are produced as a result of collisions of molecular clouds or in cloud crushing events whereas low-mass stars are produced at a steady rate in dense molecular clouds and the T-Tauri winds resulting maintain the support of these clouds against rapid collapse and fragmentation. Supernova explosions and stellar winds associated with the massive stars maintain the phase structure, and the scale height of the gas. The collective effects of these energetic processes may create a hole in the disk gas, and allow a galactic wind of metalenriched gas to develop.

In order to explore how supernova blast waves might catalyze star formation, we investigate the stability of a slab of decelerating gas of finite thickness. We examine the early work in the field by Elmegreen and Lada and Elmegreen and Elmegreen and demonstrate that it is flawed. Contrary to their claims, blast waves can indeed accelerate the rate of star formation in the interstellar medium. Also, we demonstrate that in an incompressible fluid, the symmetric and antisymmetric modes in the case of zero acceleration transform continuously into Rayleigh-Taylor and gravity-wave modes as acceleration grows more important.

Recent investigation (Parravano, 1987) shows that the diffuse phases of the ISM condense mainly by the transition from warm gas to small cool clouds (WG → SC). In this work we introduce the new hypothesis that the star formation rate (SFR) in isolated galaxies is self-regulated in such a way that it maintains Pmax close to the ISM gas pressure. Here Pmax is the gas pressure at the marginal state of stability for the transition WG → SC. This hypothesis leads to a relation between global galactic parameters which appears to be applicable to various morphological groups of isolated galaxies.

Gas mass loss in proto-dwarf galaxies can be efficiently driven out by blast waves created by the first generation of supernovae. There is, however, a threshold set by the total gravitational potential beyond which gas mass loss does not occur. This limit is in agreement with the one predicted by some Cold Dark Matter senarios.

At the conclusion of IAU Colloquium 101 a panel discussion was held. The panellists were asked to give their opinions on significant new developments presented at the Colloquium and to outline questions to be answered in the future. The editors have summarized the discussion.